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Global Warming Thesis Statement Ideas

Economic Impact of Coastal Erosion

Global warming is a complex problem that often sparks policy debates. When writing about it, stick to the facts and make sure that your thesis statement -- the central assertion of your essay -- is supported by research. Some global warming topics have produced extensive research worldwide and can serve as topical guides in formulating your thesis statement.

Manmade Causes versus Natural Causes

The causes of global warming are complex, including natural and man-made emissions of carbon dioxide and methane. Use your thesis to highlight the difference between natural sources and man-made sources. For example, according to the Environmental Protection Agency, carbon dioxide concentrations in the atmosphere have risen from 280 parts per million in the 18th century to 390 parts per million in 2010. Human activities release more than 30 billion tons of carbon dioxide each year, or 135 times as much as volcanoes. Focus your thesis on this discrepancy, how man-made carbon dioxide sources such as fossil fuel consumption, have eclipsed natural sources of the gas.

Rising Temperatures and Declining Sea Ice

Your thesis statement may focus on the relationship between rising surface temperatures and declining sea ice, specifically ice in the Arctic. For instance, since 1901, sea surface temperatures have risen at an average rate of 0.13 degrees Fahrenheit per decade, with the highest rates of change occurring in the past three decades alone, according to the EPA.

Your thesis may establish the inverse relationship between these rising surface temperatures and the shrinking ice coverage in the Arctic. Arctic sea ice extent in December 2014, for instance, was the ninth lowest in the satellite record. The rate of decline for December ice alone is 3.4 percent per decade, according to the National Snow and Ice Data Center.

Effects of Melting Glaciers on Water Supply

Along with sea ice, many of the world’s glaciers are melting due to climate change. Since the 1960s, the U.S. Geological Survey has tracked the mass of two glaciers in Alaska and one in Washington state, all three of which have shrunk considerably in the past 40 years.

Research other mountain ranges and compare the glaciological data. Use your thesis to answer the question of what melting glaciers will mean for populations dependent on the ice flows for their fresh water supply. For example, much of Peru’s population depends on Andean glaciers not only for drinking water but for hydroelectricity.

Effects of Drought on Food Production

While global warming is projected to raise sea levels and flooding in coastal regions, it’s also been credited for changes in weather patterns and extreme drought, according to the EPA. In the arid American Southwest, for example, average annual temperatures have increased about 1.5 degrees Fahrenheit over the past century, leading to decreased snowpack, extreme drought, wildfires and fierce competition for remaining water supplies.

As drought still rages in this region, your thesis can explore the relationship between global warming and agriculture, specifically in California’s Central Valley, which provides produce for much of the country. It’s possible that hotter, longer growing seasons are beneficial to California crops, but that shrinking water supplies threaten the viability of commercial agriculture.

Ocean Acidification and Global Seafood Stocks

Increased carbon dioxide emissions don't just impact our air quality. These emissions also result in increased acidity of our planet's oceans. An immense range of shellfish and other molluscs, such as clams, oysters, crabs, lobsters and more, face immediate population decline due to ocean acidification weakening their calcium carbonate shells.

Your thesis can explore the mechanics of ocean acidification as well as the potential economic impact to the fisheries that rely upon these marine animals for survival. You can also explore the potential ecosystem impact for the predators that feed upon these animals.

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• U.S. Environmental Protection Agency: Causes of Climate Change
• U.S. Environmental Protection Agency: Climate Change Indicators in the United States
• National Snow and Ice Data Center: Artic Sea Ice News and Analysis
• U.S. Geological Survey: 3-Glacier Mass Balance Summary
• National Geographic: Signs from Earth: The Big Thaw
• U.S. Environmental Protection Agency: Climate Impacts in the Southwest
• Alaska Public Media: Ocean Acidification

About the Author

Scott Neuffer is an award-winning journalist and writer who lives in Nevada. He holds a bachelor's degree in English and spent five years as an education and business reporter for Sierra Nevada Media Group. His first collection of short stories, "Scars of the New Order," was published in 2014.

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Climate Change, Health and Existential Risks to Civilization: A Comprehensive Review (1989–2013)

Associated data.

Background: Anthropogenic global warming, interacting with social and other environmental determinants, constitutes a profound health risk. This paper reports a comprehensive literature review for 1989–2013 (inclusive), the first 25 years in which this topic appeared in scientific journals. It explores the extent to which articles have identified potentially catastrophic, civilization-endangering health risks associated with climate change. Methods: PubMed and Google Scholar were primarily used to identify articles which were then ranked on a three-point scale. Each score reflected the extent to which papers discussed global systemic risk. Citations were also analyzed. Results : Of 2143 analyzed papers 1546 (72%) were scored as one. Their citations (165,133) were 82% of the total. The proportion of annual papers scored as three was initially high, as were their citations but declined to almost zero by 1996, before rising slightly from 2006. Conclusions : The enormous expansion of the literature appropriately reflects increased understanding of the importance of climate change to global health. However, recognition of the most severe, existential, health risks from climate change was generally low. Most papers instead focused on infectious diseases, direct heat effects and other disciplinary-bounded phenomena and consequences, even though scientific advances have long called for more inter-disciplinary collaboration.

1. Introduction

In 1988 the leading climate scientist James Hansen, of the National Aeronautics and Space Administration, with three other senior researchers, testified to a U.S. Congressional committee that it was 99 percent certain that the warming trend in Earth’s temperature that was then observed was not natural variation but was caused by the accumulation of carbon dioxide and other “greenhouse” gases. This testimony was reported prominently in the New York Times [ 1 , 2 ]. Hansen was criticized then, and many times since, for his “adventurous” interpretation of climate data, however the publicity which followed his testimony, itself reflecting a decade of growing agitation about the geo-political impacts of climate change [ 2 ] may have influenced health workers to think more deeply about the issues. In any case, within a year, a Lancet editorial discussed health and the “greenhouse effect” [ 3 ], possibly the first such publication in a health journal, eight years after a chapter concerning climate change and parasitic disease appeared [ 4 ]. At least six other chapters on this topic were published in the 1980s, as well as at least two reports. For details, see [ 5 ]. Two other journal articles concerning climate change and health were also published in 1989 [ 6 , 7 ].

The 1989 editorial stated “global warming, increased ultraviolet flux, and higher levels of tropospheric ozone will reduce crop production, with potentially devastating effects on world food supplies. Malnutrition (sic) might then become commonplace, even among developed nations, and armed conflicts would be more likely as countries compete for a dwindling supply of natural resources” [ 3 ]. In the New England Journal of Medicine, Leaf warned, also in 1989, of sea level rise, especially in the south-eastern U.S. state of Florida, higher precipitation, millions of environmental refugees, an increased risk of drought and the possibility that warming at higher latitudes would not fully compensate any climate change related loss of agricultural productivity towards the equator [ 6 ]. The third paper published that year [ 7 ] was even more direct, warning of “catastrophic” consequences to human health and well-being.

In the early 1990s, warnings of potentially catastrophic consequences of climate change continued to dominate. Yet, by the turn of the millennium, the author had formed the impression that the scientific publishing milieu was becoming less receptive to the message that climate change and other forms of “planetary overload” [ 8 ] pose existential, civilization-wide risks. This was disturbing, as my own confirmation bias seemed to support the case that the evidence of existential risk was continuing to rise [ 9 , 10 ].

That the health risks from climate change are indeed extraordinarily high was stressed in the 2009 publication of the lengthy (41 page) article by the Lancet and University College London Institute for Global Health Commission, which described climate change as the “biggest global health threat of the 21st century” [ 11 ]. Yet, although this paper attracted considerable attention at the time, the long-term outlook for climate change and health has since continued to deteriorate.

By existential, I mean related to the word “existence”. But it is not the continued existence of Earth that is in doubt, but instead the existence of a high level of function of civilization, one in which prospects of “health for many” (though no longer “health for all”) are realistic and even improving [ 12 ]. Existential risk does not necessarily mean that global civilization will collapse. Nor does it exclude pockets of order and even prosperity enduring for generations, from which global or quasi-global civilization may one day emerge, provided worst case scenarios are avoided, such as runaway climate change and nuclear war leading to nuclear winter [ 13 ]. Compared to today, such prospects should be recognized as catastrophic. Unchecked climate change could generate similar, or bleaker, global futures. Seeking to minimize such possibilities should be seen as a major responsibility for all workers concerned with sustaining and improving global public health.

There is reticence [ 14 , 15 ], shared by many authors, reviewers, journals, funders and media outlets to discuss the possibility of such existential risks. Nonetheless, the consequences for health are so vast that discussion is warranted. This paper seeks to do that, in the process conducting the largest review on the topic of climate change and health yet to be published.

1.1. Climate Change Science, Risk and the 2015 Paris Agreement

The scientific knowledge that gases, accumulating mainly from the burning of fossil fuels and the clearing of forests, add to the natural “greenhouse effect” has been known since the 19th century [ 16 ]. In 1957 scientists observed “human beings are now carrying out a largescale geophysical experiment of a kind which could not have happened in the past nor be reproduced in the future. Within a few hundred years we are returning to the air and oceans the concentrated organic carbon stored over hundreds of millions of years” [ 17 ].

In 2015 the Paris climate change agreement, negotiated by representatives of 196 parties (195 nations and the European Union) committed countries (thus, effectively, civilization), upon ratification, to actions that would seek to restrict average global warming to “well below” 2 °C above “pre-industrial” levels and to “pursue efforts” to limit the rise to 1.5 °C. The text of the Paris Agreement defines neither the pre-industrial temperature nor the time for this baseline, but most experts agree that it means the temperature in the late 18th or 19th century, soon after the start of the industrial revolution, when coal burning increased. This time is after the end of the Little Ice Age, which itself was accompanied by a rebound in average temperatures, independent of the slow rise in greenhouse gases (chiefly methane and nitrous oxide as well as carbon dioxide) that occurred throughout the 19th century. Estimates of global warming for the period 1861–1880 until 2015 range from 0.93 °C [ 18 ] to 1.12 °C [ 19 ].

Although the goal of 1.5 °C is widely known, there is less understanding that meeting this challenge would not guarantee safety from a climate change perspective [ 20 ]. Indeed, if it were to be more widely accepted that climate change has already contributed to the Syrian war [ 21 , 22 ], to the rise in global food prices which accompanied the 2010 drought and heatwave in Russia [ 23 , 24 ], and the 2018 wildfire season in the Northern Hemisphere, then the threshold of danger might already be widely seen as having long been exceeded.

In recent years the science concerning the physical impacts of climate has continued to expand and to disturb. Average global temperatures continue to rise [ 25 ], apparently in a process more “stepped” than as a trend [ 26 ] with record average global heat in both El Niño and La Niña years. Loss of ice from both Antarctica and Greenland is increasing and the rate of sea level rise is consequently accelerating [ 27 ]. Property values in parts of the U.S. East Coast may soon fall, due to sea level rise [ 28 ]. There is growing concern about more intense rainfall [ 29 , 30 ], fires worsened by heat and drought [ 31 ], a weakening Gulf Stream [ 32 ] and increased sinuosity of the jet stream, which can cause unusual cold at lower latitudes, even if the average global temperature is rising [ 33 , 34 ]. The projected trend toward a weaker and poleward-shifted jet stream is also consistent with projections of a significantly increased risk of worsening extreme heat and dryness in the Northern Hemisphere [ 35 ].

There is also growing evidence of greenhouse effect-intensifying feedbacks in the Earth system [ 36 ] that might release enormous quantities of carbon dioxide and methane, independent of fossil fuel combustion, agriculture or deforestation, from sources including warming tundra and increased fires, both of peat and forests [ 37 , 38 ]. Such releases could dwarf the climate saving made possible by the putative implementation of the Paris climate agreement. The strength of the oceanic carbon sink is also weakening [ 39 ]. If this intensifies it is likely to accelerate warming of the atmosphere, ocean and land.

1.2. Interaction, Attribution, and Causation

All, or virtually all, environmental health effects interact with social and technological factors as well as other “purely” environmental determinants. For example, the effects of heat upon individual health are influenced by temperature, humidity, exercise, hydration, age, pre-existing health status, and also by occupation, clothing, behavior, autonomy, vulnerability, and sense of obligation. Does the person affected by heat, perhaps a brick maker in India, have the capacity to regulate her heat exposure; or might they be an elite athlete or emergency worker voluntarily pushing their limits? Other factors influencing the heath impact of heat include housing quality, the presence of absence of affordable air conditioning and energy subsidies, if any. In turn, these factors are influenced by governance and socio-economic status. Thus, the health-harming effects of heat can be seen to have many contributing causes, of which climate change is only one. As McMichael (and before him David Hume, among others) pointed out, causal attribution is to an extent philosophical; it is influenced by the “focal depth” of the examiner’s “causal lens” [ 40 ]. Consider a mass shooting in a school: Some will see underlying social and legal factors as contributing; others may see only the shooter. Yet, a major role and goal of public health is to seek to identify and reduce “deep” or “underlying” causes [ 41 ]. A world in which only the most “proximal” causes are identified will not function well.

Attributing the fraction of human-caused (anthropogenic) climate change to physical events such as storms, floods and heatwaves is similarly contested and assumption-dependent. The contribution of climate change to more indirect, strongly socially mediated effects such as migration, famine or conflict is even more difficult and contentious [ 22 , 42 , 43 ]. Perhaps in part because of these causal complications, issues such as famine, genocide, large-scale population dislocation and conflict have, with rare exceptions [ 44 ], been peripheral to public health. This is despite the obvious large-scale adverse health effects of these phenomena.

Rigorous methods have been developed to detect and attribute the health effects of phenomena that are more directly or obviously related to climate change, such as heat and infectious diseases [ 45 ]. However, excessive caution risks a type II error, the overlooking of genuine effects [ 46 , 47 ]. To reduce this risk, the authors of a recent study on attribution acknowledged the role for “well-informed judgments, based on understanding of underlying processes and matching of patterns of health, climate, and other determinants of human well-being” [ 45 ]. This paper makes many such judgments.

1.3. Integrative Risk and the Sustainability of Civilization

Publications in health journals about nuclear war and health date at least to 1962 [ 48 ]. In 1992 the Union of Concerned Scientists coordinated the “World Scientist’s warning to humanity”, signed by over 1700 leading scientists (but no public health workers) [ 49 ]. This warning was repeated in 2017, with far more signatories (including many health workers) [ 50 ].

Many authors outside health have warned of the fragility of modern civilization [ 51 , 52 ]. However, comparatively few writers with a health background have contributed [ 9 , 10 , 53 , 54 ]. Tony McMichael, who led the first Intergovernmental Panel on Climate Change chapter on health [ 55 ] frequently wrote and spoke of eroding “life support mechanisms” [ 56 , 57 ], a term probably introduced into the health literature in 1972 by Sargent [ 58 ]. Certainly, McMichael wanted to convey, when using this term, a profound risk to human well-being and health.

If civilization is to collapse then effects such as conflict, population displacement and famine are likely to be involved. A heatwave, on its own, is unlikely to cause the collapse of civilization, nor even ruin an economy for a decade. It needs social co-factors to do this. For example, a series of heatwaves damaging crop yields and contributing to internal migration has been postulated as contributing to the Syrian civil war that started in 2011 [ 21 , 22 , 59 , 60 , 61 , 62 ]. Prolonged heat, especially if in a humid setting, could cause some regions to be completely abandoned [ 63 , 64 , 65 ].

A severely damaged health system, allied with worsening undernutrition and poverty, could provide a milieu for a devastating epidemic, including a resurgence of HIV/AIDS [ 66 ]. An increase in infectious diseases, if of sufficient scale, could contribute to integrative cascades of failure generating regional or even global civilization collapse. Infectious diseases, as well as unfavorable eco-climatic change, contributed to the collapse of the Roman Empire [ 67 ].

While such consequences may seem far-fetched to some, the prospect of sea level rise of one meter or more by 2100 (perhaps sooner), proliferating nuclear weapons, millions of refugees, xenophobia and tribalism which limits integration, and growing cases of state failure is disquieting. Few, if any, formal scenarios, as exercises by senior scientists, are as bleak, but funding and other pressures constrain the realism of such exercises [ 15 ]. Already, the number of forcibly displaced people exceeds 68 million [ 68 ], a rise that has been linked with tightening limits to growth, including climate change [ 69 ].

It is stressed, again, that the idea that any single climate related event, such as heat, drought, sea level rise, conflict or migration will cause the collapse of civilization is simplistic. It is far more plausible to conceive that collapse (or quasi-collapse) could arise via a “milieu” of multi-factorial risk, enhancing, inflaming and interacting with climate change and other factors [ 43 , 70 ].

1.4. Hypothesis

This article seeks to test the hypothesis that the early literature relevant to climate change and health was more willing to describe catastrophic, potentially civilization disrupting health effects including famine, mass migration and conflict than it was to become, at least until 2014.

To explore this hypothesis, a database of articles relevant to climate change and health was assembled, relying mainly on PubMed and Google Scholar. This had six steps (see Appendix for details). Due to limited resources, the main search was restricted to the period 1980–2013, and the terms “climate change” and health or “global warming” and health. After eliminating duplicates, remaining papers were checked to see if they met eligibility criteria (see Box 1 ).

inclusion and exclusion criteria.

Included: Articles, editorials, commentaries, journalistic pieces with bylines.

Excluded: Reports, books, book sections including e-chapters, letters, factsheets, monographs, un-credited journalistic entries, non-English publications, papers concerning stratospheric ozone depletion, podcast transcripts, journalistic pieces that could not easily be recovered.

The search was not restricted to health or to multidisciplinary journals. However, papers outside health journals had to meet more exacting requirements to be included. They had to include health (or a synonym such as nutrition) in their title, abstract, keywords or text, even if they focused on an effect with health implications, such as population displacement, conflict or food insecurity.

The title of each identified paper was read, followed by the abstract of each paper, assessed as possibly eligible. If a score was still unclear, the full text was obtained and searched for words and phrases that suggested a broader interpretation of the indirect effects of climate change, such as “population displacement”, “migration”, “conflict”, “war”, “famine”, and “food insecurity”.

Eligible papers were scored as one if they exclusively concerned an effect other than conflict, migration, population displacement or large-scale undernutrition or famine. They also needed to exclude statements (even if introductory) such as “climate change has been recognized as the greatest risk to health in the 21st century”.

Papers were scored as two if they either mentioned such an effect and/or contained statements recognizing the potentially enormous scale of the health impacts from climate change. A synonym for this understanding was the phrase eroding “life support mechanisms”.

Papers were scored as three if they included a more detailed explanation or assertion of the future (or current) existence and importance of conflict, migration or famine, perhaps suggesting an interaction among them. A score of three was more likely if they also warned of the general severity of climate change. The score was also influenced by the tone of the language, and the space devoted to these issues (see Appendix for further details).

In addition, PubMed was searched for papers published from 2014–2017 matching the criteria “climate change” and “health”. A sample of 156 of these articles was randomly selected, approximately 5% in each year, after the elimination of a proportion of ineligible articles. Each was then scored, using the method described above for papers published from 1989 to 2013 (inclusive). Bootstrapping was then used to estimate the average score and 95% confidence interval of these articles, by taking ten thousand resamples, each of 156 papers, with replacement from this set (so that in each iteration some papers will appear more than once, while others will not appear at all).

A total of 2143 unique articles and journalistic essays satisfied the inclusion criteria, for the period 1989–2013 inclusive. The full database is available in the supplementary material . This shows the year, lead author (at least), journal, title and primary search method. It also lists the number of Google Scholar citations and the date these were identified. Table A1 ( Appendix ) tabulates the primary search method of papers, by each year.

No paper published before 1989 was eligible for retention in the final database. One potential publication [ 71 ] was cited by Kalkstein and Smoyer [ 5 ] as published in 1988, but it could not be located. About half the total papers (1142 or 53%) were published since 2009 (see Figure 1 ). Most papers (1546 papers, 72%) were scored as one, while only 189 (3.3%) were scored as three. The difference in these scores is statistically significant ( p < 0.01 ANOVA). The average score of these 2143 papers was 1.37 (see Table A2 in Appendix ).

Number of papers in each category. Since 1989 the number of papers concerning climate change and health has expanded considerably, particularly since 2008. As this article did not review the entire literature, the actual number of papers published, even in English, is more than shown. The average score of these papers declined from 1.9 in the first quintile to 1.34 in the final five years.

The increase in the size of literature reflects growing awareness of the risks to health from climate change. Over 50% of the papers published in the first quintile (1989–1993) were scored as two or three, although the total number in that time (27) was small (see Figure 1 ). Since 1993 the majority of papers have focused on effects such as heat, infectious diseases, allergies or asthma. The number of papers scored as two or three increased slightly after its trough (23%) in the third quintile (1999–2004) but was only 26% for 2009–2013 inclusive.

Papers scored as three were particularly uncommon in the third quintile (1999–2003), representing only 2.6% of the total published papers in that period. Even in the first quintile (1989–1993) most citations were for papers scored as one (see Figure 2 ).

Number of citations per annum for each score of paper. Most citations were for papers scored as one. Note that in 2005–2007 three extensively cited papers were scored as two (these are discussed in the Appendix A ).

3.1. Citations

Citation data were available for 2105 papers (98%). Over 201,000 citations were identified by Google Scholar (see Table A3 in Appendix ). Thirty two percent of these citations were for papers published since 2009 (see Figure 2 ). Of these citations, the great majority (82%) were for papers scored as one, each of which was cited an average of 107 times. Papers scored 2 were cited an average of 73 times, representing 15% of the total. Papers scored as three were cited 35 times each on average and accounted for 3% of the total. The difference in these citation scores is also statistically significant ( p < 0.01 ANOVA). Citations for papers scored as three from 1995 to 2008 inclusive were even lower, accounting for less than 1% of the total citations in each year of this period (see Figure 3 ). The fraction of the literature discussing existential risk remained lower in the last 5 years of this database than in the first five years (see Figure 1 ). The shift in the ratio of annual citations from the early period to the more recent years is evident in Figure 3 . Until 1991, the majority of citations were for papers scored as three. From 1994 the fraction of citations for papers scored as three was almost zero (3% or less) in every year until 2009. In 2013 it again fell to 3%.

The proportion of citations each year for papers scored as one and three. Since 1991 most citations have been for papers scored as 1. The Lancet UCL paper published in 2009 [ 11 ] led to a resurgence of citations for papers scored as 3, but this effect declined. Three individual papers, each scored as two (published in 2005, 2006 and 2007), were disproportionately cited. In each year at least some papers scored two or three, but their proportion of citations fell steeply after the first quintile. In 2003 no paper was scored as three, and for almost a decade (1997–2005 inclusive) virtually no papers scored as three were cited.

3.2. Coverage of Topics

All papers published in 1989 discussed multiple potential health effects of climate change. However, from 1990, journal articles focusing exclusively on infectious diseases and climate change appeared [ 72 , 73 , 74 ]. Early papers also focused on heat [ 75 ] and allergies [ 76 ]. From 2000, the foci of concerns expanded greatly. Additional topics included reduced micronutrient concentrations in food [ 77 ], asthma [ 78 ], thunderstorm asthma [ 79 ], chronic diseases and obesity [ 80 ], toxin exposure (such as from increased concentrations in Arctic mammals [ 81 ] and increased algal blooms [ 82 ]), forest fires [ 83 ], mental health [ 84 ] and respiratory [ 85 ], cardio-vascular [ 86 ], renal [ 87 ], fetal [ 88 ], genito-urinal [ 89 ] and skin conditions [ 90 ]. By 2000, papers were also appearing arguing that the impact of climate change for malaria was overstated [ 91 , 92 ].

Articles also appeared on the impact of climate change on groups such as indigenous people [ 93 ], children [ 94 ], the elderly [ 95 ] and regions and locations, including cities [ 96 ], the Arctic [ 97 ] and small island states [ 98 ] as well as many individual nations. Other themes appeared, including on how the health sector might reduce its carbon footprint [ 99 ], on “co-benefits” [ 100 ], on climate change as a great opportunity to improve public health [ 101 ], on medical education [ 102 ], pharmaceuticals [ 103 ] and on the health risks of adaptation and geoengineering, including of carbon capture and storage [ 104 ].

3.3. The Leadership Role of Some Journals

Many journals played prominent, even campaigning roles, especially the Lancet, BMJ and Environmental Health Perspectives. Several journals had special issues, including Global Health Action, the American Journal of Preventive Medicine, the Asia Pacific Journal of Public Health and Health Promotion International. Seven journals published at least 28 articles each, including editorials and news items (see Table A4 in Appendix ). At least 34 journals published editorials, which, with an average score of 2.2, were more likely to be scored as two or three than journal articles (average score 1.3). News items and other journalistic pieces had an average score of 1.6. At least 21 articles were published in nursing journals, with an average score of 1.67.

3.4. Papers for the Period 2014–2017

A total of 3377 papers were identified by PubMed as published from 2014–2017. Of these, 346 were found to be ineligible, although the true number would be higher, if all candidates were examined. Of the potentially eligible remainder, 113 papers were published in 2018, but recorded by PubMed as e-published in 2017. Slightly over five percent of the articles for each year was randomly selected, resulting in 156 articles (see Table A5 in Appendix ). Their average score and 95% confidence interval, estimated by bootstrapping, was 1.29 (95% CI 1.21–1.39) (see Figure A2 in Appendix ). Details of these 156 papers are in the supplementary material . Note that their citations were not checked.

4. Discussion

This paper describes the first published analysis of the extent to which the literature on climate change and health has described or in other ways engaged with “existential” risk. By including 2000 articles, 60 editorials and 83 news items (2143 “papers” in total) on climate change and health, it is by far the largest review of the climate change and health literature to have so far been published. Lack of resources currently prevents an extension of the fuller analysis to more recent years. However, a randomly selected sample of 156 articles for papers identified by PubMed as published in the period 2014–2017 found that these papers had an average score lower than the average score for any quintile from 1989–2013, other than for 1999–2003 (see Table A2 and Table A5 in Appendix ).

Several systematic and other reviews of topics related to climate change and health have been published, but on a much smaller scale, and with different research questions. Ford and Pearce systematically reviewed 420 papers, published between 1990 and 2009, exploring the topic of climate change vulnerability in the Canadian western Arctic [ 105 ]. Two systematic reviews concerned heat. Huang et al. [ 106 ] searched for papers published between 1980 and July 2010, projecting the heat related mortality under climate change scenarios. Only 14 papers were included in their final analysis. Xu et al. [ 107 ] explored the relationship between heat waves and children’s health, but selected twelve, an even small number. A systematic review into dengue fever and climate change (for the period 1991–2012) included 16 studies [ 108 ].

Nichols et al. (2009) [ 109 ] undertook a systematic review on health, climate change and energy vulnerability, searching for papers published in English between 1998 and 2008. They retrieved 114 papers but included only 36 in their final analysis. Bouzid et al. (2013) undertook a “systematic review of systematic reviews” to explore the effectiveness of public health interventions to reduce the health impact of climate change [ 110 ]. This identified over 3100 unique records, but of these, only 85 full papers were assessed, with 33 included in the final review.

This may also be the first review paper concerning climate change and health to use a citation analysis [ 111 ] as an indicator of influence. Citations in Google Scholar were used for convenience and cost. Although such citations are prone to error, and include essays in the gray literature, they still reflect influence. Some reports in the gray literature may be more widely read and more influential than more scholarly work.

4.1. Selection and Other Forms of Bias

A systematic review was not undertaken. However, all papers identified by searching using PubMed and at least 100 papers for each year identified by Google Scholar were considered for inclusion. The search term relevant to health was restricted to a single word, rather than synonyms such as “disease”, “morbidity”, “illness”, or “mortality”. Undoubtedly, a search using additional terms will identify more papers, as would a systematic review.

To examine the possibility that a more extensive search strategy would alter the conclusions, PubMed was also searched for the terms “climate change” and “morbidity” for papers published in 2013. This strategy identified 261 papers, compared to 496 when searching for “climate change” and “health”. Of these 261 papers, 30 had not previously been identified by the other search methods used, and met the other inclusion criteria. However, all of these additional papers were scored as one. Their inclusion in the final analysis was considered likely to bias the paper away from the null hypothesis, by accentuating the fraction of papers not scored as two or three. This bias towards papers scored one (i.e., identified by searching for “morbidity”) seems plausible because the term morbidity may be more likely to be associated with specific diseases than the term “health”. These papers therefore were not added to the analysis.

The search was supplemented by the addition of 17 papers first identified from the author’s own database, but not later found by the search strategy using Google Scholar or PubMed (steps 2–3) as described in Figure A1 . Eight of these 17 papers, five of which the author wrote or co-wrote, were scored as three. Their average score was 2.17, far higher than for the balance (1.23). This group also includes two editorials, one published in the Lancet, one in the BMJ. The inclusion of one of these editorials (scored as three, published in 1989) has biased the findings in favor of the hypothesis that highly scored papers were more common in the early period of this literature. Note, however, that no citations were recorded for this editorial.

The inclusion of these higher scoring papers later in the period of analysis has biased the result to the null, that is, away from the hypothesis that fewer such papers were published from about 2000. The most influential of these 17 papers, judged by Google Scholar citations, was cited 272 times. It was the first to report that rising levels of carbon dioxide depress micronutrient concentrations in food [ 77 ]. The other 16 papers were cited 405 times between them, an average of 25, which is low compared to the average citation number (94). Twenty eight other papers were included, mostly identified from special issues. Their average score was 1.9. One paper was identified post-review, by chance. It was scored as two (perhaps generously) and was included because it was judged that to exclude it would bias the result away from the null hypothesis.

Bias is also likely to have been introduced in the scoring process, but not to the extent that it could challenge the main conclusions. The rigor of this paper would be improved if the scores could be checked by a third party, blind to the first score. Unfortunately, no resources were available for this purpose. Some classification errors are likely, especially for papers for which the author had no previous familiarity, and if published after 2009, when, due to time pressure, many papers were scored rapidly. On the other hand, in the process of ranking over 2000 papers the author became skilled at making rapid decisions, especially for most papers scored as one. The difference between papers scored one and two was generally more apparent than for papers scored between two and three. In cases of doubt a higher score was always selected.

The likelihood of bias and error is unlikely to explain the difference in the character of the papers in the early period and those which later dominated. Although the widely cited paper by Costello et al. [ 11 ] (1583 citations as of June 2018) may have refreshed appreciation of the potentially catastrophic nature of climate change, the majority of papers and their citations published between 2010 and 2013 continued to focus on specific issues. This trend appears to have persisted in the years since, judged by the analysis of a randomly selected sample, identified by PubMed as published between 2014 and 2018.

4.2. Reasons for the Apparent Conservatism of the Literature

There are several plausible, overlapping and interacting explanations for the decline in the proportion of papers scored as two or three (and for their comparatively fewer citations) following 1996, and also in the failure for papers published since 2009 to fully amplify the most severe warnings. One likely contributing explanation is self-censorship. The topic of climate change and health is unfamiliar territory for many health editors and writers. Climate change has become politicized in many English-speaking countries, especially in the U.S. and Australia. Although comparatively few health workers have expertise concerning climate change and health, the readership of some health journals seems judged, by their editor, to be skeptical of, or even to reject climate science. For example, one editor, defending the decision to publish a paper (scored, possibly generously, as two) [ 112 ] seemed almost apologetic, writing “On its face, the paper by Hess and colleagues is largely a political commentary and a departure from the types of articles found in Academic Emergency Medicine” [ 113 ].

Thus, for some health workers and editors, even broaching the topic of climate change and health may be a courageous act. The publication of papers in health journals that describe potential pathways that could threaten civilization would appear even bolder. It is unsurprising that such papers are still fairly uncommon, at least until 2014, and particularly in journals which do not yet have a long tradition of publishing papers or editorials on this topic.

In the early period of the climate and health literature (1989–1993) some of the most outspoken articles were editorials. Perhaps at that time, there was a certain sense of shock concerning climate change, which has since waned. It was also a time when concerns about overpopulation were slightly less taboo [ 114 , 115 , 116 ]. However, editorials in more years also tend to have a higher index of concern than other articles.

Another likely contributor to the comparative degree of restraint is the view, backed by some research, that an excess of fear is counter-productive [ 117 ]. However, the smell of smoke in a theater requires the sounding of a vigorous alarm. Compounding the difficulty of communicating the risk over climate change is the lag between the whiff of smoke and the onset of visible fire. Hansen warned of great danger over thirty years ago, and he, with others, have issued many warnings since [ 118 ]. Sceptics are still waiting to see the metaphorical “flames” of climate change, even disputing the link between literal flames (fires) and climate change.

On the other hand, science, though not infallible, has delivered countless miracles such as antisepsis, anesthesia, penicillin and the jet engine. It has long warned of the physical changes of climate change. We who work in health should not be amazed if the predictions of climate and Earth scientists prove broadly accurate. Social science is less precise than climatology [ 43 ], however the links between food insecurity, drought, sea level rise, migration and, in some places, conflict are, also, surely not far-fetched. Papers that fail to express appreciation of the extraordinary risks we face as civilization may be judged by people of the future as having failed in their duty of care to protect health.

Another likely reason for the general restraint in the literature is the fragmentation of science and limited funding for multidisciplinary work. Comparatively few authors, other than if collaborating in large, multidisciplinary teams (rare for most authors primarily concerned with health), are rewarded or funded for thinking systemically. This problem is possibly worsening. Related to this, many recent papers are by sub-disciplines of health that have not previously published on the topic of climate change. Such papers are probably less likely to discuss existential risk.

As the effects of climate change have become increasingly clear the need for adaptation has become overwhelming. A stress on adaptation does not necessarily reflect any underestimation of the eventual severity of climate change. However, a stress on adaptation at the expense of mitigation may do so. In many countries, political leadership favors adaptation.

5. Conclusions

In 1989, thirty two years after the International Geophysical Year, the first papers on global warming and health appeared in the world’s leading medical journals [ 3 , 6 , 7 ]. All three of these early papers warned of severe, even existential risk and were each scored as three.

In 1990 McCally and Cassel warned that “progression of these environmental changes could lead to unprecedented human suffering” [ 119 ]. Also, in 1990, Fiona Godlee, then deputy editor of the BMJ, wrote “Countries in the developing world would suffer both the direct effects of drought and flood and the knock-on effect of agricultural and economic decline in the West. The already present problems of feeding the world’s growing population would be compounded by the increasing numbers of displaced people unable to grow their own food” [ 120 ]. In 1992 Powles observed “It is possible that adverse lagged effects of current industrial (and military) activities will disrupt the habitat of future generations of our species through processes such as stratospheric ozone depletion, global warming and others as yet unpredicted” [ 121 ]. However, in the following years, this sense of urgency largely dissipated, until the long paper by Costello et al. in 2009 [ 11 ].

Conditioned by growing up during the Cold War, the author has long been apprehensive about civilization’s survival. However, my timeline for global health disaster has always been multi-decadal. Civilizational collapse, if it is to occur, will not necessarily be in my own lifetime [ 54 ]. My concerns are not based solely on climate change. Climate change, by itself, is most unlikely to cripple civilization. A well-functioning global society, motivated to do so, could easily eliminate hunger and poverty, not only today, but under all but worst-case climate change. Refugees from inundated islands, war-torn Syria or the drought-stricken Chad basin [ 122 ] could easily be accommodated in more fertile and more elevated parts of the world. Unfortunately, humans currently do not co-operate on such a scale, and this behavior may, in part, be driven by inborn, “hard-wired”, evolutionary-shaped traits [ 123 ]. If civilization is to endure we may need to collectively overcome our seemingly deep wiring for tribalism and separation.

Acknowledgments

My thanks to John Potter for his help with locating obscure references, and to Andy Morse and Kristie Ebi for their very helpful comments, and Joseph Guillaume for his statistical advice. I especially thank Ivan Hanigan for the bootstrap analysis. I also thank three anonymous reviewers.

Supplementary Materials

The following are available online at http://www.mdpi.com/1660-4601/15/10/2266/s1 .

Appendix A.1. Detailed Methods and Results

The search method had six steps (see Figure A1 ). Initial exploration used the author’s Endnote database, of over 35,000 references, to find relevant articles. The second step was to search, using Google Scholar, for up to the first 100 results for each year in the search period (1980–2013), using the terms “climate change” and health or “global warming” and “health”. For the first decade in which relevant articles were found (1989–1998) both pairs of terms were used, but from 1999 to 2013 inclusive, only the former terms were used (“climate change” and “health”). In the third step, the search was expanded by seeking the same terms, using PubMed, for the same period; 1980–2013 (inclusive). After eliminating duplicates, all remaining papers were checked to ensure that they met the eligibility criteria listed in Box 1 . In stage 4, several papers were included if they appeared in special issues of journals, together with articles identified by PubMed, or suggested by colleagues. In stage 5, the BMJ database for news items about climate change and health was searched explored, because although PubMed found a few the proportion it identified was low. Finally, in stage 6, several other papers were found by chance, such as in reviews, in the references of cited papers, or by searching for other papers.

Outline of the six stage search strategy for papers published from 1989–2013.

Appendix A.2. Further Scoring Details

The following details are provided in order to provide additional information about the scoring process. It discusses the scoring process for three highly cited papers (from 2005–2007), each of which was scored as two. The first (cited 2059 times) had no mention of population displacement or conflict, but included the sentence “Projections of the effect of climate change on food crop yield production globally appear to be broadly neutral, but climate change will probably exacerbate regional food supply inequalities” [ 124 ]. This statement was assessed as accepting the possibility of a degree of food scarcity judged to be more severe than that described by many papers (particularly concerning the Arctic) which discuss a likely impairment in regional nutrition, but do not forecast insufficient calories or nutrients, let alone famine. Although the conclusion regarding overall global food security in this paper was reassuring, there are already four acknowledged famines in African nations and one in Yemen [ 125 ]. Any exacerbation of regional food supply inequalities is therefore likely to result in aggravated famines, unless future famines are eliminated; an unlikely prospect. Because this paper was cited so frequently a lower score would impact the overall result. If there is a bias from scoring this paper as two it is towards the null hypothesis.

In 2006 a widely cited paper [ 126 ] stated “Other important climatic risks to health, from changes in regional food yields, disruption of fisheries, loss of livelihoods, and population displacement (because of sea-level rise, water shortages, etc.) are less easy to study than these factors and their causal processes and effects are less easily quantified”. This is a more comprehensive list of civilization-endangering effects than the paper discussed above, but the language is restrained and brief. It was scored as a two.

In 2007 another widely cited paper included the sentences “Climate change will, itself, affect food yields around the world unevenly. Although some regions, mostly at mid-to-high latitude, could experience gains, many (e.g., in sub-Saharan Africa) are likely to be adversely affected, with impairment of both nutrition and incomes. Population displacement and conflict are also likely, because of various factors including food insecurity, desertification, sea-level rise, and increased extreme weather events” [ 127 ]. Of the three papers discussed here this provided the most comprehensive list of such effects and also explores their interaction. However, it did not speculate about civilization collapse, nor describe climate change as the biggest threat to global public health.

A gradient exists between papers scored two or three, rather than a clear threshold. Papers were not scored as three simply by including a more detailed explanation or assertion of the existence and importance of conflict, migration or famine, even if an interaction among them was suggested. They needed something extra. For example, one paper [ 128 ] stated (referring to Costello et al. [ 11 ]) “a watershed paper … suggests that climate change represents the biggest potential threat to human health in the twenty-first century … a recent report … also estimates that four billion people are vulnerable and 500 million people are at extreme risk”. This paper was scored as three even though the paper focused on medical education. Although the phrase “the biggest potential threat to human health in the twenty-first century” can, with repetition, lose its capacity to shock, its meaning, if taken literally, is surely sufficiently dire to be scored as three.

Another paper (scored as three) stated “global health, population growth, economic development, environmental degradation, and climate change are the main challenges we face in the 21st century” [ 129 ]. It also stated that “significant mass migration is likely to occur in response to climate change”.

The interpretation of papers was not excessively generous. For example, a paper that noted: “Changes in the frequency and intensity of extreme weather and climate events have had profound effects on both human society and the natural environment” was scored as one because there was no discussion of this aspect in the abstract or further in the text. It was also considered that the words “have had profound” was insufficiently clear. Nor did the paper discuss conflict, migration or famine.

In contrast, two papers about climate change and health in Nepal were scored as two, as they included the statements “Climate change is becoming huge threat to health especially for those from developing countries” (sic) [ 130 ] and “Climate change is a global issue in this century which has challenged the survival of living creatures affecting the life supporting systems of the earth: atmosphere, hydrosphere and lithosphere” [ 131 ].

Appendix A.3. Sources (Detailed)

Seventeen articles were identified from the author’s database, but not found via PubMed or Google Scholar. Other sources are shown in Table A4 .

This shows the primary source of the 2146 included articles. 18 articles were from special issues, 5 were found accidentally, 1 was from a review and 1 was from a colleague. Many articles were found using multiple methods. The papers listed here in the GS column were not found by PM but may also have been identified by CB. Abbreviations: PM = PubMed, GS = Google Scholar, CB = Colin Butler.

Appendix A.4. Score, Citation and Journal Details

This shows the number of articles and their average score for each quintile from 1989–1993.

This shows the number of papers and citations in each category divided into five quintiles for the 25 years of analysis. Note that in the third quintile (1999–2003) only 5 articles were ranked as three. Ironically, the paper scored as three in 2002 was a news item which quoted Andrew Sims, policy director of the New Economics Foundation as lamenting “Health is not even being talked about here [Delhi], although the potential health impact is a devastating one, almost unimaginable” [ 132 ].

Ten journals published at least 22 articles on climate change and health in the period 1989–2013.

Appendix A.5. Additional Papers 2014–2018

PubMed was searched for the terms “climate change” and “health” for the period 2014–2017 inclusive. This found 3377 papers, which were grouped by year of publication and listed alphabetically, by surname of the first author. Every 20th paper (in each year) was then examined. If a paper was found to be ineligible, successive consecutive (alphabetical) candidates were examined until at least 5% of the total maximum number for each year had been found eligible and analyzed. In total, 156 papers were scored. This sample represented 5.1% of the 3036 papers which remained after 341 of the original pool had been eliminated. More would be excluded, given a more thorough inspection. The average score of these 156 articles and their 95% confidence interval, determined by bootstrapping, was 1.29 (1.21–1.39). The average score of these papers is lower than for the papers published from 2009–2013 (1.37). Although the 95% confidence interval for the period 2014–2018 overlaps with this, there is no evidence to suggest that the more recent literature better recognizes existential risk. See Table A5 and Figure A2 .

This shows the number, number analyzed and scores for the 156 papers that were analyzed for the period 2014–2018, tabulated by year. Note that some of the candidate papers would be culled after further examination.

This shows the density of means and distributions for each year (2014–2017), based on 10,000 bootstrapped resamples (with replacement from the set for each year) and also for papers from 2013–2018 inclusive.

This research received no external funding.

Conflicts of Interest

The author declares no conflict of interest.

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Heat waves: a hot topic in climate change research

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• Published: 03 September 2021
• Volume 146 , pages 781–800, ( 2021 )

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• Werner Marx   ORCID: orcid.org/0000-0002-1763-5753 1 ,
• Robin Haunschild   ORCID: orcid.org/0000-0001-7025-7256 1 &
• Lutz Bornmann   ORCID: orcid.org/0000-0003-0810-7091 1 , 2  

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Research on heat waves (periods of excessively hot weather, which may be accompanied by high humidity) is a newly emerging research topic within the field of climate change research with high relevance for the whole of society. In this study, we analyzed the rapidly growing scientific literature dealing with heat waves. No summarizing overview has been published on this literature hitherto. We developed a suitable search query to retrieve the relevant literature covered by the Web of Science (WoS) as complete as possible and to exclude irrelevant literature ( n  = 8,011 papers). The time evolution of the publications shows that research dealing with heat waves is a highly dynamic research topic, doubling within about 5 years. An analysis of the thematic content reveals the most severe heat wave events within the recent decades (1995 and 2003), the cities and countries/regions affected (USA, Europe, and Australia), and the ecological and medical impacts (drought, urban heat islands, excess hospital admissions, and mortality). An alarming finding is that the limit for survivability may be reached at the end of the twenty-first century in many regions of the world due to the fatal combination of rising temperatures and humidity levels measured as “wet-bulb temperature” (WBT). Risk estimation and future strategies for adaptation to hot weather are major political issues. We identified 104 citation classics, which include fundamental early works of research on heat waves and more recent works (which are characterized by a relatively strong connection to climate change).

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1 Introduction

As a consequence of the well-documented phenomenon of global warming, climate change has become a major research field in the natural and medical sciences, and more recently also in the social and political sciences. The scientific community has contributed extensively to a comprehensive understanding of the earth’s climate system, providing various data and projections on the future climate as well as on the effects and risks of anticipated global warming (IPCC 2014; CSSR 2017; NCA4 2018; and the multitude of references cited therein). During recent decades, climate change has also become a major political, economic, and environmental issue and a central theme in political and public debates.

One consequence of global warming is the increase of extreme weather events such as heat waves, droughts, floods, cyclones, and wildfires. Some severe heat waves occurring within the last few decades made heat waves a hot topic in climate change research, with “hot” having a dual meaning: high temperature and high scientific activity. “More intense, more frequent, and longer lasting heat waves in the twenty-first century” is the title of a highly cited paper published 2004 in Science (Meehl and Tebaldi 2004 ). This title summarizes in short what most climate researchers anticipate for the future. But what are heat waves (formerly also referred to as “heatwaves”)? In general, a heat wave is a period of excessively hot weather, which may be accompanied by high humidity. Since heat waves vary according to region, there is no universal definition, but only definitions relative to the usual weather in the area and relative to normal temperatures for the season. The World Meteorological Organization (WMO) defines a heat wave as 5 or more consecutive days of prolonged heat in which the daily maximum temperature is higher than the average maximum temperature by 5 °C (9 °F) or more ( https://www.britannica.com/science/heat-wave-meteorology ).

Europe, for example, has suffered from a series of intense heat waves since the beginning of the twenty-first century. According to the World Health Organization (WHO) and various national reports, the extreme 2003 heat wave caused about 70,000 excess deaths, primarily in France and Italy. The 2010 heat wave in Russia caused extensive crop loss, numerous wildfires, and about 55,000 excess deaths (many in the city of Moscow). Heat waves typically occur when high pressure systems become stationary and the winds on their rear side continuously pump hot and humid air northeastward, resulting in extreme weather conditions. The more intense and more frequently occurring heat waves cannot be explained solely by natural climate variations and without human-made climate change (IPCC 2014; CSSR 2017; NCA4 2018). Scientists discuss a weakening of the polar jet stream caused by global warming as a possible reason for an increasing probability for the occurrence of stationary weather, resulting in heavy rain falls or heat waves (Broennimann et al. 2009 ; Coumou et al. 2015 ; Mann 2019 ). This jet stream is one of the most important factors for the weather in the middle latitude regions of North America, Europe, and Asia.

Until the end of the twentieth century, heat waves were predominantly seen as a recurrent meteorological fact with major attention to drought, being almost independent from human activities and unpredictable like earthquakes. However, since about 1950, distinct changes in extreme climate and weather events have been increasingly observed. Meanwhile, climate change research has revealed that these changes are clearly linked to the human influence on the content of greenhouse gases in the earth’s atmosphere. Climate-related extremes, such as heat waves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability to climate change as a result of global warming.

In recent years, research on heat waves has been established as an emerging research topic within the large field of current climate change research. Bibliometric analyses are very suitable in order to have a systematic and quantitative overview of the literature that can be assigned to an emerging topic such as research dealing with heat waves (e.g., Haunschild et al. 2016 ). No summarizing overview on the entire body of heat wave literature has been published until now. However, a bibliometric analysis of research on urban heat islands as a more specific topic in connection with heat waves has been performed (Huang and Lu 2018 ).

In this study, we analyzed the publications dealing with heat waves using appropriate bibliometric methods and tools. First, we determined the amount and time evolution of the scientific literature dealing with heat waves. The countries contributing the most papers are presented. Second, we analyzed the thematic content of the publications via keywords assigned by the WoS. Third, we identified the most important (influential) publications (and also the historical roots). We identified 104 citation classics, which include fundamental early works and more recent works with a stronger connection to climate change.

2 Heat waves as a research topic

The status of the current knowledge on climate change is summarized in the Synthesis Report of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2014, https://www.ipcc.ch/report/ar5/syr/ ). This panel is the United Nations body for assessing the science related to climate change. The Synthesis Report is based on the reports of the three IPCC Working Groups , including relevant Special Reports . In its Summary for Policymakers , it provides an integrated view of climate change as the final part of the Fifth Assessment Report (IPCC 2014, https://www.ipcc.ch/site/assets/uploads/2018/02/AR5_SYR_FINAL_SPM.pdf ).

In the chapter Extreme Events , it is stated that “changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions … It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-twentieth century. It is likely that human influence has more than doubled the probability of occurrence of heat waves in some locations” (p. 7–8). Under Projected Changes , the document summarizes as follows: “Surface temperature is projected to rise over the twenty-first century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions” (p. 10).

With regard to the USA, the Climate Science Special Report of the U.S. Global Change Research Program (CSSR 2017, https://science2017.globalchange.gov/ ) mentions quite similar observations and states unambiguously in its Fourth National Climate Assessment (Volume I) report ( https://science2017.globalchange.gov/downloads/CSSR2017_FullReport.pdf ) under Observed Changes in Extremes that “the frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s (very high confidence). The frequency and intensity of extreme heat and heavy precipitation events are increasing in most continental regions of the world (very high confidence). These trends are consistent with expected physical responses to a warming climate [p. 19]. Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence) [p. 20]. There are important regional differences in trends, with the largest increases occurring in the northeastern United States (high confidence). Recent droughts and associated heat waves have reached record intensity in some regions of the United States … (very high confidence) [p. 21]. Confidence in attribution findings of anthropogenic influence is greatest for extreme events that are related to an aspect of temperature” (p. 123).

Among the key findings in the chapter on temperature changes in the USA, the report states that “there have been marked changes in temperature extremes across the contiguous United States. The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s (very high confidence). Extreme temperatures in the contiguous United States are projected to increase even more than average temperatures. The temperatures of extremely cold days and extremely warm days are both expected to increase. Cold waves are projected to become less intense while heat waves will become more intense (very high confidence) [p. 185]. Most of this methodology as applied to extreme weather and climate event attribution, has evolved since the European heat wave study of Stott et al.” (p. 128).

Heat waves are also discussed in the Fourth National Climate Assessment (Volume II) report (NCA4 2018, https://nca2018.globalchange.gov/ ). The Report-in-Brief ( https://nca2018.globalchange.gov/downloads/NCA4_Report-in-Brief.pdf ) for example states: “More frequent and severe heat waves and other extreme events in many parts of the United States are expected [p. 38]. Heat waves and heavy rainfalls are expected to increase in frequency and intensity [p. 93]. The season length of heat waves in many U.S. cities has increased by over 40 days since the 1960s [p. 30]. Cities across the Southeast are experiencing more and longer summer heat waves [p. 123]. Exposure to hotter temperatures and heat waves already leads to heat-associated deaths in Arizona and California. Mortality risk during a heat wave is amplified on days with high levels of ground-level ozone or particulate air pollution” (p. 150).

In summary, climate change research expects more frequent and more severe heat wave events as a consequence of global warming. It is likely that the more frequent and longer lasting heat waves will significantly increase excess mortality, particularly in urban regions with high air pollution. Therefore, research around heat waves will become increasingly important and is much more than a temporary research fashion.

3 Methodology

3.1 dataset used.

This analysis is based on the relevant literature retrieved from the following databases accessible under the Web of Science (WoS) of Clarivate Analytics: Web of Science Core Collection: Citation Indexes, Science Citation Index Expanded (SCI-EXPANDED), Social Sciences Citation Index (SSCI), Arts & Humanities Citation Index (A&HCI), Conference Proceedings Citation Index—Science (CPCI-S), Conference Proceedings Citation Index—Social Science & Humanities (CPCI-SSH), Book Citation Index—Science (BKCI-S), Book Citation Index—Social Sciences & Humanities (BKCI-SSH), Emerging Sources Citation Index (ESCI).

We applied the search query given in Appendix 1 to cover the relevant literature as completely as possible and to exclude irrelevant literature. We practiced an iterative query optimization by identifying and excluding the WoS subject categories with most of the non-relevant papers. For example, heat waves are also mentioned in the field of materials science but have nothing to do with climate and weather phenomena. Unfortunately, WoS obviously assigned some heat wave papers related to climate to materials science-related subject categories. Therefore, these subject categories were not excluded. By excluding the other non-relevant subject categories, 597 out of 8,568 papers have been removed, resulting in a preliminary publication set of 7,971 papers (#2 of the search query). But this is no safe method, since the excluded categories may well include some relevant papers. Therefore, we have combined these 597 papers with search terms related to climate or weather and retrieved 62 relevant papers in addition, which we added to our preliminary paper subset, eventually receiving 8,033 publications (#3 to #5 of the search query).

Commonly, publication sets for bibliometric analyses are limited to articles, reviews, and conference proceedings as the most relevant document types and are restricted to complete publication years. In this study, however, we have included all relevant WoS document types for a better literature coverage of the research topic analyzed. For example, conference meetings and early access papers may well be interesting for the content analysis of the literature under study. Such literature often anticipates important results, which are published later as regular articles. Furthermore, we have included the literature until the date of search for considering the recent rapid growth of the field. Our search retrieved a final publication set of 8,011 papers indexed in WoS until the date of search (July 1, 2021) and dealing with heat waves (#6 of the search query). We have combined this publication set with climate change-related search terms from a well-proven search query (Haunschild et al. 2016 ) resulting in 4,588 papers dealing with heat waves in connection with climate change or global warming (# 11 of the search query). Also, we have selected a subset of 2,373 papers dealing with heat waves and mortality (#13 of the search query). The complete WoS search query is given in Appendix 1.

The final publication set of 8,011 papers dealing with heat waves still contains some non-relevant papers primarily published during the first half of the twentieth century, such as some Nature papers within the WoS category Multidisciplinary Sciences . Since these papers are assigned only to this broad subject category and have no abstracts and no keywords included, they cannot be excluded using the WoS search and refinement functions. We do not expect any bias through these papers, because their keywords do not appear in our maps. Also, they normally contain very few (if any) cited references, which could bias/impact our reference analysis.

3.2 Networks

We used the VOSviewer software (Van Eck and Waltman 2010 ) to map co-authorship with regard to the countries of authors (88 countries considered) of the papers dealing with heat waves ( www.vosviewer.com ). The map of the cooperating countries presented is based on the number of joint publications. The distance between two nodes is proportionate to the number of co-authored papers. Hence, largely cooperating countries are positioned closer to each other. The size of the nodes is proportionate to the number of papers published by authors of the specific countries.

The method that we used for revealing the thematic content of the publication set retrieved from the WoS is based on the analysis of keywords. For better standardization, we chose the keywords allocated by the database producer (keywords plus) rather than the author keywords. We also used the VOSviewer for mapping the thematic content of the 104 key papers selected by reference analysis. This map is also based on keywords plus.

The term maps (keywords plus) are based on co-occurrence for positioning the nodes on the maps. The distance between two nodes is proportionate to the co-occurrence of the terms. The size of the nodes is proportionate to the number of papers with a specific keyword. The nodes on the map are assigned by VOSviewer to clusters based on a specific cluster algorithm (the clusters are highlighted in different colors). These clusters identify closely related (frequently co-occurring) nodes, where each node is assigned to only one cluster.

3.3 Reference Publication Year Spectroscopy

A bibliometric method called “Reference Publication Year Spectroscopy” (RPYS, Marx et al. 2014 ) in combination with the tool CRExplorer ( http://www.crexplorer.net , Thor et al. 2016a , b ) has proven useful for exploring the cited references within a specific publication set, in order to detect the most important publications of the relevant research field (and also the historical roots). In recent years, several studies have been published, in which the RPYS method was basically described and applied (Marx et al. 2014 ; Marx and Bornmann 2016 ; Comins and Hussey 2015 ). In previous studies, Marx et al. have analyzed the roots of research on global warming (Marx et al. 2017a ), the emergence of climate change research in combination with viticulture (Marx et al. 2017b ), and tea production (Marx et al. 2017c ) from a quantitative (bibliometric) perspective. In this study, we determined which references have been most frequently cited by the papers dealing with heat waves.

RPYS is based on the assumption that peers produce a useful database by their publications, in particular by the references cited therein. This database can be analyzed statistically with regard to the works most important for their specific research field. Whereas individual scientists judge their research field more or less subjectively, the overall community can deliver a more objective picture (based on the principle of “the wisdom of the crowds”). The peers effectively “vote” via their cited references on which works turned out to be most important for their research field (Bornmann and Marx 2013 ). RPYS implies a normalization of citation counts (here: reference counts) with regard to the research area and the time of publication, which both impact the probability to be cited frequently. Basically, the citing and cited papers analyzed were published in the same research field and the reference counts are compared with each other only within the same publication year.

RPYS relies on the following observation: the analysis of the publication years of the references cited by all the papers in a specific research topic shows that publication years are not equally represented. Some years occur particularly frequently among the cited references. Such years appear as distinct peaks in the distribution of the reference publication years (i.e., the RPYS spectrogram). The pronounced peaks are frequently based on a few references that are more frequently cited than other references published in the same year. The frequently cited references are—as a rule—of specific significance to the research topic in question (here: heat waves) and the earlier references among them represent its origins and intellectual roots (Marx et al. 2014 ).

The RPYS changes the perspective of citation analysis from a times cited to a cited reference analysis (Marx and Bornmann 2016 ). RPYS does not identify the most highly cited papers of the publication set being studied (as is usually done by bibliometric analyses in research evaluation). RPYS aims to mirror the knowledge base of research (here: on heat waves).

With time, the body of scientific literature of many research fields is growing rapidly, particularly in climate change research (Haunschild et al. 2016 ). The growth rate of highly dynamic research topics such as research related to heat waves is even larger. As a consequence, the number of potentially citable papers is growing substantially. Toward the present, the peaks of individual publications lie over a broad continuum of newer publications and are less numerous and less pronounced. Due to the many publications cited in the more recent years, the proportion of individual highly cited publications in specific reference publication years falls steadily. Therefore, the distinct peaks in an RPYS spectrogram reveal only the most highly cited papers, in particular the earlier references comprising the historical roots. Further inspection and establishing a more entire and representative list of highly cited works requires consulting the reference table provided by the CRExplorer. The most important references within a specific reference publication year can be identified by sorting the cited references according to the reference publication year (RPY) and subsequently according to the number of cited references (N_CR) in a particular publication year.

The selection of important references in RPYS requires the consideration of two opposing trends: (1) the strongly growing number of references per reference publication year and (2) the fall off near present due to the fact that the newest papers had not sufficient time to accumulate higher citation counts. Therefore, we decided to set different limits for the minimum number of cited references for different periods of reference publication years (1950–1999: N_CR ≥ 50, 2000–2014: N_CR ≥ 150, 2015–2020: N_CR ≥ 100). This is somewhat arbitrary, but is helpful in order to adapt and limit the number of cited references to be presented and discussed.

In order to apply RPYS, all cited references ( n  = 408,247) of 216,932 unique reference variants have been imported from the papers of our publication set on heat waves ( n  = 8,011). The cited reference publication years range from 1473 to 2021. We removed all references (297 different cited reference variants) with reference publication years prior to 1900. Due to the very low output of heat wave-related papers published before 1990, no relevant literature published already in the nineteenth century can be expected. Also, global warming was no issue before 1900 since the Little Ice Age (a medieval cold period) lasted until the nineteenth century. The references were sorted according to RPY and N_CR for further inspection.

The CRExplorer offers the possibility to cluster and merge variants of the same cited reference (Thor et al. 2016a , b ). We clustered and merged the associated reference variants in our dataset (which are mainly caused by misspelled references) using the corresponding CRExplorer module, clustering the reference variants via volume and page numbers and subsequently merging aggregated 374 cited references (for more information on using the CRExplorer see “guide and datasets” at www.crexplorer.net ).

After clustering and merging, we applied a further cutback: to focus the RPYS on the most pronounced peaks, we removed all references ( n  = 212,324) with reference counts below 10 (resulting in a final number of 3,937 cited references) for the detection of the most frequently cited works. A minimum reference count of 10 has proved to be reasonable, in particular for early references (Marx et al. 2014 ). The cited reference publication years now range from 1932 to 2020.

In this study, we have considered all relevant WoS document types for a preferably comprehensive coverage of the literature of the research topic analyzed. The vast majority of the papers of our publication set, however, have been assigned to the document types “article” ( n  = 6.738, 84.1%), “proceedings paper” ( n  = 485, 6.1%), and “review” ( n  = 395 papers, 4.9%). Note that some papers belong to more than one document type.

4.1 Time evolution of literature

In Fig.  1 , the time evolution between 1990 and 2020 of the publications dealing with heat waves is shown (there are only 109 pre-1990 publications dealing with heat waves and covered by the WoS).

Time evolution of the overall number of heat wave publications, of heat wave publications in connection with climate change, and of heat wave publications in connection with mortality, each between 1990 and 2020. For comparison, the overall number of publications (scaled down) in the field of climate change research and the total number of publications covered by the WoS database (scaled down, too) are included

According to Fig.  1 , research dealing with heat waves is a highly dynamic research topic, currently doubling within about 5 years. The number of papers published per year shows a strong increase: since around 2000, the publication output increased by a factor of more than thirty, whereas in the same period, the overall number of papers covered by the WoS increased only by a factor of around three. Also, the portion of heat wave papers dealing with climate change increased substantially: from 16.1 in the period 1990–1999 to 25.7% in 2000, reaching 66.9% in 2020. The distinct decrease of the overall number of papers covered by the WoS between 2019 and 2020 might be a result of the Covid-19 pandemic.

With regard to the various impacts of heat waves, excess mortality is one of the most frequently analyzed and discussed issues in the scientific literature (see below). Whereas the subject specific literature on heat waves increased from 2000 to 2020 by a factor of 33.6, literature on heat waves dealing with mortality increased from 2000 to 2020 by a factor of 51.5. The dynamics of the research topic dealing with heat waves is mirrored by the WoS Citation Report , which shows the time evolution of the overall citation impact of the papers of the publication set (not presented). The citation report curve shows no notable citation impact before 2005, corresponding to the increase of the publication rate since about 2003 as shown in Fig.  1 .

4.2 Countries of authors

In Table 1 , the number of papers assigned to the countries of authors with more than 100 publications dealing with heat waves is presented, showing the national part of research activities on this research topic. For comparative purposes, the percentage of overall papers in WoS of each country is shown. As a comparison with the overall WoS, we only considered WoS papers published between 2000 and 2020, because the heat wave literature started to grow substantially around 2000.

The country-specific percentages from Table 1 are visualized in Fig.  2 . Selected countries are labeled. Countries with a higher relative percentage of more than two percentage points in heat wave research than in WoS overall output are marked blue (blue circle). Countries with a relative percentage at least twice as high in heat wave research than in overall WoS output are marked green (green cross), whereas countries with a relative percentage at most half as much in heat wave research than in overall WoS output are marked with a yellow cross. Only Japan has a much lower output in heat wave research than in WoS overall output, as indicated by the red circle and yellow cross. Most countries are clustered around the bisecting line and are marked gray (gray circle). China and the USA are outside of the plot region. Both countries are rather close to the bisecting line. Some European countries show a much larger activity in heat wave research than in overall WoS output. Australia shows the largest difference and ratio in output percentages as shown by the blue circle and green cross.

Publication percentages of countries in Table 1 . Countries with large deviations between heat wave output and overall WoS output are labeled. Countries with an absolute percentage of more than two percentage points higher (lower) in heat wave research than in overall WoS output are marked blue (red). Countries with a relative percentage at least twice as high (at most half as much) in heat wave research than in overall WoS output are marked green (yellow)

The results mainly follow the expectations of such bibliometric analyses, with one distinct exception: Australia increasingly suffers from extreme heat waves and is comparatively active in heat wave research—compared with its proportion of scientific papers in general. The growth factor of the Australian publication output since 2010 is 8.5, compared to 5.3 for the USA and 3.3 for Germany.

Figure  3 shows the co-authorship network with regard to the countries of authors of the papers dealing with heat waves using the VOSviewer software.

Co-authorship overlay map with regard to the countries of authors and their average publication years from the 8,011 papers dealing with heat waves. The minimum number of co-authored publications of a country is 5; papers with more than 25 contributing countries are neglected; of the 135 countries, 89 meet the threshold, and 88 out of 89 countries are connected and are considered (one country, Armenia, that is disconnected from the network has been removed). The co-authorship network of a single country can be depicted by clicking on the corresponding node in the interactive map. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/3ywkwv8t

According to Fig.  3 and in accordance with Table 1 , the USA is most productive in heat wave research. This is not unexpected, because the US publication output is at the top for most research fields. However, this aside, the USA has been heavily affected by heat wave events and is leading with regard to the emergence of the topic. Australia appears as another major player and is strongly connected with the US publications within the co-authorship network and thus appears as a large node near the US node in the map. Next, the leading European countries England, France, Germany, Italy, and Spain appear.

The overlay version of the map includes the time evolution of the research activity in the form of coloring of the nodes. The map shows the mean publication year of the publications for each specific author country. As a consequence, the time span of the mean publication years ranges only from 2014 to 2018. Nevertheless, the early activity in France and the USA and the comparatively recent activity in Australia and China, with the European countries in between, become clearly visible.

4.3 Topics of the heat wave literature

Figure  4 shows the keywords (keywords plus) map for revealing the thematic content of our publication set using the VOSviewer software. This analysis is based on the complete publication set ( n  = 8,011). The minimum number of occurrences of keywords is 10; of the 10,964 keywords, 718 keywords met the threshold. For each of the 718 keywords, the total strength of the co-occurrence links with other keywords was calculated. The keywords with the greatest total link strength were selected for presentation in the map.

Co-occurrence network map of the keywords plus from the 8,011 papers dealing with heat waves for a rough analysis of the thematic content. The minimum number of occurrences of keywords is 10; of the 10,964 keywords, 718 meet the threshold. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/enrdbw

According to Fig.  4 , the major keywords are the following: climate change, temperature, mortality, impact, heat waves (searched), and variability. The colored clusters identify closely related (frequently co-occurring) nodes. The keywords marked red roughly originate from fundamental climate change research focused on the hydrological cycle (particularly on drought), the keywords of the green cluster are around heat waves and moisture or precipitation, the keywords marked blue result from research concerning impacts of heat waves on health, the keywords marked yellow are focused on the various other impacts of heat waves, and the keywords of the magenta cluster are around adaptation and vulnerability in connection with heat waves.

The clustering by the VOSviewer algorithm provides basic categorizations, but many related keywords also appear in different clusters. For example, severe heat wave events are marked in different colors. For a better overview of the thematic content of the publications dealing with heat waves, we have assigned the keywords of Fig.  4 (with a minimum number of occurrences of 50) to ten subject categories (each arranged in the order of occurrence):

Countries/regions: United-States, Europe, France, China, Pacific, Australia, London, England

Cities: cities, city, US cities, Chicago, communities

Events: 2003 heat-wave, 1995 heat-wave

Impacts: impact, impacts, air-pollution, drought, soil-moisture, exposure, heat-island, urban, islands, photosynthesis, pollution, heat-island, air-quality, environment, precipitation extremes, biodiversity, emissions

Politics: risk, responses, vulnerability, adaptation, management, mitigation, risk-factors, scenarios

Biology: vegetation, forest, diversity, stomatal conductance

Medicine: mortality, health, stress, deaths, morbidity, hospital admissions, public-health, thermal comfort, population, heat, sensitivity, human health, disease, excess mortality, heat-stress, heat-related mortality, comfort, behavior, death, stroke

Climate research: climate change, temperature, climate, model, simulation, energy, projections, simulations, cmip5, ozone, el-nino, parametrization, elevated CO 2 , models, climate variability, carbon, carbon-dioxide

Meteorology: heat waves, variability, precipitation, summer, heat-wave, weather, ambient-temperature, waves, extremes, wave, cold, water, rainfall, circulation, heat, air-temperature, extreme heat, climate extremes, heatwaves, temperature extremes, temperatures, temperature variability, high-temperature, ocean, extreme temperatures, atmospheric circulation, interannual variability, sea-surface temperature, oscillation, surface temperature, surface

Broader terms (multi-meaning): trends, events, patterns, growth, performance, time-series, indexes, system, dynamics, association, index, tolerance, productivity, ensemble, resilience, increase, quality, prediction, frequency, particulate matter, future, framework, 20 th -century, time, reanalysis, systems

Although allocated by the database provider, the keywords are not coherent. For example, the same keyword may appear as singular or plural, and complex keywords are written with and without hyphens.

In order to compare the thematic content of the complete publication set with the earlier literature on heat waves, we have analyzed the pre-2000 publications ( n  = 297) separately. Figure  5 shows the keywords (keywords plus) map for revealing the thematic content of the pre-2000 papers.

Co-occurrence network map of the keywords plus from the 297 pre-2000 papers dealing with heat waves for a rough analysis of the thematic content. The minimum number of occurrences of keywords is 1; of the 389 keywords, 277 keywords are connected, and all items are shown. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/u2zzr399

The major nodes in Fig.  5 are heat waves (searched), temperature, United States, and mortality, with climate change appearing only as a smaller node here. Obviously, the connection between heat waves and climate change was not yet pronounced, which can also be seen from Fig.  1 . Compared with Fig.  4 , the thematic content of the clusters is less clear and the clusters presented in Fig.  5 can hardly be assigned to specific research areas. For a better overview of the thematic content of the early publications dealing with heat waves, we have assigned the connected keywords of Fig.  5 to seven subject categories:

Countries/regions: United-States, Great-Plains

Cities: St-Louis, Athens, Chicago

Events: 1980 heat-wave, 1995 heat-wave

Impacts: impacts, responses, drought, precipitation, comfort, sultriness

Climate research: climate, climate change, model, temperature, variability

Medicine: cardiovascular deaths, mortality, air pollution

Meteorology: atmospheric flow, weather, heat, humidity index

4.4 Important publications

Figures  6 – 8 show the results of the RPYS analysis performed with the CRExplorer and present the distribution of the number of cited references across the reference publication years. Figure  6 shows the RPYS spectrogram of the full range of reference publication years since 1925. Figure  7 presents the spectrogram for the reference publication year period 1950–2000 for better resolving the historical roots. Figure  8 shows the spectrogram for the period 2000–2020, comprising the cited references from the bulk of the publication set analyzed.

Annual distribution of cited references throughout the time period 1925–2020, which have been cited in heat wave-related papers (published between 1964 and 2020). Only references with a minimum reference count of 10 are considered

Annual distribution of cited references throughout the time period 1950–2000, which have been cited in heat wave-related papers (published between 1972 and 2020). Only references with a minimum reference count of 10 are considered

Annual distribution of cited references throughout the time period 2000–2020, which have been cited in heat wave-related papers (published between 2000 and 2020). Only references with a minimum reference count of 10 are considered

The gray bars (Fig.  6 ) and red lines (Figs. 7 – 8 ) in the graphs visualize the number of cited references per reference publication year. In order to identify those publication years with significantly more cited references than other years, the (absolute) deviation of the number of cited references in each year from the median of the number of cited references in the two previous, the current, and the two following years (t − 2; t − 1; t; t + 1; t + 2) is also visualized (blue lines). This deviation from the 5-year median provides a curve smoother than the one in terms of absolute numbers. We inspected both curves for the identification of the peak papers.

Which papers are most important for the scientific community performing research on heat waves? We use the number of cited references (N_CR) as a measure of the citation impact within the topic-specific literature of our publication set. N_CR should not be confused with the overall number of citations of the papers as given by the WoS citation counts (times cited). These citation counts are based on all citing papers covered by the complete database (rather than a topic-specific publication set) and are usually much higher.

Applying the selection criteria mentioned above (minimum number of cited references between 50 and 150 in three different periods), 104 references have been selected as key papers (important papers most frequently referenced within the research topic analyzed) and are presented in Table 2 in Appendix 2. The peak papers corresponding to reference publication years below about 2000 can be seen as the historical roots of the research topic analyzed. Since around 2000, the number of references with the same publication year becomes increasingly numerous, usually with more than one highly referenced (cited) paper at the top. Although there are comparatively fewer distinct peaks visible in the RPYS spectrogram of Fig.  8 , the most frequently referenced papers can easily be identified via the CRE reference listing. Depending on the specific skills and needs (i.e., the expert knowledge and the intended depth of the analysis), the number of top-referenced papers considered key papers can be defined individually.

Table 2 lists the first authors and titles of the 104 key papers selected, their number of cited references (N_CR), and the DOIs for easy access. Some N_CR values are marked by an asterisk, indicating a high value of the N_TOP10 indicator implemented in the CRExplorer. The N_TOP10 indicator value is the number of reference publication years in which a focal cited reference belongs to the 10% most referenced publications. In the case of about half of the cited references in Table 2 ( n  = 58), the N_TOP10 value exceeded a value of 9. The three highest values in our dataset are 24, 21, and 20.

Out of the 104 key papers from Table 2 , 101 have a DOI of which we found 101 papers in the WoS. Three papers have no DOI but could be retrieved from WoS. The altogether 104 papers were exported and their keywords (keywords plus) were displayed in Fig.  9 for revealing the thematic content of the key papers from the RPYS analysis at a glance.

Co-occurrence network map of the keywords plus of the 104 key papers dealing with heat waves selected applying RPYS via CRE software and listed in Table 2 . The minimum number of occurrences of keywords is 2; of the 310 keywords, 91 meet the threshold. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/4vwpc4t2

Overall, the keywords mapped in Fig.  9 are rather similar to the keywords presented in Fig.  4 . Besides climate change, temperature, weather, and air-pollution, the keywords deaths, health, mortality, and United-States appear as the most pronounced terms.

The key papers presented in Table 2 can be categorized as follows: (1) papers dealing with specific heat wave events, (2) the impact of heat waves on human health, (3) heat wave-related excess mortality and implications for prevention, (4) the interaction between air pollution and high temperature, (5) circulation pattern and the meteorological basis, (6) future perspectives and risks, and (7) climate models, indicators, and statistics.

5 Discussion

Today, the hypothesis of a human-induced climate change is no longer abstract but has become a clear fact, at least for the vast majority of the scientific community (IPCC 2014; CSSR 2017; NCA4 2018; and the multitude of references cited therein). The consequences of a warmer climate are already obvious. The rapidly growing knowledge regarding the earth’s climate system has revealed the connection between global warming and extreme weather events. Heat waves impact people directly and tangibly and many people are pushing for political actions. Research on heat waves came up with the occurrence of some severe events in the second half of the twentieth century and was much stimulated by the more numerous, more intense, and longer lasting heat waves that have occurred since the beginning of the twenty-first century.

As already mentioned in Sect.  1 , the more intense and more frequently occurring heat waves cannot be explained solely by natural climate variations but only with human-made climate change. As a consequence, research on heat waves has become embedded into meteorology and climate change research and has aimed to understand the specific connection with global warming. Scientists discuss a weakening of the polar jet stream as a possible reason for an increasing probability for the occurrence of heat waves (e.g., Broennimann et al. 2009 ; Coumou et al. 2015 ; Mann 2019 ). Climate models are used for projections of temperature and rainfall variability in the future, based on various scenarios of greenhouse gas emissions. As a result, the corresponding keywords appear in the maps of Figs. 4 and 9 . Also, the application of statistics plays a major role in the papers of our publication set; some of the most highly referenced (early) papers in Table 2 primarily deal with statistical methods. These methods provide the basis for research on heat waves.

Our analysis shows that research on heat waves has become extremely important in the medical area, since severe heat waves have caused significant excess mortality (e.g., Kalkstein and Davis 1989 ; Fouillet et al. 2006 ; Anderson and Bell 2009 , 2011 ). The most alarming is that the limit for survivability may be reached at the end of the twenty-first century in many regions of the world due to the fatal combination of rising temperatures and humidity levels (e.g., Pal and Eltahir 2016 ; Im et al. 2017 ; Kang and Eltahir 2018 ). The combination of heat and humidity is measured as the “wet-bulb temperature” (WBT), which is the lowest temperature that can be reached under current ambient conditions by the evaporation of water. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature and is different at lower humidity levels. For example, an ambient temperature of 46 °C and a relative humidity of 50% correspond to 35 °C WBT, which is the upper limit that can kill even healthy people within hours. By now, the limit of survivability has almost been reached in some places. However, if global warming is not seriously tackled, deadly heat waves are anticipated for many regions that have contributed little to climate change.

According to high-resolution climate change simulations, North China and South Asia are particularly at risk, because the annual monsoon brings hot and humid air to these regions (Im et al. 2017 ; Kang and Eltahir 2018 ). The fertile plain of North China has experienced vast expansion of irrigated agriculture, which enhances the intensity of heat waves. South Asia, a region inhabited by about one-fifth of the global human population, is likely to approach the critical threshold by the late twenty-first century, if greenhouse gas emissions are not lowered significantly. In particular, the densely populated agricultural regions of the Ganges and Indus river basins are likely to be affected by extreme future heat waves. Also, the Arabic-speaking desert countries of the Gulf Region in the Middle East and the French-speaking parts of Africa are expected to suffer from heat waves beyond the limit of human survival. But to date, only 12 papers have been published on heat waves in connection with wet-bulb temperature (#15 of the search query); no paper was published before 2016. Some papers report excess hospital admissions during heat wave events (e.g., Semenza et al. 1999 ; Knowlton et al. 2009 ), with the danger of a temporary capacity overload of local medical systems in the future. Presumably, this will be an increasingly important issue in the future, when more and larger urban areas are affected by heat waves beyond the limit of human survival indicated by wet-bulb temperatures above 35° C.

The importance of heat waves for the medical area is underlined by the large portion of papers discussing excess hospital admissions and excess mortality during intense heat wave events, particularly in urban areas with a high population density. As was the case during the boom phase of the Covid-19 pandemic, local medical health care systems may become overstressed by long-lasting heat wave events and thus adaptation strategies are presented and discussed. Finally, the analysis of the keywords in this study reveals the connection of heat wave events with air pollution in urban regions. There seems to be evidence of an interaction between air pollution and high temperatures in the causation of excess mortality (e.g., Katsouyanni et al. 1993 ). Two more recent papers discuss the global risk of deadly heat (Mora et al. 2017 ) and the dramatically increasing chance of extremely hot summers since the 2003 European heat wave (Christidis et al. 2015 ).

Another important topic of the heat wave papers is related to the consequences for agriculture and forestry. Reduced precipitation and soil moisture result in crop failure and put food supplies at risk. Unfortunately, large regions of the world that contribute least to the emission of greenhouse gases are affected most by drought, poor harvests, and hunger. Some more recent papers discuss the increasing probability of marine heat waves (Oliver et al. 2018 ) and the consequences for the marine ecosystem (Smale et al. 2019 ).

The results of this study should be interpreted in terms of its limitations:

We tried to include in our bibliometric analyses all relevant heat wave papers covered by the database. Our long-standing experience in professional information retrieval has shown, however, that it is sheer impossible to get complete and clean results by search queries against the backdrop of the search functions provided by literature databases like WoS or others. Also, the transition from relevant to non-relevant literature is blurred and is a question of the specific needs. In this study, we used bibliometric methods that are relatively robust with regard to the completeness and precision of the publication sets analyzed. For example, it is an advantage of RPYS that a comparatively small portion of relevant publications (i.e., an incomplete publication set) contains a large amount of the relevant literature as cited references. The number of cited references is indeed lowered as a consequence of an incomplete publication set. However, this does not significantly affect the results, since the reference counts are only used as a relative measure within specific publication years.

As most literature databases, the WoS does not cover each and every scientific journal but only a carefully selected set of core journals most important for scientific disciplines. The coverage or comprehensiveness of the database can be estimated by comparing the number of all cited references with the number of the linked cited references (i.e., the references, which correspond to papers appearing in publications covered by the database as publication records). Based on the publication years 1990, 1995, 2000, 2005, and 2010, about 70% of all references in the natural sciences are linked references (Marx and Bornmann 2015 ). Thus, about 30% of the cited literature of these disciplines is not covered by the database in the form of paper records, presumably many non-English publications. It may be true that the publication set analyzed is biased toward mid-latitude developed countries, disadvantaging countries with most people suffering from humid heat waves. Parts of the most extreme heat waves occur in the French-speaking parts of Africa and the Arabic-speaking desert countries. Presumably, relevant literature like national reports discussing for example the local impact of extreme heat waves is not included in this analysis. However, if such documents were highly relevant, they should be cited in the literature covered by the WoS. In this case, our RPYS analysis would have discovered them. Therefore, we are confident that at least the highly relevant documents of the heat wave literature are considered in our analysis.

Two other limitations of this study refer to the RPYS of the heat wave paper set:

There are numerous rather highly cited references retrieved by RPYS via CRExplorer but not considered in the listing of Table 2 due to the selection criteria applied. Many of these non-selected papers have N_CR values just below the limits that we have set. Therefore, papers not included in our listing are not per se qualified as much less important or even unimportant.

In the interpretation of cited references counts, one should have in mind that they rely on the “popularity” of a publication being cited in subsequent research. The counts measure impact but not scientific importance or accuracy (Tahamtan and Bornmann 2019 ). Note that there are many reasons why authors cite publications (Tahamtan and Bornmann 2018 ), thus introducing a lot of “noise” in the data (this is why RPYS focuses on the cited reference peaks).

Our suggestions for future empirical analysis refer to the impact of the scientific heat wave discourse on social networks and funding of basic research on heat waves around topics driven by political pressure. Whereas this paper focuses on the scientific discourse around heat waves, it would be interesting if future studies were to address the policy relevance of the heat waves research.

Data availability

Not applicable.

Code availability

Change history, 23 february 2022.

The original version of this paper was updated to add the missing compact agreement Open Access funding note.

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Marx, W., Haunschild, R. & Bornmann, L. Heat waves: a hot topic in climate change research. Theor Appl Climatol 146 , 781–800 (2021). https://doi.org/10.1007/s00704-021-03758-y

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The role of climate change education on individual lifetime carbon emissions

Roles Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Writing – original draft

* E-mail: [email protected]

Affiliation Department of Meteorology and Climate Science, San José State University, San José, California, United States of America

Roles Formal analysis, Writing – review & editing

Roles Conceptualization, Methodology, Writing – review & editing

Affiliation Department of Communication Studies, San José State University, San José, California, United States of America

• Eugene C. Cordero, 
• Diana Centeno, 
• Anne Marie Todd

• Published: February 4, 2020
• https://doi.org/10.1371/journal.pone.0206266
• Reader Comments

Strategies to mitigate climate change often center on clean technologies, such as electric vehicles and solar panels, while the mitigation potential of a quality educational experience is rarely discussed. In this paper, we investigate the long-term impact that an intensive one-year university course had on individual carbon emissions by surveying students at least five years after having taken the course. A majority of course graduates reported pro-environmental decisions (i.e., type of car to buy, food choices) that they attributed at least in part to experiences gained in the course. Furthermore, our carbon footprint analysis suggests that for the average course graduate, these decisions reduced their individual carbon emissions by 2.86 tons of CO 2 per year. Surveys and focus group interviews identify that course graduates have developed a strong personal connection to climate change solutions, and this is realized in their daily behaviors and through their professional careers. The paper discusses in more detail the specific components of the course that are believed to be most impactful, and the uncertainties associated with this type of research design. Our analysis also demonstrates that if similar education programs were applied at scale, the potential reductions in carbon emissions would be of similar magnitude to other large-scale mitigation strategies, such as rooftop solar or electric vehicles.

Citation: Cordero EC, Centeno D, Todd AM (2020) The role of climate change education on individual lifetime carbon emissions. PLoS ONE 15(2): e0206266. https://doi.org/10.1371/journal.pone.0206266

Editor: Francesco S. R. Pausata, Universite du Quebec a Montreal, CANADA

Received: October 5, 2018; Accepted: January 7, 2020; Published: February 4, 2020

Copyright: © 2020 Cordero et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: Data are available at OSFHOME, and access to the data can be found at ( https://osf.io/an4ht/ ), with this reference: DOI 10.17605/OSF.IO/AN4HT .

Funding: The National Science Foundation under grant 1513332 provided support for the partial salary of DC, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section. The commercial company Green Ninja did not provide any financial support related to this research, nor did they have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests: EC is the majority owner of Green Ninja, an education company that produces middle school curriculum. A conflict of interest plan has been established through San Jose State University. This does not alter our adherence to PLOS ONE policies on sharing data and materials as detailed online in our guide for authors http://journals.plos.org/plosone/s/competing-interests .

1. Introduction

In 1992, the United Nations Framework Convention on Climate Change (UNFCC) stated, “Education is an essential element for mounting an adequate global response to climate change” [ 1 ]. Few would argue against the importance of education in providing an informed response to environmental problems. Solutions to climate change tend to focus on mitigation and adaptation measures, and successful implementation of either strategy requires an informed and educated citizenry. Interest in education and climate change has increased in recent years [ 2 ] in part due to leadership efforts from organizations like the United Nations Education, Scientific, and Cultural Organization (UNESCO) that continue to advocate for educational efforts to respond to climate change [ 3 ]. Yet despite the notion of education’s importance in responding to climate change, education is rarely mentioned in discussions of today’s major climate solution strategies. One reason that education programs may not feature prominently in discussions about climate change mitigation is that few studies verify the effective reductions in carbon emissions resulting from education programs. Although several studies have linked environmental education and environmental quality (e.g., Education and water quality [ 4 ]; Education and air quality [ 5 ]; and Education and energy reduction [ 6 ]), the environmental education literature is relatively sparse [ 7 ]. And while the potential to reduce carbon emissions through behavior programs is clear (e.g., [ 8 ]), connections to education over time have not been as well established [ 9 ]. This is in contrast to technologies such as renewable energy generation and the electrification of automobiles that can demonstrate reductions in carbon emissions using more easily accessible data. Should education be shown to be an effective tool to reducing emissions via changes in attitudes and behavior, it would seem likely that funding and interest in such methods would become more widespread and well supported.

Education has been found to be one method for promoting behavior change, but only under certain circumstances (e.g., [ 10 ]; [ 11 ]). The environmental education literature offers insights into the connections between education and behavior change, and it also provides guidance on how to encourage pro-environmental behavior [ 12 ]; [ 13 ]; [ 14 ]; [ 15 ]. The notion that knowledge leads to awareness and then to action has been countered with studies that document that knowledge and skills are not enough to change behavior (e.g., [ 16 ]). The literature suggests that more personal factors such as a deep connection to nature, personal relevance to the issue and personal agency towards action are important elements that contribute to successful behavior change programs (e.g., [ 10 ]; [ 17 ]; [ 18 ]; [ 19 ]). Even among successful programs, the question of how long the intended behavior is sustained can vary depending on the type of intervention, with longer and more sustained engagements tending to have more long-lasting impacts [ 20 ]. This previous research informs educational research programs towards designs that not only focus on information but also promote the personal qualities that can support sustained action.

A growing base of literature is developing around climate change education as national standards move towards inclusion of this subject in the core curriculum [ 21 ], and educators negotiate the teaching of this sometimes ‘controversial’ subject (e.g., [ 22 ]; [ 23 ]; [ 24 ]). While there are similarities to the teaching of other environmental topics, climate change includes some unique education challenges that make teaching this topic especially difficult [ 25 ]; [ 26 ]; [ 27 ]. The science is highly complex and spans various areas in the natural and physical sciences, and yet the implications of our changing climate and the role of human activities make this scientific topic both a social and a political issue. Despite the goals of environmental education organizations like the UNESCO, relatively few climate change education programs remain that have successfully demonstrated the type of behavior change needed to effectively respond to climate change [ 23 ]; [ 28 ]; [ 29 ]. Further, even among existing climate change education resources offered in textbooks and through government programs, it appears there are opportunities to promote more effective emission-reduction strategies [ 30 ].

The purpose of this paper is to evaluate the impact of an intensive university climate change course on individual long-term carbon emissions. The design of the course is described including the background research framework that was employed to help students develop a deep connection with climate change and climate solutions. Five years of graduates from the course were surveyed at least five years after they took the course. The results of both survey data and focus group interviews provide an indication of the long-term impact of the course, and they contribute to our understanding of the potential role that education can play in long-term behaviors and attitudes. We then quantify the reductions in annual carbon emissions resulting from graduates’ pro-environmental behavior, and we compare the reductions achieved through this education program with other climate change mitigation measures. Additional discussion is provided about the educational approach and the factors we felt were critical to the success of the education program.

The San Jose State University IRB committee has approved this human subject research (F15035) and all participants have provided written consent.

2.1. University course and students

In fall 2007, a new course was offered at San José State University (SJSU) that satisfied all three subject areas of the upper division general education (GE) requirements, plus the campus upper division writing requirement. The course, COMM/ENVS/GEOL/HUM/METR 168 & 168W: Global Climate Change I & II (hereafter referred to as COMM 168), is taught over an academic year, with six credit hours in the fall semester, and three credit hours in the following spring semester. The course is team taught by three faculty members from different departments with expertise in the core themes of climate science, climate mitigation and environmental communication. Although different professors taught the course during the five-year study period, the syllabus was consistent through the five years. During this same five-year period, student enrollment came from a broad distribution of the campus colleges, as shown in Table 1 . The course uses a number of design approaches to impact students in ways that maximize effects on students’ personal and professional lives, and this is described in more detail in section 3, Course Design. COMM 168 has been taught every year since 2007 and continues to be a well-enrolled class at SJSU.

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https://doi.org/10.1371/journal.pone.0206266.t001

2.2. Survey and focus groups

An 18-item survey instrument (provided in the S1 Text ) was developed to study participants’ beliefs about climate change and whether their own personal actions to mitigate climate change could be associated with taking the COMM 168 course. The survey was broadly based on questions about climate change drawn from [ 31 ] and [ 32 ], and included questions that used a five-element Likert scale (strongly agree, agree, don’t know, disagree, or strongly disagree), multiple choice, and free response. A draft survey was trialed at SJSU by other educators and was revised based on their feedback. Of the more than 500 students who took the course, 104 students from the five different course iterations between 2007 and 2012 completed the survey. We emphasize that the survey was given to students at least five years after they completed the course, and no surveys were given before participants took the course. The categories of questions focused on participants’ a) attitudes and beliefs about global warming and whether they perceive it to affect them personally, and b) whether any of the participants’ current pro-environmental behaviors can be attributed to taking the COMM 168 course. The survey data was collected using an online platform where participant email was used to ensure only one response was collected per participant. Once the data was collected, spreadsheet statistical techniques, including pivot tables, were used to analyze participant data based on responses to different items.

After evaluating the survey responses and noting themes in the utility of the course and personal climate change mitigation strategies, we followed up with focus group interviews to gain more in-depth understanding of the enduring influence of the course on students’ personal and professional lives. Including a qualitative approach, such as focus group interviews, can complement the survey analysis and ultimately enhance the quality of the resulting analysis [ 33 ].

Focus group participants were randomly selected from the 100+ survey respondents. We conducted two focus groups with a total of five participants in a classroom at San José State University. Participants were asked a series of open-ended questions about the course and its impact on their current lives. Once the focus group interviews were completed, focus group transcripts were analyzed according to thematic analysis. The goal of a thematic analysis was to identify patterns in the data to bring clarity to the research questions. First, we interpreted patterns in the focus group responses by identifying themes in the transcripts that were common across the interviewees in different focus groups. Then select quotes and phrases were chosen to illustrate the identified themes. These quotes and phrases were woven into a narrative to describe the focus group responses in a coherent way. This exploratory approach to thematic analysis enabled us to present a rich description of student experiences in the course and perceptions of climate change issues. Copies of the survey, focus group scripts, and focus group protocols are provided in S1 and S3 Texts.

2.3. Estimating carbon emission reductions from the survey responses

Once responses to the survey questions were obtained, the potential carbon reductions from the decisions made by participants were estimated. Details of the procedure used are provided in S2 Text , but we briefly describe the method here. We use the CoolClimate Calculator [ 34 ] an online household carbon footprint calculator that has been well documented and verified in a number of studies (e.g., [ 35 ]; [ 36 ]; [ 37 ]; [ 38 ]). The carbon footprint calculator is used to estimate how a particular action attributed to taking COMM 168 would impact individual annual carbon emissions. We start by calculating the annual carbon emissions for an average person in California. Then, based on the response to a particular question (e.g., participant attributed their current purchasing of renewable energy from their utility to the COMM 168 course), we use the calculator to determine the reduction in annual carbon emissions due to that particular action (e.g., participant reduced emissions by 1.38 tons/year by purchasing renewable energy from their utility). This procedure is repeated for each of the actions identified in the survey, and thus allows us to estimate how particular actions have changed individual carbon emissions. We acknowledge that although participants attributed particular actions to the COMM 168 course, other experiences either before or after the course may have also contributed towards these pro-environmental attitudes and behaviors. Our notion is that this intensive one-year class on climate change played a key or leading role in the development of these attitude and behaviors.

3. Course design

The COMM 168 course was designed to promote lasting responsible environmental behavior through an educational model broadly based on the environmental education research of [ 17 ]. In this research, Hungerford and Volk identified three predictor variables or factors that contribute to pro-environmental behavior. The first factor is labeled as an entry-level variable and describes the importance of an empathetic perspective towards nature and the environment. The second factor is labeled as the ownership variables and describes the importance of both in-depth knowledge about the issue and a personal connection to the issue. The third factor is the empowerment variable, and this describes the understanding and skills around solutions to the issue, together with a sense of personal agency. As described in various later studies (e.g., [ 10 ]; [ 39 ]; [ 40 ], these three factors are important components to successful behavior change educational programs.

To illustrate the theoretical connection between the design elements of the course and the expected outcomes, we use a conjecture map in Fig 1 [ 41 ] to illustrate what we believe are the most salient connections between the primary conjecture, key elements of the intervention design, the measurable mediating processes and the intervention outcomes. This framework outlines the intermediate processes that support learning, and offers opportunities to measure the effectiveness of these mediating processes towards the intervention outcomes.

In the course design column, the item superscripts indicate an alignment with predictor variables (i.e., 1—entry level; 2—ownership; 3—empowerment).

https://doi.org/10.1371/journal.pone.0206266.g001

The course design includes two primary tools that aim to provide students with the key learning experiences that will lead to the intended outcomes. The first tool is a series of activities where students explore connections between their personal and professional lives and climate change. The second tool is the community action project, where student teams design and implement plans to reduce carbon emissions in a community of their choice. Each of these tools, together with other learning experiences in the class, are structured around the three key focus areas of climate science, climate solutions and communication. Examples of the primary tools followed by the mediating processes are provided below.

The COMM 168 course used a series of activities to help students develop a stronger connection to climate change and to leverage the predictor factors that have been found to promote behavior change. We provide three examples of learning activities that leveraged each of these predictor factors. In one activity focused on careers, students write a paper, supported by research, about the importance of climate change in their specific discipline. The audience of the paper are peers in their field, and students identify at least three reasons why climate change would be important in their discipline. This career activity is most closely aligned with the ownership variable. In another activity focused on individual action, students use an online calculator to compute their own carbon footprint based on their lifestyle, and then they develop a plan for how to reduce their carbon footprint by 10%. Students then implement their carbon reduction plan for a week and report on their experiences. This activity is most closely aligned with the empowerment variable. In a third activity, students participate in a multi-day United Nations (UN) climate negotiation simulation, where students play the role of a delegate representing a specific nation or bloc of nations. This activity provided students with unique perspectives on the impacts of climate change on vulnerable communities, and this activity was most strongly associated with the entry-level variable.

The other primary tool used in the course design is the community action project (CAP), a year-long culminating experience that threads through the two semesters. In the CAP, student teams build on their course knowledge to develop, design and implement projects that respond to climate change in local communities. During the first semester, student teams are formed and develop proposals for their community action project, while in the second semester, student teams are focused on developing and implementing their projects. Examples of CAPs include developing community gardens in the local neighborhood, presenting climate lessons in schools, and creating campaigns to help individuals and businesses move towards some type of climate action. At the end of the second semester, a panel of external judges comprising local government and industry award prizes to the teams with the most innovative and successful projects. The CAP allows students the opportunities to apply their learning in a way that is meaningful and impactful, and there is strong alignment between CAP projects and the predictor variables described above.

Supporting these two instructional tools are the three key focus areas of climate science, climate solutions and environmental communication. For the focus area of climate science, the instruction provides an understanding of the natural and anthropogenic factors that affect the Earth’s climate. Students study the past climate to understand natural factors, and then they focus on the current climate where human activities are the dominant contributor to contemporary changes. Tools like radiative forcing and climate models are used to help students identify evidence connecting human activities and climate change.

For the focus area of climate solutions, students study how both policy mechanisms and personal actions can help mitigate climate change. Through various case studies, students look at the role that local, state and national policies can have on improving environmental conditions. Related issues such as environmental justice and the slow uptake of climate action in government are also discussed. Other areas of climate change mitigation include studies of personal behavior around subjects like food, transportation and home energy use.

For the focus area of environmental communication, students look at marketing and communication strategies and the ideas around framing for particular audiences. Students study various media campaigns and develop experience creating their own communication tools designed for a particular audience. A component of this also focuses on analyzing the current public discourse around climate change and how various stakeholders play a role in shaping these discussions.

As referenced in the conjecture map of Fig 1 , these course design elements support a number of mediating processes that ultimately can lead to actions and behaviors that reduce carbon emissions. Aspects of the mediating processes and intervention outcomes can be measured using various tools. In this study we have used surveys and focus group interviews to explore students’ knowledge and attitudes about climate change at least five years after completing the course.

The design elements of the course were developed to achieve the stated outcome of developing a personal connection to climate change and participating in behaviors that reduce carbon emissions. As is the case in many educational settings, along the way faculty made adjustments to the course and their teaching to help promote student engagement. However, the primary course design tools and key focus areas were constant throughout the five study years. A copy of the original syllabus is provided in S4 Text .

Finally, when developing this course more than 10 years ago, we were focused on creating a contemporary and action-based learning experience. Only later did we realize that this learning environment was creating unique outcomes, worthy of further study. Although it would have been preferable to have also collected data before and during the course experience, the type of longitudinal analyses presented here is rare in environmental education, and our methodology, although subject to some limitations, provides a unique opportunity to investigate the long-term role of education on personal behavior.

As described in Sections 2.2 and 2.3, we use surveys and focus groups to study the attitudes and behaviors of graduates of COMM 168 after more than five years following the course completion. These results are analyzed in the below sections.

4.1. Survey

The first part of the survey focused on participants’ attitudes and beliefs about global warming. A large majority of participants (83%) agreed with the statement, “Most scientists think global warming is happening.”, and most participants (84%) also felt that global warming would affect their lives “a great deal” or “a moderate amount.” This is notable since the general public often discounts the impacts that global warming will have on them personally [ 42 ]; [ 43 ]. Most participants (84%) also strongly agreed or agreed with the statement, “I have personally experienced the effects of global warming.”, and when asked about how global warming will affect future generations, 91% said “a great deal.” Because these results are quite different from the average U.S. general public (e.g., [ 44 ]), this suggests that the course may have had an influence on students’ long-term beliefs about climate change. Even so, we cannot rule out the possibility that a socially-agreeable bias may be present in participant responses, as described further in the Section 7.

The second group of questions asked about personal actions to reduce climate change and whether the COMM 168 course had any effect on those actions. The general areas of climate action included waste reduction, home energy conservation, transportation and food choices. Each question asked participants to reflect on how participation in COMM 168 may have affected their actions today in those areas.

A summary of the results for the different categories is provided in Fig 2 . In the waste and home energy conservation categories, a large percentage of participants described engaging in some actions to reduce waste or reduce energy use in their home that they attribute to taking the COMM 168 course. This included recycling more often (95%), changing to more energy efficient light bulbs (86%), giving away or donating products so they can be reused (75%), buying products that have less packaging (64%), and purchasing energy-efficient appliances (59%). Fewer participants reported actions such as composting food scraps (48%), purchasing renewable energy from their utility (18%) and installing solar panels (4%).

The blue bars represent the percentage of students who agreed with the survey response, while the orange bars represent the impact in carbon emissions in percent relative to the total reductions.

https://doi.org/10.1371/journal.pone.0206266.g002

In the transportation category, about 25% of participants reported some behavior to reduce emissions that is attributed to the COMM 168 course. This included using public transportation more (35%), using a bicycle for transportation (26%) and carpooling regularly (22%). And in the food choices category, most participants (80%) reported that at least occasionally they made food choices based on reducing carbon emissions.

The survey responses reported here suggest that participant behavior was influenced by the COMM 168 course in ways that continue to impact daily life. The types of actions studied here can be divided into two groups: one-time actions and recurring actions. For example, the purchase of an energy-efficient light bulb or automobile is a one-time action, and these decisions will shape energy use for years into the future. In contrast, recurring actions such as recycling or food choices are made every day, and thus require more consistent engagement or behavior response. In reality, pro-environmental behavior includes both types of actions, and their impacts on carbon emissions can vary depending on the type of action and whether recurring actions become part of an individual’s lifestyle. Given the number of years that elapsed between the course and the survey, the survey provides a glimpse into behaviors that have likely become habitual. In the waste and food categories, some recurring actions were noted by most participants. Although recycling may be viewed as a fairly common action in many Californian communities, food choices and the connection with carbon emissions is not as widely known by the general public (e.g., [ 30 ]; [ 31 ]). Given that 80% of participants reported some changes to their food choices, it appears that the course did have an impact on decision-making in this category even years after the course.

4.1.1. Estimated carbon emissions.

Using the survey responses about the actions that participants took, we estimate the reductions in carbon emissions for all participants using a household carbon footprint calculator. Fig 2 also shows the contribution of each of the survey questions to the total reductions in carbon emissions. While changes to behavior around reducing waste and energy conservation at home were the most common actions taken, the largest reduction in participant-averaged carbon emissions came through transportation decisions. For example, while only 31% of participants reported purchasing a more gas-efficient car, this single action accounted for 18% of all carbon emission reductions observed. In contrast, while over 90% of participants reported that they recycle more often, the combined reduction in carbon emissions only accounted for 11% of the total reductions.

As shown in Fig 3 , the average reduction in carbon emissions based on the participant survey responses is 3.54 tons of CO 2 /year, with most participants between 2 and 5 tons of CO 2 /year. About 5% of students reported almost no change (0–1 ton of CO 2 /year), and about 10% reported between 6 and 8 tons of CO 2 /year. Of the four primary categories of carbon emission reductions, changes in transportation were responsible for 40% of the total carbon emission reductions, while waste reduction, food choices and home energy contributed 33%, 13% and 12% respectively of the achieved total carbon emission reductions.

https://doi.org/10.1371/journal.pone.0206266.g003

4.1.2. Understanding how personal relevance and carbon emissions are related.

Given that one of the goals of the course is to help students develop a personal connection between global warming and their lives, we explore the connections between participant beliefs and total carbon emission reductions through analysis of grouped data. In Fig 4 , we show the relationship between individual carbon emission reductions with personal beliefs about how global warming will influence them or future generations. We find that participants who believe that global warming will harm them personally, or will harm future generations, have larger reductions in carbon emissions compared to participants who do not believe there will be a strong impact on them or future generations. Thus, it appears that in most cases, participants were at some level influenced by how they perceived the impact of global warming on their own well-being, or the well-being of future generations, when making personal decisions related to the environment.

The percentage of the total responses for that question is also given above each bar.

https://doi.org/10.1371/journal.pone.0206266.g004

Further, of the participants who agreed (strongly agreed or agreed) to the statement, “I have personally experienced the effects of global warming.” their reductions in carbon emissions were 3.7 tons of CO 2 /year, while for the participants who did not agree with that statement (disagreed, strongly disagreed or neutral), their reductions were only 2.9 tons of CO 2 /year (see Fig 5 ).

https://doi.org/10.1371/journal.pone.0206266.g005

4.2. Focus groups

Responses from focus group participants converged around two themes: the importance of daily decisions to mitigate their climate change impact and the importance of engaging their community through climate change communication. Examples of these themes from focus groups responses are provided below, together with relevant connection to the three predictor variables used to inform the course design.

4.2.1. Impact on daily decisions.

A hallmark of conversations with graduates from the course was the consideration of climate in daily decisions. Fundamentally, focus group participants recognized the pervasiveness of climate change. As Tara, a focus group participant, noted, “Almost every activity we choose can affect [climate change] in some way, whether we choose to take the bus or drive to work or whether we choose to buy food that’s grown on land that was cleared from rainforests….Since it is in every aspect of our life pretty much, that automatically makes it relevant to all those different aspects.” Other participants agreed and described daily actions that centered on transportation, waste and food choices. Melissa noted, “I think about it all the time…. Definitely how I think about and go about my days, making decisions, even just from using plastic.” And Elaine commented about buying a car after she paid off her student loans “I ended up choosing a Prius C for a lot of reasons. At the time it was pricey, but it just seemed energy efficient. It had what I was looking for while still being helpful for the environment.” These responses exemplify a common theme in the group—the knowledge of climate change gained in the course prompted them to think about the impact of their actions.

The focus group participants noted that they go out of their way to take action because they feel as if they are making a difference. Billy noted, “When everyone does something to mitigate climate change, it will have a huge impact.” Tara concurred, “Almost everything I do can affect the climate somehow. If you start realizing how everything ties together, then pretty much everything you do, every choice you make can affect it in some way.” She continued, “I think every small step does make a difference…. One little step at a time; it all adds up. I’d like to think we’re making a difference. I feel like I am when I contribute a little bit.” Participants suggested that the interdisciplinary focus of the course allowed them to see the connections between their actions and broader climate forcings.

Participant comments demonstrate that environmental actions are not just because of sacrifice but that people feel good about taking action. Lolitta explained, “So when we started the global climate change class, for a week we had to do something eco-friendly. I’m like okay, I’m gonna be a vegan. And I did it totally wrong. I just ate vegetables and fruits all day, and I was starving. But it got me to become vegan, and for a couple years I was. Now I’m a vegetarian, and I’ve stuck to it. I feel good about how I’m living my life, and I’m excited by all the changes that I’m making, and I will continue making these changes because they make me feel great.” Billy noted proudly that he acts because “it’s like a moral obligation.” Ultimately, participants concurred that daily actions matter, and they cited this belief as the reason they continue to take actions. Their comments suggest they are empowered to act because they see themselves as part of the solution.

4.2.2. Community engagement through communication.

Overall, focus group participants noted that this course helped them develop experience communicating with other people about their actions and why they are taking them. Participants cited the community action class project as a key element in their understanding of the impact of community engagement. Billy described the lasting impact of “the hands-on approach” of the project: “Those experiences, I think for me, I carry those longer than / more than being in the classroom…. Being with people, doing something that’s going to translate into what I have to do work-wise in the future, [the project was a] translatable experience to the workforce.” Melissa described the lasting impression of the project as a crucial aspect to seeing the impact of action: “It actually bridged the gap between the course and what the community itself is doing.”

Participants noted the impact of the course on life beyond the home. One focus group participant, Elaine, is a manager at Walmart, and she credited the course for her “awareness in an industry with high consumption…. It’s so interesting how much I’ve been able to use just from this course.” She noted her focus as a manager is how to “reduce your inventory, reduce the waste, sell what you need to.” Elaine views Walmart’s waste issues both as a climate issue and a management problem: “I see the huge amounts that they’re throwing away because they’re not managing their business correctly, because they’re not managing their production versus what they need, what they don’t. So that’s one of the things that I work on.” Elaine’s comments exemplify how many graduates of COMM 168 viewed the importance of taking action.

Billy noted the course explained how to make “big issues” like climate change “resonate with your audience… That’s what I do now.” He explains that in his job at the utility company, Pacific Gas and Electric, one of his roles is communicating about energy issues, “That’s my biggest takeaway from this class: messaging. [I now understand] the importance of communicating about climate change in a way where people who don’t have a background in that subject can understand.” Other participants concurred that the course made them experts in climate change, and they now have to think about how to communicate with people who don’t have such extensive knowledge.

Participants also noted the importance of communicating with others about the actions they take. Tara noted: “It doesn’t really help unless you try to bring it out there. If I only ever walk places, no one will ever know unless I try to let them know why I walk places…. If you’re going to make a point by breaking the rules, you first have to know the rules because otherwise it doesn’t mean anything. If I want to rebel by not using a car, I first have to know that everyone thinks using a car is a normal thing to do.” Participants agreed that talking about their own actions helped in discussing climate change issues with others.

The community action project was a key part of the course in giving students experience outside of class in creating change. It also gave them some agency over this issue. Participants described their attempts to make a difference, both in their personal and professional lives. Participants noted the community action project allowed them to see the importance of communication in the design of their projects. The focus group responses suggest that interdisciplinary education including aspects of communication can give students the skills and experience necessary to create change in their own communities.

The outcome of the themes that emerged from the focus groups are broadly aligned with the methodology outlined in the course design (Section 3) and as described in the conjecture map ( Fig 1 ) In particular, students noted a personal connection with climate change (ownership variable), and they demonstrated specific ways either through personal actions or through communication that they could take action (empowerment variable). The entry level variable, which describes a sensitivity or empathy for the environment, was present in some of the focus group remarks, but did not emerge as a central theme.

5. Educational approach

We describe a number of key design elements that stood out as critical to the success of the education program we developed and that have sustained student engagement over many years. These include a) connecting climate science to students’ lives, b) providing students with experience creating change in a community of their choice and c) creating a culture devoted to stewardship and action. We found that these elements of the course helped students to connect with the subject in ways that extended into their personal and professional lives, and are broadly aligned with some of the predictor variables that we used to design the course. These elements were not isolated from each other, or from other important elements of the course, including a solid focus on climate science, climate solutions and environmental communication. These elements are in line with the models suggested by other researchers, including personal relevance and empowerment [ 16 ]; [ 23 ]. We now review each of these elements in more detail to provide insights into how these ideas may be applied to other educational settings.

5.1. Connecting science to students’ lives

Various activities in the course were designed to help connect climate change with students’ lives and align with the ownership variable discussed in Fig 1 . One project asked students to reflect on how climate change would affect their personal and professional lives. Another project had students track their personal energy use, and then implement a plan to reduce their energy use in their home using data from their home smart meters. These elements appeared to have some lasting impact, as various focus group participants reflected on how the course materials affected their personal and professional lives.

In addition to the actions that were identified in the survey data, open-ended feedback also revealed that the course affected other major decisions, such as where to live and how many children to have. In fact, two of the participants mentioned their decisions to adopt a child or not to have children were influenced by the course. This implies that at least for some of the students, the course content and the implications of climate change affected their personal lives deeply. It appears that some of the high-impact actions identified by [ 30 ], such as having fewer children, did resonate with the COMM 168 students.

In a recent study by [ 23 ], a systematic review of the climate change education literature identified themes common in successful programs. One of the primary themes identified was a focus on making climate change personally relevant and meaningful for learners. It is noted that this is also a common practice in environmental education and science education, but as we found in our own work here, it can be made especially meaningful given the personal connection that climate change can have to students’ lives.

5.2. Creating change in a community of their choice

Another design element of the course was to provide students with real-world experience creating and implementing an action plan to reduce carbon emissions, an activity aligned with the empowerment variable. The Community Action Project (CAP) was the culminating experience where student teams competed to develop the most impactful community-based project. The goal of the CAP was to give students real-world experience developing solutions to climate change. It was our intention that through this experience, students would not only better understand some of the challenges associated with creating change but also gain confidence that change can happen through well-designed efforts. [ 45 ] found that using issue investigation and action training was an effective way to promote pro-environmental behavior. And [ 46 ] found that students were deeply affected by their service-learning course even years after the experience. The COMM 168 course was focused around the year-long CAP, and feedback from the focus groups shared how impactful the project was for some of the students, as a majority of the focus group participants mentioned the CAP as the most memorable part of the course. Our conclusion that the CAP promoted engagement and student empowerment has also been recognized in various other climate change education programs as a key element in creating effective learning experiences [ 23 ].

5.3. Creating a culture devoted to stewardship and action

• Encouraging group discussions with different students: We did a lot of group work in class, and with 80–120 students per class, we took special efforts to mix students for their group work. This helped students work with new students and be exposed to new ideas. By giving students some challenging subjects to discuss (i.e., how does climate change affect their current or future lives), or challenging situations (i.e., during a UN simulation on climate change where students represented different countries negotiating a climate treaty), we gave students the opportunity to exchange personal ideas about climate. We felt this helped students see multiple views across the class, and if an emerging interest and dedication to climate change arose through the class, it could spread.
• Faculty committed to climate action: The faculty who taught this course were all deeply committed to climate change solutions, and they were encouraged to share their own personal and professional journeys towards reducing carbon emissions. And because students got to know the faculty fairly well, given the course was taught over an academic year, students had the opportunity to connect with the faculty at a personal level. For example, when faculty reflected on their own personal challenges in reducing emissions associated with driving or eating, students could relate to this. Role models are important in creating social change, and we suggest that having professors committed to environmental solutions was also a factor in creating a social culture for the class that encouraged pro-environmental thinking and behavior. For example, one of the focus group participants mentioned that as a result of the class culture, her ownership of an SUV grew uncomfortable given her shifting connection to the environment. She admitted to deliberately concealing her vehicle type from the faculty, even though the faculty attempted to create a culture of acceptance without judgement. Later after graduating, this participant purchased a hybrid as her next vehicle. This is an example of the social norms that were established in the class that may have extended to students’ lives outside of school and over time.

6. Potential role of education on carbon emission reductions

Given the reductions in carbon emissions calculated in Section 4.1.1 (and shown in Fig 3 ), we now explore the potential role of education as a climate change mitigation strategy. We start by estimating the participant reductions in carbon emissions compared to a control group. The control group is created by using California’s per capita carbon emissions data as estimated by the California Air Resources Board (CARB) [ 49 ]. We choose to use California’s per capita carbon emissions for two reasons. First, we do not have a good way to access course participant’s prior behavior retrospectively, and second, we assume that behavior after graduating from the course could change as students become professionals resulting in a potential dramatic lifestyle change. The California per capita carbon emission data show that by 2014, per capita carbon emissions for the average Californian declined by 0.68 tons/year compared to 2009, the midpoint when students had graduated from SJSU. By contrast, the participants in COMM 168 reduced their per capita emissions by 3.54 tons/year. Thus, if we subtract the emission reductions for the average Californian (0.68) from our participants (3.54), we find that the net reduction above the average citizen is 2.86 tons/year (3.54–0.68 = 2.86).

We now use the net reduction in carbon emissions observed for graduates of COMM 168 to compare the potential role of education as a climate change mitigation strategy with other climate change mitigation strategies. For this comparison, we employ the methodology outlined in Project Drawdown, where 80 different technologies or strategies are evaluated based on the potential to cumulatively reduce carbon emissions by 2050 [ 50 ].

The following procedure and set of assumptions are used to calculate carbon emission reductions associated with climate change education, as shown in Fig 6 . We first assume that a modest investment in climate change education would allow students of secondary school age from middle and high income countries (where their carbon emissions are highest) to receive a specialized climate change education (i.e., using similar educational methodologies as we have described in this paper), and that students who receive this education would each reduce their carbon emissions by 2.86 tons of CO 2 /year (i.e., as in the COMM 168 course), for that year, and for each year following. Further, we assume that such a program would start small at 1 million students and grow by 13% per year until 2050, when the program reaches over 38 million participants. We use 2015 data from the United Nations Educational, Scientific and Cultural Organization (UNESCO) [ 51 ] to estimate the number of students of secondary school age from high income and upper middle income as 298 million. This allows us to estimate the percentage of students participating in this specialized climate change education program in 2020 and 2050, assuming the population of secondary students in these countries does not change.

The potential role of climate change education programs is calculated using the per student carbon reductions estimated from the COMM 168 course.

https://doi.org/10.1371/journal.pone.0206266.g006

In Fig 6 , six of the solutions presented in Project Drawdown are compared with our own estimate for using education as a climate change mitigation strategy. For the solution scenarios developed by Project Drawdown, each of these represents ambitious and yet also technically and economically feasible plans for reducing carbon levels. Technical details and reference literature for all these solutions are presented at www.drawdown.org . As examples, the Rooftop Solar scenario grows the percentage of electricity generated by rooftop solar from 0.4% today to 7% by 2050, while the Electric Vehicles scenario grows the percentage of passenger miles from electric vehicles from less than 1% today to 16% by 2050. For the Climate Change Education scenario we assume that a) each student reduces their carbon emissions by 2.86 tons of CO 2 , similar to the COMM 168 course and b) the adoption of this type of education grows from less than 1% of all secondary students today to 16% of all secondary students by 2050 (note: the number of secondary students is restricted to only high income and upper-middle income countries where residents have higher carbon emissions).

The results of this comparison show that education, if designed appropriately, can potentially be as effective as other established climate change mitigation techniques. Based on the scenario we developed, the implementation of climate change education over a 30-year period (2020–2050) could reduce emissions by 18.8 GT of CO 2 eq, an amount that would rank in the top quarter (15 out of 80) of the presented solutions in Project Drawdown. Although at scale, the use of education as a climate change mitigation technique is still untested, our analysis suggests that if the educational approach is sound, and if we take the effort to measure the impact of education, we may realize the potential to reduce carbon emissions using education. We also acknowledge that although barriers to developing a successful large-scale climate change education program exist, significant social and political challenges exist with most large-scale solutions to climate change.

7. Uncertainties and study limitations

In the following section, we describe a number of uncertainties and study limitations that are important to the interpretation of the results. Although we believe the COMM 168 course provides unique insight into the long-term role that education can have on individual behaviors, especially given the lack of existing studies that look at how education can shape behavior over many years, we also acknowledge the potential limits of such a research design, and thus we are careful here to identify uncertainties and describe limitations in the study. The exposure of such uncertainties and limitations provides the research with a context for interpreting the results and also provides an avenue for researchers to undertake additional studies to investigate the impact that education can have on long-term behavior change.

7.1. Uncertainties

The study methodology and analysis include a number of assumptions that contribute to the uncertainties associated with this study, and these are discussed below.

Student enrollment.

Because the course is titled “Global Climate Change,” students interested in environmental issues may have self-selected into the course. These students may respond more favorably to the course design, and may be more willing to change their behavior in the future given their initial interest in the environment. Although the impact of incentives on bias is not clearly understood [ 52 ], the year-long course included a 3-unit incentive where a passing grade in the 9-unit course provided students with an additional 3 units of general education credit. We heard that many students reported that they signed up for the course because of the extra requirements satisfied. Further, we note that when this incentive was removed from the course design in 2014, the initial broad distribution of majors who enrolled in the class declined quite dramatically. In the earlier years with the 3-unit incentive (2007–2013), four colleges (i.e, Social Sciences, Humanities and the Arts, Business, and Health and Human Sciences) each had at least 10% of the registered students. However, once the 3-unit incentive was removed in 2014, only two colleges (i.e., Social Sciences, Humanities and the Arts) had significant enrollment (i.e., more than 10% of students), with the course now having more enrollment from Environmental Studies and Communication Studies. The 3-unit incentive thus appears to have been effective in drawing students from across campus, and this suggests that the course topic was not the only reason students enrolled in the course.

Energy calculations.

The household carbon footprint calculator was used to estimate how student responses would impact carbon emissions. Although the calculator has been used in a number of studies, various assumptions were made as described in Table 1 of S2 Text . It is clear that some of the carbon reductions attributed to the course experience may have inherent uncertainties. For example, actions such as carpooling regularly, making food choices to reduce emissions and buying energy-star appliances all suggest actions to reduce emissions, and yet the actual reduction amount depends on specifics of the action that are difficult to obtain without a more detailed survey tool. In contrast, the goal of this analysis was to document actions attributable to the course and develop a practical methodology for estimating the carbon reductions using the best tools available.

Behavior changes.

Another uncertainty that this research only partially uncovered was the motivation for the reported changes. Did participants make lifestyle changes because of environmental concerns or for other reasons, such as financial considerations or ethical concerns? In our focus group, participants reported that pro-environmental outcomes were the primary reason for their choices, but we do not know if this was also the case with all students. Further, without a more detailed survey, it is difficult to understand whether other factors (e.g., social circumstances) also contributed to these changes. This is one reason we chose to use the California per capita emission reduction as a control group, so that pro-environmental trends seen throughout California could be accounted for.

Other considerations.

We also acknowledge a number of other uncertainties in the design of this study. Participants were surveyed at least five years after taking the course, and we recognize the limits of human memory may skew some of their responses. There may be students who incorrectly remember aspects of the course, and this may have influenced some of our conclusions. This is in part why we chose to do a focus group to more accurately investigate aspects of the course that may have been important.

7.2. Study limitations

One limitation in this study is the lack of a control group or a pre-survey. We acknowledge that without such accompanying data, determining the precise relationship between students’ participation in the course and their current attitudes and behaviors is difficult. We did attempt to control for how pro-environmental behaviors in California have become more common over the last decade, but we do not have any data that measured student attitudes or behavior before taking the class. Although further studies should consider the various ways to measure changes in participant attitudes and behaviors, measuring such changes over many years remains a challenge.

Another limitation in the study is the potential for selection bias. Although we attempted to determine whether students self-selected into the course based on their environmental leanings or the 3-unit incentive, we do not have independent data to quantify the role that selection bias had on student enrollment. If students did select this course because of their initial interest in environmental stewardship, this could bias the outcomes of the study.

Another concern is related to biases in participant responses to survey and focus group questions. We acknowledge that a socially-agreeable response bias with regard to behaviors being attributed to the course may exist in the participant responses to surveys and focus group questions. Although we took measures in our survey design and focus group protocol to reduce such biases, it cannot be ruled out that such self-reporting response biases may be present and could influence the reliability of the results.

Finally, we recognize that among the uncertainties identified in section 7.1, none of them have been adequately quantified. Although some of these uncertainties, such as the reliability of the carbon footprint calculator and the related carbon emissions, probably would not influence the primary outcomes of the study, other uncertainties such as initial attitudes of participating students may have a larger influence on the study results.

As we have generally described, establishing linkages between an educational campaign and long-term behavior can be challenging. Other studies that attempt to establish causal links between education and environmental quality also faced similar challenges (e.g., [ 5 ]), and yet the insights gained from such work provide a strong motivation for environmental education and this type of research [ 7 ]; [ 53 ]. Our work is similar. Despite the limitations we have identified, our analysis provides important insights into understanding the role that well-designed climate change education can play on long-term attitudes and behavior.

8. Conclusions

The potential role of education on individual carbon emissions was studied using data from students who completed an intensive university course on climate change. Students were surveyed at least five years after having taken the course, and their responses were used to provide both qualitative and quantitative measures of the impact of the course on their attitudes and behavior regarding solutions to climate change. The university course was designed to be impactful, including various elements from the environmental education literature to engage students around personal and social activism. In open-ended feedback and the focus group interviews, students recounted how the course has changed their lives, both personally and professionally. Examples of personal changes included the type of car they drive and the type of food they eat. Examples of professional changes included how they create environmental benefits through their job. The results from the survey data also suggest that the course was impactful, even many years later. Student behavior related to waste decisions, home energy decisions, transportation and food choices all showed significant behavior change that was attributed to the COMM 168 course, and these changes were quantified using a reputable online carbon emissions calculator. The estimated reductions in carbon emissions attributed to the COMM 168 graduates are 3.54 tons/year, compared with the carbon emissions for an average California resident of 25.1 tons/year. It was found that the participants who had personally experienced the effects of global warming, or felt that global warming will harm them personally, had the largest reductions in carbon emissions. Although a number of studies have established links between educational programs and environmental quality, such as water or air quality [ 7 ], far fewer studies have established causal links between education and carbon emissions [ 5 ].

This study suggests that the design of the COMM 168 course provides elements of the three crucial factors that [ 17 ] identify as contributing to pro-environmental behavior: entry-level, ownership, and empowerment variables. Surveys and focus group interviews reveal that graduates of the course feel a lasting personal connection to the issue and have confidence in the success of their actions. This strong sense of personal obligation and the perceived individual agency to address climate change suggest that education that leverages these design elements including community engagement may provide a public benefit. The authors also note that social norms, established through a year-long course and emphasized through various classroom activities, also may have contributed to students’ pro-environmental attitudes and behaviors. However, while previous studies have demonstrated that factors such as having a personal connection (e.g., [ 23 ]) and perceived self-efficacy (e.g., [ 54 ]) can influence individual behaviors, we acknowledge that other factors are also likely important (e.g., [ 55 ]), and understanding how these factors contribute to individual behavior change is complex [ 39 ]; [ 56 ]. We also acknowledge that there may be cases where structural factors, such as size of home or distance of commute, may obscure the intentions of pro-environmental behavior [ 57 ].

The potential to use education as a climate change mitigation measure would be valuable and in line with other mitigation measures if such reductions as achieved in the COMM 168 course could be achieved in other classrooms. We illustrate this through comparisons with other climate change solutions, and show that at scale, climate change education can be as effective in reducing carbon emissions as other solutions such as rooftop solar or electric vehicles. The notion that education is an important part of responding to climate change is not novel (e.g., [ 29 ]; [ 58 ]), and yet rarely has it been quantified and measured [ 53 ]. This paper sheds light on how such measurements could be taken, and it offers a pedagogical insight for how to make education an effective climate change mitigation strategy.

At present, the authors are using similar design approaches to develop a comprehensive science curriculum focused around environmental stewardship and climate action (e.g., [ 59 ]; [ 60 ]) for middle schools. The Next Generation Science Standards now emphasize applying integrative science fields to solving real-world problems, and this serves as an ideal platform for applying the type of educational platform developed in COMM 168 towards a broader science curriculum for schools. The middle school science curriculum [ 61 ] is currently being used in a number of school districts in California, and studies examining changes in student attitudes and behavior will be reported in the future.

Supporting information

S1 text. survey instrument used for the graduates of the comm 168 course..

https://doi.org/10.1371/journal.pone.0206266.s001

S2 Text. Procedure for estimating reductions in carbon emissions from the survey responses.

https://doi.org/10.1371/journal.pone.0206266.s002

S3 Text. Focus group protocol.

https://doi.org/10.1371/journal.pone.0206266.s003

S4 Text. COMM 168 syllabus.

https://doi.org/10.1371/journal.pone.0206266.s004

Acknowledgments

We are grateful to the students involved in this study for their time and participation, and Dr. Elizabeth Walsh for her helpful suggestions on our data analysis. We also thank Liz Palfreyman for her help in gathering some of the student demographic data.

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• Open access
• Published: 15 October 2020

Climate change: Does international research fulfill global demands and necessities?

• Doris Klingelhöfer   ORCID: orcid.org/0000-0003-0716-9872 1 ,
• Ruth Müller 1 , 2 ,
• Markus Braun 1 ,
• Dörthe Brüggmann 1 &
• David A. Groneberg 1  

Environmental Sciences Europe volume  32 , Article number:  137 ( 2020 ) Cite this article

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Climate change is safe to be one of the biggest challenges of mankind. Human activities, especially the combustion of fossil fuels, contribute to the increase of greenhouse gases in the atmosphere and thus to the pace of climate change. The effects of climate change are already being felt, and the resulting damage will most likely be enormous worldwide. Because global impacts vary widely and will lead to very different national vulnerability to climate impacts, each country, depending on its economic background, has different options to ward off negative impacts. Decisions have to be made to mitigate climate consequences according to the preparedness and the vulnerability of countries against the presumed impacts. This requires a profound scientific basis. To provide sound background information, a bibliometric study was conducted to present global research on climate change using established and specific parameters. Bibliometric standard parameters, established socioeconomic values, and climate change specific indices were used for the analyses. This allowed us to provide an overall picture of the global research pattern not only in terms of general aspects, but also in terms of climate change impacts, its effects and regional differences. For this purpose, we choose representative indices, such as the CO 2 emissions for the responsibility of countries, the global climate risk index as a combination value for the different types of damage that countries can expect, the increase in sea level as a specific parameter as a measure of the huge global environmental impacts, and the readiness and vulnerability index for the different circumstances of individual countries under which climate change will take place. We hope to have thus made a comprehensive and representative selection of specific parameters that is sufficient to map the global research landscape. We have supplemented the methodology accordingly.

In terms of absolute publication numbers, the USA was the leading country, followed by the UK, and China in 3rd place. The steep rise in Chinese publication numbers over time came into view, while their citation numbers are relatively low. Scandinavian countries were leading regarding their publication numbers related to CO 2 emission and socioeconomic indices. Only three developing countries stand out in all analyses: Costa Rica, the Fiji Atoll, and Zimbabwe, although it is here that the climate impact will be greatest. A positive correlation between countries’ preparedness for the impacts of climate change and their publication numbers could be shown, while the correlation between countries’ vulnerability and their publication numbers was negative.

Conclusions

We could show that there exists an inequity between national research efforts according to the publication output and the demands and necessities of countries related to their socioeconomic status. This inequity calls for a rethink, a different approach, and a different policy to improve countries' preparedness and mitigation capacity, which requires the inclusion of the most affected regions of the world in a strengthened international cooperation network.

Particularly in the western world, public awareness of the consequences of climate change has reached a high level. Before the appearance of the coronavirus pandemics (SARS-CoV 2), hardly any news broadcast in the western world could do without commentary on climate change. Every week millions of pupils and students around the world demonstrated all for a strict ecological regimen of all governments to ensure the 2 °C target of the Paris Agreement [ 42 ]. In “Corona times” the effects of climate change seem almost forgotten by the public, although many scientists have already explained the connection between climate change and the increase in zoonoses [ 24 , 36 ]. Besides, the negative effects of climate change will certainly be more permanent and severe than the temporary damage of a pandemic; however, severe it may be.

Climate change will undoubtedly affect the entire planet and calls for international collective action. Shifts in wind patterns, the average temperature, or the amount of precipitation and frequency of extreme weather events will endanger the health, the food, and the water supply for humans. Those risks are directly linked to the reduction in biological diversity and the extinction of species that challenge most parts of the world. The impacts of climate change will lead to socioeconomic and political instability, which will change the living conditions of many communities.

The global climate has always been changing. However, the enormous problems are caused by the speed with which changes due to human intervention are progressing, and greenhouse gas concentrations have reached levels never before experienced by mankind. Although climate change has officially been considered the most hazardous global risk so far, the recent Conference of Parties (COP) in Madrid failed to achieve binding measures for nations.

But time is running. Solutions must be found to mitigate the consequences of climate change. Governments must react and be prepared for the worst future scenarios that require strategies without national borders. Climate change affects every country in different ways, and the ways in which countries can prepare for it or mitigate its impacts vary widely.

But what has actually happened so far? Anthropogenic activities, in particular the combustion of fossil fuels, have accelerated the rise in carbon dioxide emissions and thus the increase in global warming, with tangible impacts on humans, animals, and the ecological balance around the world [ 45 ]. The immediate environmental consequence of global warming is the increase in natural disasters, e.g., melting glaciers, more extreme and more frequent floods, wildfires, storms, and droughts or heatwaves. The indirect consequences include threats to human health, and the reduction of biodiversity and habitable areas, leading to migration and deterioration of community, public health, and socioeconomic conditions in most countries of the world [ 45 ].

Reliable estimation of the extent of these impacts is at the heart of research and forms the basis for all mitigation strategies at governmental, economic, scientific, or personal levels.

A sound research database is necessary for sustainable approaches for assessing and mitigating climate change impacts. The research on the climate change focuses on a wide range of areas and modeling approaches to consider different future carbon dioxide (CO 2 ) emission scenarios to assess local and regional global warming. CO 2 is a major component of the global carbon cycle and both a natural part of the atmosphere and an essential greenhouse gas. It is mainly through the combustion of fossil fuel that humans influence the amount of CO 2 emission and thus contributes to global warming.

For this, experts who cover all areas of climate change are in demand. These areas range from ecology, life sciences, meteorology, health care, social, and economic sciences, mathematics and computer science to energy, food, and transport. Interdisciplinary approaches deliver huge amounts of data to create reliable future scenarios. They should provide a comprehensive understanding of the problem and possible measures at all levels. All models show significant geographical differences and illustrate the enormous burden on many developing countries. However, there is no in-depth analysis evaluating global research efforts on climate change including climate change-specific parameters, that provides a comprehensive picture with specific geographical and chronological patterns of scientific publications and the resulting needs and requirements for scientific action. Therefore, the present study focuses on the evaluation of the global and national publication output on climatic change to depict structures and international developments using bibliometric analyses. Metadata analysis allows a comprehensive assessment of the global scientific landscape because all countries are vulnerable to the impacts of climate change to varying degrees because of their natural and socioeconomic conditions.

Building on other bibliometric studies [ 2 , 35 ], which also show the publication output of countries in the field of climate change, this analysis interprets global scientific output using country-specific indices relevant to climate change to present the world map accordingly [ 9 , 13 , 33 ]. The resulting implications help to answer the question of whether international research and networking on climate change meet global requirements and necessities given the current and predicted impacts on all regions and all areas of life. Thus, the interpretation of the results can enable decision makers, funders, scientists, and other stakeholders to develop concepts for future research based on carefully evaluated metadata.

Methodological platform

A representative and qualitative database has been built up, providing comprehensive metadata on the past and present scientific landscape of climate change research, its incentives, its benchmarks, and its challenges and requirements. The applied method is integrated into the bibliometric platform New Quality and Quantity Indices in Science (NewQIS), which was initiated in 2009 to provide in-depth data of the publication output on a variety of life science and biomedical topics [ 14 , 17 ]. The approach combines the application of publication and contextual factors with state-of-the-art visualization techniques. The Core Collection Indices of Web of Science (WoS), which represent one of the most important scientific literature databases, are used as data sources. In addition, WoS provides citation parameters for advanced data interpretation and quality assessment via the Journal Citation Report (JCR) and the Journal Impact Factor (JIF).

Search strategy, data acquisition, and correction

The quality of the database depends on the appropriateness of the search strategy applied. The search term must involve all important synonyms. For this study the terms: “climat* change”, “global warming”, and “greenhouse effect” were applied. The asterisk acts as a wild card and was used to search for terms with different endings. To retrieve only the original research publications, only data from the publication type “Articles” was downloaded. The Art and Humanity entries were excluded. No limitation of the evaluation period was made so that all articles from 1900 to 2020 were included in the analysis (Fig.  1 ).

Procedure for generating the analysis database

The aim was to decimate thematically incorrect entries and maximize correct ones. The risk of an unrepresentative database has been reduced by searching in the title of the manuscripts, even if e-data resulting from the search strategy cannot include all indexed articles. The metadata, sorted by various keyed information, was downloaded and saved as an MS-Access database. To unify different designations of data, e.g., the names of authors and their institutional affiliation, a standardization had to be carried out with the help of a specially developed application. For the standardization of institutions, a quantity of at least 200 articles in a regional context must be achieved. A threshold of at least 20 articles on climate change was set for authors. By applying those thresholds, it was possible to completely adjust all entries for institutions and authors above this value. Also, the names of the assigned subject areas had to be adapted and standardized due to missing spaces or typing errors. In doing so, all entries could be corrected without using a threshold value.

Analysis parameters

The resulting database consists of a large number of bibliometric parameters. The research topics were clustered based on the keywords that occur at least 650 times (threshold) using the application VOSviewer [ 44 ].

Chronological analyses were carried out to evaluate the development of research (number of articles), research incentives (number of citations). In addition, geographical analyses were conducted to identify the main actors (countries with the most cited publications, most publishing institutions), and their international networking. The average citation rate of the countries is calculated by dividing the number of citations received by the number of the publication on climate change.

However, the evaluation of the absolute numbers does not allow an assessment of the development of publication shares and the current distribution of countries’ research output on climate change issues. Therefore, the evaluation period of the last 30 years was divided into 5-year intervals for further analysis, and the ten most publishing countries were analyzed.

By linking socioeconomic characteristics and citation parameters, important additional statements on country-specific publication activities on climate change can be made.

The country-specific number of articles was put in relation to (1) Demography: total population in million inhabitants ( R POP ) [ 39 ], (2) Socioeconomic status: gross domestic product (GDP) in billion US-Dollars ( R GDP ) [ 38 ], and (3) Research investment parameters: number of researchers in FTE (full-time equivalents) [ 40 ], expenditures on research and development (R&D) (personnel in FTE) [ 40 ], and gross expenditures for R&D (GERD) in PPP$ (purchasing power parity in US-Dollars) [ 40 ]. For all ratios, a minimum threshold of at least 30 articles was applied to avoid distortions due to extreme values.

For a more specific assessment of the national research contribution, it seems appropriate to include relevant country-specific indicators related to climate change. For this purpose, we select representative indices to put them in relation to the research output of the countries. CO 2 emissions represent the responsibility of countries, the Global Climate Risk Index acts as a composite value for the different types of damage that countries can expect, the rise in sea level as a measure of the enormous global environmental impact, and the readiness and vulnerability index for the different circumstances of the individual countries under which climate change will occur.

Carbon dioxide (CO 2 ) emission in tons per year [ 33 ]: The integration of the CO 2 emissions of the countries was done by calculating the relation of the number of articles to CO 2 emissions in billions of tons (threshold = 300).

The Global Climate Risk Index (CRI): The CRI was launched by German Watch and published in its 15th edition 2020. It assesses the extent to which countries have been suffering from weather incidences [ 9 ]. The CRI provides data for the last 20 years as an average value and also for individual years. The existing data on the weather vulnerability, measured in fatalities per country, and losses in US dollars could indicate the expected increase in extreme events due to climate change and help to mitigate the impacts.

Sea-level rise: For the analysis of the number of people living on vulnerable land due to prognosticated sea level rise [ 19 ], we have taken the values of the average number of fatalities per 100,000 inhabitants from 1999 to 2018 as reference quantity. To estimate the resulting sea-level rise, the US National Aeronautics and Space Administration (NASA) created the digital elevation model (DEM) SRTM ( Shuttle Radar Topography Mission ). The here utilized CoastalDEM is a development based on the neural networks to reduce SRTM errors resulting from its limitation with respect to terrain elevations (important for densely populated areas) by regression analysis [ 19 ]. There are several prospective scenarios, based on the 5th IPCC report [ 15 ], which are based on the Representative Concentration Pathways (RCP) models 2.6, 4.5, and 8.5 leading to different degrees of global temperature rise. These scenarios presuppose different greenhouse gas concentrations. RCP 8.5 would lead to a rise of 4 °C, while RCP 4.5 would lead to a rise of 2.6 °C, and the target limit of 2 °C set by the Paris Agreement can be realized by the RCP 2.6 scenario—always as compared to pre-industrial times [ 10 ]. In addition, the Sea Level Rise Modell K17 is a nonprobabilistic projection that incorporates physical models of ice sheet dynamics [ 18 ]. Furthermore, the applied model data refer to the forecast for the year 2100 and include the local 1-year coastal flood return level [ 19 ]. For our analysis, we chose the K17 model, CoastalDEM, RCP 4.5 for the year 2100.

Readiness and vulnerability index: To assess the differences between the individual countries, the Notre Dame Global Adaptation Initiative (ND-GAIN) developed a country index that provides data on countries’ vulnerability to climate disruption and their readiness to improve resilience by “leverage of private and public sector investments”. The index combines 74 variables to define the ranking for 192 countries [ 27 ].

Visualization of results

The results of the keyword cluster analysis were presented using the VOSviewer software developed by van Eck and Waltman [ 44 ]. The occurrence of keywords was visualized by a network of nodes and connecting lines representing the different colored clusters and their combinations.

The geographical findings of this study were partially visualized by the creation of anamorphic cartograms using Gastner and Newman’s method of density equalizing map projections (DEMP) [ 12 ]. Methodically, these DEMPs reduce or enlarge the country sizes according to the value of the evaluation parameter, following the physical principle of density compensation by diffusion balance in each country. To maintain the basic structure of the world map, mean values are calculated and assigned to oceans and Antarctica. With an ArcGIS tool (mapping software for geographic information systems), which is based on the algorithms of the DEMP method, geographic data can be visualized by generating distorted maps. The DEMPs generated in this way allow a quick visual acquisition of the extensive data and concentration on the essential.

Methodological limitations and strengths

Although being a sophisticated and widely applied method, some limiting points need to be recognized and discussed.

The quality and representativeness of the retrieved metadata depend on the one hand on the technical and bibliographic conditions of the source database and on the other hand on the care taken in generating the search strategy. In this case, WoS was used as a data source. It should be noted that WoS is English biased, as most of the indexed journals are English-language journals. Furthermore, the citation number given is prone to various errors, e.g., incorrect citation behavior or self-citation, so that its significance for the quality of research needs to be discussed. Although the strategy of searching only in the title of the publications resulted in a reduced data quantity, this is justified by the higher representativeness of the data sets. The additional search in the abstracts and keywords would lead to the inclusion of a large number of false entries that would not provide valid figures. Therefore, choosing a title search strategy allows the creation of a valid, albeit not all-encompassing, database.

Some data records had to be corrected manually, e.g., institutions and subject areas. Although the unification of the subject areas in the overall database could be carried out exactly, the merging of different labeled affiliations belonging together was not 100% possible. Therefore, a threshold has been applied in a geographical approach, so that only those geographical entity, e.g., cities, with at least 200 articles on climate change were subject to in-depth corrections. However, the exact number of publishing institutions could not be determined.

The visualization of the results utilizing DEMPs is limited by the physical principles of the technique so that some small island countries could not be represented in the respective figures.

All the results are based on the evaluation database, which consists of a total of 40,062 articles on climate change identified and extracted from WoS.

Research focal points

In total, 45 keywords from three main clusters could be identified (Fig.  2 a). First, articles relating to environmental and ecological issues can be grouped together, with "impacts" being the most commonly used term in the cluster. Secondly, all articles dealing with modeling and simulation can be grouped. In this second keyword cluster, the terms “temperature”, “model”, and “variability” appeared most frequently. Thirdly, all articles on social, political, and management issues can be grouped in one cluster. The umbrella term “climate change” was assigned to this group and is the most frequently used keyword in the analysis. In addition, the terms “adaption” and “vulnerability” have been used most frequently in the third keyword cluster.

Research foci. a Clusters of author’s keywords with at least 650 occurrences. Red: environmental and ecological issues, green: modeling and simulation issues, blue: social and management issues. b Most assigned subject areas according to Web of Science categories with number of articles and average citation rate (number of citations / number of articles)

The main subject areas (WoS research areas) are shown in figure Fig.  2 b with the numbers of articles ( n ) assigned to them and their average citation rates. By far the most assigned subject area was Environmental Science and Ecology ( n  = 15,741). Meteorology and Atmospheric Science ( n  = 6522) followed with less than half of assigned articles. Ranks 3 to 5 were occupied by Geology ( n  = 3806), Water Resources ( n  = 3247), and Science and Technology—Other Topics ( n  = 2916). Apart from ecological issues, the most frequently assigned subject areas ( Business and Economics: n  =  1710 , Government and Law: n  =  1493 , Public Administration: n  =  936 ) focus on economics and political issues, which represent the blue cluster in Fig.  1 a. In principle, the articles are distributed over the three main subject areas clusters that distinguish between scenario modeling, risk analysis, and mitigation, respectively, adaption measurements. From these results, the main foci of climate change research can be identified. In summary, articles on modeling and simulation of scenarios for consequences of climate change under different conditions have been developed resulting in ecological and socioeconomic impacts, which in turn form the basis for mitigation and adaption measures on climate change.

It is noteworthy concerning the average citation rate (cr) of the research areas that the highest rates reached the areas Science and Technology—Other Topics (cr = 59.37), Biodiversity and Conservation (cr = 39.01), Geography (cr = 38.85), and Meteorology and Atmospheric Science (cr = 35.54), while the most assigned area Environmental Science and Ecology achieved an average of only cr = 26.80. Among the ten most frequently assigned subject areas, Public Administration (cr = 13.49) and Government and Law ranked last (cr = 10.69).

Evolution of publication output over time

The vast majority of articles on climate change (92.17%) has been published since the year 2000 ( n  = 36,925) (Fig.  3 ). However, the first publication that meets the search criteria was published as early as 1910. Annual publication numbers remained in single digits until the mid-1970s. Only at the end of the 1980s, the numbers reach yearly amounts above n  = 100. A steep increase in research activity can be observed from 2003 onwards when the trend followed an exponential course, which reached a small peak in 2011 and is still rising exponentially until today (Fig.  3 a). This development can be illustrated even more clearly by looking at the numbers in relation to the absolute number of articles indexed in the Science Citation Index (SCI) (Fig.  2 b). The gradual increase in research interest is also reflected in the steep relative increases in these years, calculated with the annual number of articles on climate change per 10,000 articles listed in the Science Citation Index (SCI) (Fig.  3 b). Until 1988 and between 1992 and 2003, the upward trend of climate change research is similar to that for all articles indexed in the SCI.

Chronological development of articles on climate change from 1970 to 2018. a Number of articles on climate change and their citations. Dashed line: Cited Half-Life. b Number of all indexed SCI articles (Science Citation Index of Web of Science) and number of articles on climate change per 10,000 SCI articles

Analog to the development of the number of articles, the number of citations (c) also increased significantly since 1988, with peaks in the years 1991 ( c  = 10,106), 2000 ( c  = 32,612), 2004 ( c  = 45,177), and the preliminary maximum of c  = 88,747 in 2010. Afterward, the citation numbers dropped again significantly. This is because little time has elapsed since the articles were published to generate citations. This effect is known as Cited Half-Life (CHL) and refers to a period of about eight years for the life sciences, which is needed for the articles to reach half of the total number of citations (Fig.  3 a) [ 21 ].

Among the ten most frequently cited articles in the database, 80% stem can from the USA ( n  = 8) and 20% from the UK ( n  = 2). All of those ten articles were published after 2000, and mainly in the renowned journals Nature ( n  = 5) and Science ( n  = 2) (Tables 1 , 2 ). The publication years 2000, 2003, and 2010 can be logically associated with the research increase shown in Fig.  2 a.

Leading institutions

The 15 most publishing institutions on climate change are located exclusively in the northern hemisphere (Table 2 ). Almost half of the most publishing institutions are US-American (7 institutions), 3 others are British, 2 are Dutch, and 1 institution is located in China, Switzerland, and Germany respectively. The Chinese Academy of Science (CAS) was the most publishing institution on climate change with n  = 1333 articles, followed by the University of London ( n  = 680), which published only half the amount. The US Department of Agriculture (USDA) followed with n  = 588 articles. In 4th place was the British University of Oxford ( n  = 452), followed by the Dutch Wageningen University ( n  = 442) and the US University of Washington ( n  = 426). When looking at the average citation rate of the most publishing institutions, the order is different. With the highest value of almost 100, the US National Center for Atmospheric Research (cr = 98.94) led the ranking, followed by the British University of East Anglia (cr = 89.16), and the US Columbia University (cr = 75.16). The articles of the CAS ranked last among the leading 15 institutions (cr = 21.29).

Global landscape of publication output

Not all articles out of the entire database could be assigned to a country of origin due to missing metadata before 1973. Coming from 186 countries or autonomous regions, n  = 38,917 articles could thus be included in the database and analyzed in terms of geographical parameters.

The most publishing country was the USA with n  = 12,637 articles on climate change, followed by the United Kingdom (UK) with less than half as many articles ( n  = 5524). China was placed 3 rd with n = 3508, followed by Australia ( n  = 3349), Germany ( n  = 3238, and Canada ( n  = 3126) (Fig.  4 a).

The most publishing countries. a Density equalizing map projection of the number of articles. b Relative share of the most publishing countries in 5-year intervals from 1998 to 2019

Looking at the share of the most publishing countries in 5-year intervals (Fig.  4 b), the USA conducted more than 50% of the research on climate change in the first evaluation interval from 1989 to 1994. In the last interval from 2015 to 2019, the share of US articles decreased to 30%, whereas the absolute numbers increased almost tenfold. The relative share of the UK fell also from 20.64% to 12.42% between 1995 and 2019, during which time it lost its second rank to China that contributed an increasing share from 1.23% to 13.27% throughout the whole evaluation period. The share of Australian, German, Spanish, and Indian articles also increased slightly over time, while the shares of Canadian, French, and Netherlandic articles remained more or less the same.

The distribution of the number of citations follows a similar pattern with the exception of China, which here falls to rank 8 ( c  = 66,844). The USA received by far the most citations ( c  = 513,888), followed by the UK ( c  = 243,261), Australia ( c  = 108,054), Canada (c = 107,713), and Germany ( c  = 107,335) (Fig.  5 a).

Citation-specific parameters for articles on climate change. a Number of citations per country. b Articles/Citation rate of articles on climate change per country (threshold 30 articles)

When evaluating the average citation rate (cr) per country with more than 30 articles on climate change (threshold), Costa Rica was in first place (cr = 93.89, n  = 67), followed by Estonia (cr = 55, n  = 66), Iceland (cr = 50.15, n  = 47), Austria (cr = 46.77, n  = 668), and Switzerland (cr = 45.94, n = 1126). The UK ranked 16th (cr = 44.04), the USA 21st (cr = 40.66), Canada 35th (cr = 34.46), Germany 40th (cr = 33.15), and Australia 41st (cr = 32.26) (Fig.  5 b).

Inclusion of socioeconomic parameters

The analysis of socioeconomic parameters of the publishing countries on climate change showed a divergent ranking.

In terms of the inclusion of the countries’ population size [ 39 ] (number of articles/population in million inhabitants =  R POP ) the following order emerged: Norway ( R POP  = 174.16), Australia ( R POP  = 145.65), Denmark ( R POP  = 142.12), Iceland ( R POP  = 139.93), Switzerland ( R POP  = 137.66). The most publishing countries were ranked lower: the USA ranked 20th ( R POP  = 39.00), the UK ranked 13th ( R POP  = 85.73), China ranked 67th ( R POP  = 2.55), and Germany ranked 18th ( R POP  = 40.11) (Fig.  6 a).

Ratio of socio-economic parameters (threshold 30 articles). a Country-specific ratios of the number of articles on climate change and the countries’ population size in million inhabitants [39]. B) Country-specific ratios of the number of articles on climate change and the Gross Domestic Product (GDP) in 1000 billion US-Dollars [38]

In terms of the economic status, the South Pacific island state Fiji led the range of countries with more than 30 articles on climate change (threshold) with a ratio of the numbers of articles and the GDP in billion US-Dollars [ 38 ] ( R GDP ) with R GDP  = 6329.11, followed by Denmark ( R GDP  = 3002.26), New Zealand ( R GDP  = 2991.99), Iceland ( R GDP  = 2910.22), and Australia ( R GDP  = 2816.65) (Fig.  6 b). It was also surprising that the African country Zimbabwe was placed among the top ten countries and reached 7th place ( R GDP  = 2541.48). In terms of socioeconomic analysis, other developing countries such as Nepal and some African countries (Kenya, Benin) achieved also ranks among the leading 20 countries.

Besides Australia, the UK reached the second highest ratio of the most publishing countries and ranked 11th, Canada ranked 13th, Germany 30th. The USA was only in 37th position.

The inclusion of science-related parameters [ 40 ]. listed New Zealand first (Table 3 ) with R GERD (number of articles/gross expenditure for research and development in current PPP (purchasing power parity) US dollars) = 244.34. Australia ranked 2nd ( R GERD  = 157.98), followed by Norway ( R GERD  = 133.77), South Africa ( R GERD  = 120.36), and UK ( R GERD  = 115.54). Germany only achieved rank 22nd ( R GERD  = 25.47), and the USA ranked 23rd ( R GERD  = 23.26).

New Zealand published also the highest number of articles per researcher (full-time equivalent FTE/1000) with R RES  = 27.92, followed by Norway, South Africa, and Switzerland. Here the USA ranked 16th ( R RES  = 9.22) and Germany 19th ( R RES  = 7.83). Unfortunately, the data for Australia was not available.

Inclusion of climate change indices

Carbon dioxide emission.

The linkage of country-specific number of publications on climate change with the countries’ CO 2 emission shown in Table 4 discloses Sweden as the leading country ( R CO2  = 29.28), followed by Switzerland ( R CO2  = 28.10), Denmark ( R CO2  = 23.01), Norway ( R CO2  = 20.47) and New Zealand ( R CO2  = 14.52). In this analysis, the most publishing countries fell sharply behind. UK ranked 6 th ( R CO2  = 14.36), Germany 17th ( R CO2  = 4.05), and the USA 19th ( R CO2  = 14.52).

Global climate risk index

For reasons of comparison, reference is made here to the results of the Global Climate Risk Index (CRI) [ 9 ]: The average ranking shows Puerto Rico, Myanmar, and Haiti as the most affected countries, while the assessment for 2018 ranked Japan, the Philippines, and Germany as the most affected countries [ 9 ]. The figures for the average number of deaths per 100,000 inhabitants from 1999 to 2018 as a reference point put some small island developing states (SIDS), such as St. Kitts and Nevis, Tuvalu, Kiribati, Seychelles, Marshall Islands, and the Maldives in the first place. Armenia, Iceland, Singapore, and Qatar also held leading positions. (Fig.  7 a).

Global Climate Risk Index (1999–2018) [ 9 ]. a Average number of fatalities per 100,000 inhabitants. b Number of articles on climate change pro average number of fatalities per 100,000 inhabitants

The linkage of the number of publications on climate change to the expected increase in extreme events due to climate change discloses France as the leading country ( R CRI  = 215.50), followed by the USA ( R CRI  = 162.01), Spain ( R CRI  = 145.80), Italy ( R CRI  = 144.44), Germany ( R CRI  = 140.78), and UK ( R CRI  = 215.01). Of the countries most affected by climate change, Myanmar ranked 17th ( R CRI  = 12.00), followed by Japan on rank 18 ( R CRI  = 11.89), and the Philippines on rank 20 ( R CRI  = 8.19) (Fig.  7 b). There was no correlation between the number of articles and the average number of fatalities per 100,000 inhabitants on average.

Sea-level rise

For reasons of comparison, reference is made here to the results of Kulp and Strauss [ 19 ]: According to their findings of the working group, China is by far the country with the highest number of people living on vulnerable land in million according to the CoastalDEM scenario (we here label it: P vul  = 151.6) (Fig.  8 a). With P vul  = 73, Bangladesh’s population is the second most affected, followed by India ( P vul  = 151.6), Vietnam, and Indonesia ( P vul  = 151.6). In addition to these absolute figures, the working group of Kulp and Strauss [ 19 ] put the number of affected people in relation to the total population. This results in a different picture (Fig.  8 b), with the small island states (Maldives, Marshall Islands, Tokelau, and Tuvalu) most affected, where more than 70% of the population will live on vulnerable land in 2100. In the South-American countries of Suriname and Guyana, more than 60% will live on vulnerable land, followed by Kiribati, Cayman Islands, and the Bahamas with more than 50% affected people. The Netherlands is the first European country in the ranking, where 55% of the inhabitants will be exposed to vulnerable land. The here determined most publishing countries on climate change, were following far behind: USA (2.3%), UK (9%), China (11%), Australia (4%), and Germany (2.5%) [ 19 ].

Estimated number of people exposed to vulnerable land in 2100 (CoastalDEM scenario: Sea Level Rise Modell K17, RCP 4.5, 95 percentile) [ 19 ]. a Number of people living on vulnerable land in mill. b Relative number of people (per 1000 inhabitants) living on vulnerable land. c Relation of the number of articles on climate change and the number of people living on vulnerable land in mill. High values of SIDS (Small Island Developing States) cannot be shown. The highest values have Maldives (87%), Marshall Island (85%), Tokelau (78%), Tuvalu (73%). d Relation of the number of articles on climate change and the relative number of people (per 1000 inhabitants) living on vulnerable land in mill

Here we have calculated the ratio of countries’ publication performance on climate change in relation to Kulp et al.’s absolute ( R absolute ) and relative figures ( R relative ) of Kulp and Strauss [ 19 ] (Fig.  7 c, d). In terms of the relation of articles on climate change to the absolute numbers, Sweden was leading ( R absolute  = 15,187), followed by Canada ( R absolute  = 4597), Romania ( R absolute  = 4366), Australia ( R absolute  = 3940), South Africa ( R absolute  = 3054), and Lithuania ( R absolute  = 3050). The most publishing countries ranked as follows: The USA ranked 11th ( R absolute  = 1805), Germany 14th ( R absolute  = 1619), and UK 17th ( R absolute  = 986), while China followed far behind on rank 88 ( R absolute  = 23).

The analysis of the relative ratios led to the following ranking: Finland ( R relative  = 2123), USA ( R relative  = 549), Russia ( R relative  = 157), South Africa ( R relative  = 150), Canada ( R relative  = 149). In terms of most publishing countries, Germany was ranked 8th ( R relative  = 130), UK 18th ( R relative  = 61), and China 25th ( R relative  = 32).

A significant correlation could be shown between the absolute numbers of people living on vulnerable land and the number of articles ( p  < 0.001), while the relative numbers did not correlate with the number of articles ( p  < 0.53).

Vulnerability and readiness

Correlation analysis of the two ND-GAIN indices (readiness and vulnerability) of 2017 and the number of articles were both significant ( p  < 0.0001), but with different slopes. The correlation of the readiness index and the number of articles was significantly positive (Fig.  9 a), and the correlation of the vulnerability index and the number of articles was significantly negatively correlated ( p  < 0.001) (Fig.  9 b).

Correlation of the number of articles and indices of the ND-GAIN 2017 (Notre Dame Global Adaption Initiative) [27] regarding countries. a Readiness index, positive correlation ( p  < 0.001). b Vulnerability index, negative correlation ( p  < 0.001)

International networking

A total of n  = 11,626 (29%) international cooperation articles were identified. Of these, n  = 7995 were bilateral and n  = 3165 trilateral collaborations, respectively. Four articles were worked out with at least 20 collaboration countries.

The first international cooperation in our database was published in 1975. Over time, the number of international partnerships increased exponentially, similar to the total number of articles, until it reached its maximum in 2014 with n  = 1425 international collaboration articles.

The USA as core country of the international networking participated in the 5 strongest partnerships (Fig.  10 ): USA/UK ( n  = 905), USA/China ( n  = 830), USA/Canada ( n  = 722), USA/Australia ( n  = 563), and USA/Germany ( n  = 534). Of the US articles, 37% were international collaborations, while more than half of the British articles and almost half of the Canadian and Australian articles were developed in international collaboration. Germany even conducted more than 60% of its studies with another country.

Network of internationally co-authored articles on climate change with numbers in brackets (number of articles/number of cooperation articles). The width of connecting lines represents the quantity of common articles (threshold: 40 collaboration articles between countries)

Progress of publications on climate change

The first article on climate change identified by our approach was published as early as 1910. It is an article published in Nature and asked the question of whether the Indian climate changed [ 20 ]. This early publication already addressed the causal link between climate change and anthropogenic influence. The author asked whether there are causal links of increased irrigation and forest loss, also in comparison to statements by Gilbert Walker, the General Director of Indian Observatories, who made connections between the air pressure in South America and the intensity of Monsoon in India, thus negating links between climate change in India and human interference.

In 1947, an English article raised the question of whether there was a connection between the retreat of glaciers and climate change [ 4 ].

In 1956, the carbon dioxide theory was confirmed by a US-American article, which referred to a series of articles published as early as the end of the nineteenth century [ 31 ]. The authors of these articles formulated the carbon dioxide theory and thus provided the most widely accepted explanation for the climate change already recognized at that time. However, this was later denied until it turned out to be true. In his study, Gilbert N. Plass from John Hopkins University has already seen the impact of human activities on the CO 2 balance through the combustion of fossil fuels, deforestation, and land management. In contrast to today’s threat awareness, the problem he discussed was the risk of new glacial formation due to the decrease of CO 2 caused by a changed balance in the atmosphere–ocean system [ 31 ].

A German article from 1961 also argues that "man-made effects on climate change “should not be underestimated as well as "the danger that such effects will work irreversibly against human benefit” [ 11 ].

A study on the astronomical theory , also known as the Milankovitch hypothesis of climate change, raised in 1969 the problem awareness of the scientific world with its Barbados data [ 23 ].

In 1988, the Intergovernmental Panel on Climate Change (IPCC), an intergovernmental body of the United Nations (UN), was established at the first world climate conference in Geneva with the aim to provide a wide range of scientific information on climate change to support governmental decisions [ 16 ]. As of this first world climate conference, the number of publications has risen firstly to a three-digit figure, which can also be seen from the sharp increase in relative numbers per 10,000 SCI articles.

Thereafter, the numbers increased steadily until 2003, when an exponential increase could be observed that was also reflected by the steep rise in relative numbers. At the COP in Milan (Italy) in 2003, all parties agreed to the Adaption Fund, which was primarily founded to support developing countries in their capacity to respond to the consequences of climate change. In the same year, an enormous heatwave caused many thousands of deaths in Europe [ 37 ]. Since European countries, in particular, are among the most publishing nations, this regional climate catastrophe has certainly contributed to a strong increase in research interest on climate change.

Also, in 2003, the most cited article of this study was published. By analyzing more than 1700 species, the meta-analysis of C. Parmesan and G. Yohe shows that biological trends are in line with predictions of climate change [ 30 ]. This successful publication certainly contributed to the fact that the highest average citation rate per year was achieved in 2003, initiating an exponential growth in publication output.

The citation numbers increased adequately to the publication numbers with some outstanding years, e.g., 1991, 2000, 2004, and 2010, latter the year with the highest number of citations so far. Many of the high impact articles are published in these years so that an association can be assumed.

Geographical aspects of publications on climate change

The USA, the UK, China, Australia, and Germany could be identified as the most publishing countries on climate change. This is not surprising, as it shows that mostly scientifically well-structured countries conduct most of the research, not only on climate change issues, as previous studies also have shown [ 17 ]. China, in particular, was catching up in the recent years due to its targeted research policy, which is represented by the enormous increase in expenditures on R&D [ 28 ].

The USA government, which is the most publishing country on climate change so far, is not exactly famous for its climate change–conscious attitude. The rejection of binding targets and the denial to sign the Paris Agreement confirms this. The USA is still the country with the highest expenditures on R&D and certainly one of the most preferred places to work for the most renowned scientists in the world. Despite the government’s attitude, its leading position in terms of publication output is not unique for climate change research and certainly not astonishing.

The results also show a clear dominance of European countries in the publication numbers on climate change. Also, Europe has a very good scientific infrastructure at its disposal. In contrast, most European countries signed up to the binding targets of the Paris Agreement to reduce emissions by at least 40% by 2030 compared to 1990 [ 10 ]. Denmark event targeted for a 70% reduction [ 7 ].

To evaluate the scientific landscape on climate change in greater depth, we extended the analyses to other, more differentiated parameters.

The Scandinavian countries have to be highlighted due to their leading position concerning various additional evaluation parameters, e.g., socioeconomic ratios. In general, Scandinavian countries have established good conditions for researchers and spend a lot on R&D. This is why research on climate change has also proven to be no exception. Sweden and Norway were leading in the analysis of their publication numbers in terms of national CO 2 emission, with Switzerland in between in 2nd place. This parameter had been chosen for the analysis in order to establish a link with countries’ obligations under the polluter-pays principle. In 2017, the highest emissions rates were released by China, USA, India, Russia, Japan, Germany, Iran, and Saudi Arabia. Sweden ranks first when putting the number of published articles in relation to the emission rate (threshold 300 articles), followed by Switzerland, Denmark, Norway, and New Zealand.

The Scandinavian countries are known to be “early adopters of renewable energy”. The share of renewable energy in Iceland is 77%, in Sweden 63%, in Norway 51% (despite the oil production capacity), in Finland 47%, and in Denmark 33%, in contrast to the EU28 with a proportion of only 21% [ 26 ]. Also, in terms of the relation to the number of people living on exposed land to sea-level rise, Sweden led in the evaluation of absolute numbers and Finland of relative numbers. With 1215 articles, Sweden ranked 12th regarding its absolute publication numbers, Norway 15th, Denmark 16th, and Finland 20th.

Switzerland, which is ranked second in terms of inclusion of CO 2 emission, is affected to a considerable extent by climate change due to its location in the European Alps and the progressive melting of glaciers and permafrost. Especially since tourism—above all skiing—is an important economic sector. Therefore, it is not astonishing that the focus of Swiss research is mainly on problems related to the Alpine region [ 6 ].

In terms of science-related parameters, such as GERD or number of researchers, both Australia and New Zealand came into focus. Since they are located close to each other, the intensity of cooperation in climate change research is understandable. The location near Antarctica on the one hand and the immense heatwaves with extraordinary effects on ecosystems and biosphere on the other hand form the background for relatively high investments in climate change research.

Looking at the average citation rate of the publishing countries ( n  ≥ 30), Costa Rica occupied a prominent position. With 67 articles, Costa Rica is far behind in absolute terms. Nevertheless, these articles were cited 6291 times. Nearly half of the studies of Costa Rica are worked in collaboration with the USA. The Tropical Science Center (TSC) affiliated with the Monteverde Cloud Forest Biological Reserve in Costa Rica participated in a US-American and Costa Rican collaboration, with the 5th most cited article of this analysis, that deals with the impacts of climate change on wildlife [ 34 ]. The Center is also taking part in two other high-profile publications, which, like the most cited article, are also published in Nature . They all deal with the risk of extinction caused by global warming. Alan Pounds, biological scientist since 1996 at the TSC and focusing on the biological impact of climate change, found, e.g., a correlation between amphibian die-offs and rising average temperatures [ 32 ]. Previously, he worked at the Department of Zoology, University of Florida, USA, where he already collaborated with colleagues from Costa Rica. The pattern of the successful partnership of international networks can be seen in this example, which stands for mutual benefit for both cooperating countries.

In terms of citation rates, Estonia also took a leading position, as it is part of a Europe-wide meta-analysis on changes in phenology using data on more than 125,000 observational series of plants and animals to assess their response to climate change [ 22 ]. The resulting article, which was published in 2006 in Global Change Biology, received almost half of the Estonian citations.

The third country that should be highlighted regarding the citation rate of its articles is Iceland. Its articles were not counted among the high-impact publications. Instead, many of its articles achieved recognition with above-average citation rates. The location far in the north, close to Greenland and the Arctic Circle, is an advantage for all climate change projects that focus on melting glaciers in these regions, and the glacial retreat is here more and more evident. It has been assumed that all Islandic glaciers will be disappeared by the year 2200 [ 26 ]. Therefore, the most cited Icelandic article is the result of an international collaboration focusing on the regional differences in the last glacial period to better understand climate dynamics [ 3 ].

The comparison of the countries’ results in relation to the GDP put the insular state of Fiji, which consists of more than 300 islands, at the top of the evaluation. Currently, almost one million people are living in an area of about 18,000 square kilometers north of New Zealand in the South Pacific. Fiji has been selected to chair the 23rd climate summit 2017 in Germany. In the same year, the number of articles from Fiji reached its maximum, which seems to be associated. Nearly half of the articles are collaboration works with Australia. One of the advantages of research cooperation on climate change is the existence of Fiji’s coral reefs, their vulnerability, and their importance for coastal protection.

Worthy to note is also the rank of Denmark in terms of socioeconomic influence. In addition to Denmark’s otherwise equally good scientific infrastructure, its position in climate change research is certainly influenced by Greenland’s affiliation and the direct and immediate effects of climate change in this region located closest to the Arctic. The direct association to Greenland or the Arctic can be found in more than 200 Danish articles mostly focusing on Geoscience . The Niels Bohr Institute at the University of Copenhagen is leading in the climate change research based on ice cores. The ice core collection is considered as a “national treasure” and contains a deep drill core of more than 15 km in length [ 43 ].

In connection with the socioeconomic analysis, it is also remarkable that the African developing country Zimbabwe ranked 7th among the top 10. Unlike other African countries, it is relatively industrialized and produces twice the average amount of greenhouse gases [ 5 ]. Nevertheless, Zimbabwe—like other African countries—has to cope with droughts, freshwater and food shortages, diminished biodiversity, vector-borne diseases, and dry ups as a result of climate change. Zimbabwe was among the first countries to sign and ratify the UN Framework Convention on Climate Change (UNFCCC) in 1992 [ 29 , 41 ]. In 2011, it participated in the REDD+ program ( Reducing Emissions from Deforestation and Forest Degradation ), which aims to avoid 52 million tons of CO 2 over 30 years in Zimbabwe and in return to support the communities with financial aid for agriculture, fire prevention, and production methods to preserve forest areas [ 5 ].

France, which ranks 7th in terms of absolute publication numbers, led when the ratio between publication numbers and fatalities due to climate events of the CRI index is assessed. More than half of the articles on climate change are worked out with the participation of the French state research organization Centre National de la Recherche Scientifique (CNRS). The CNRS was ranked 4th by the Nature Index in 2017 regarding the largest contributors, behind the CAS (in this study identified as most publishing institute on climate change), Harvard University USA, and Max Planck Society Germany [ 25 ]. Plus, the majority of its articles is worked out as international collaboration (66.59%). The share of the other most publishing countries is considerably lower: USA (36.98%), UK (51.90%), China (48.97%), Australia (47.27%) Germany (61.10%), Canada (46.77%).

Nevertheless, the share of collaboration articles is relatively high in comparison to other research fields. This may be due to the majority of articles published after 2000, considering that the share of collaboration articles generally increases over time due to the international awareness of its benefits [ 1 ].

Articles on climate change focused on three main thematic groups, leading from the modeling of future scenarios to the environmental and socioeconomic impacts and the corresponding mitigation and adaptation measures. The readiness of countries and their vulnerability are inversely related to the number of articles published on climate change. Our results show the dominance of the Northern hemisphere in terms of publication output on climate change. Taking into account socioeconomic, research, and climate-specific characteristics, the order of the leading countries shifts, but the main actors remain the same with only a few exceptions. Only Costa Rica, Fiji, and Zimbabwe as developing countries came to play a role in the evaluation of the results. In principle, Africa, Asia, and South America are extremely under-represented. Many scientists are becoming aware of the advantages of international networking, which is of mutual benefit to all participating countries. However, particularly regarding climate change research, these benefits should be more frequently shared with developing countries, as the involvement of these most affected nations still is sparse.

In this context, the term “equity” is certainly familiar to all those interested in climate change research. There is a heated debate in the scientific community on whether scientific cooperation with developing countries should be called for. Many researchers see this as limiting the freedom of research. However, the principle of research responsibility should also be taken into account in this context. This should or must lead to a global risk-indexed joint planning because all scientists have only one planet to take care of. In this context, Prof. Drenth, Emeritus, Psychometrics and Organizational Psychology, Free University Amsterdam [ 8 ], asked the following questions for any scientist dealing with climate change: “Risks for whom? How far does the right to know go? What is the balance between self-determination and the interests of larger groups or the society as a whole? How certain does the scientist have to be before warning, especially in the case of irreversible developments?”.

The spread and economic impact of the current COVID 19 pandemic has reduced public and media interest in climate change issues. All the more reason to urgently press for the causes and consequences of climate change to once again become the focus of interest, while at the same time dealing with the consequences of the pandemic. Climate change must continue to be recognized as one of the most urgent global challenges. This makes it necessary to reconcile future scientific direction with the long-term environmental, social and economic consequences of the impacts of climate change that all countries are facing.

Availability of data and materials

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DK, DAG contributed to the development of the methodological platform. DK concepted, designed, drafted the initial manuscript, carried out the literature search and the analyses. DAG contributed to conception, design, and analyses. RM, MB, DB contributed to the literature search, interpretation, design of results. RM, MB corrected the draft. RM, MB, DB contributed with important expert content. All authors agree to be accountable for all aspects of the work. All authors read and approved the final manuscript.

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Klingelhöfer, D., Müller, R., Braun, M. et al. Climate change: Does international research fulfill global demands and necessities?. Environ Sci Eur 32 , 137 (2020). https://doi.org/10.1186/s12302-020-00419-1

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• Global warming
• Greenhouse effect
• Bibliometrics
• Socioeconomic indices
• Climate inequity
• Research investment

Climate Change: Evidence and Causes: Update 2020 (2020)

Chapter: conclusion, c onclusion.

This document explains that there are well-understood physical mechanisms by which changes in the amounts of greenhouse gases cause climate changes. It discusses the evidence that the concentrations of these gases in the atmosphere have increased and are still increasing rapidly, that climate change is occurring, and that most of the recent change is almost certainly due to emissions of greenhouse gases caused by human activities. Further climate change is inevitable; if emissions of greenhouse gases continue unabated, future changes will substantially exceed those that have occurred so far. There remains a range of estimates of the magnitude and regional expression of future change, but increases in the extremes of climate that can adversely affect natural ecosystems and human activities and infrastructure are expected.

Citizens and governments can choose among several options (or a mixture of those options) in response to this information: they can change their pattern of energy production and usage in order to limit emissions of greenhouse gases and hence the magnitude of climate changes; they can wait for changes to occur and accept the losses, damage, and suffering that arise; they can adapt to actual and expected changes as much as possible; or they can seek as yet unproven “geoengineering” solutions to counteract some of the climate changes that would otherwise occur. Each of these options has risks, attractions and costs, and what is actually done may be a mixture of these different options. Different nations and communities will vary in their vulnerability and their capacity to adapt. There is an important debate to be had about choices among these options, to decide what is best for each group or nation, and most importantly for the global population as a whole. The options have to be discussed at a global scale because in many cases those communities that are most vulnerable control few of the emissions, either past or future. Our description of the science of climate change, with both its facts and its uncertainties, is offered as a basis to inform that policy debate.

A CKNOWLEDGEMENTS

The following individuals served as the primary writing team for the 2014 and 2020 editions of this document:

• Eric Wolff FRS, (UK lead), University of Cambridge
• Inez Fung (NAS, US lead), University of California, Berkeley
• Brian Hoskins FRS, Grantham Institute for Climate Change
• John F.B. Mitchell FRS, UK Met Office
• Tim Palmer FRS, University of Oxford
• Benjamin Santer (NAS), Lawrence Livermore National Laboratory
• John Shepherd FRS, University of Southampton
• Keith Shine FRS, University of Reading.
• Susan Solomon (NAS), Massachusetts Institute of Technology
• Kevin Trenberth, National Center for Atmospheric Research
• John Walsh, University of Alaska, Fairbanks
• Don Wuebbles, University of Illinois

Staff support for the 2020 revision was provided by Richard Walker, Amanda Purcell, Nancy Huddleston, and Michael Hudson. We offer special thanks to Rebecca Lindsey and NOAA Climate.gov for providing data and figure updates.

The following individuals served as reviewers of the 2014 document in accordance with procedures approved by the Royal Society and the National Academy of Sciences:

• Richard Alley (NAS), Department of Geosciences, Pennsylvania State University
• Alec Broers FRS, Former President of the Royal Academy of Engineering
• Harry Elderfield FRS, Department of Earth Sciences, University of Cambridge
• Joanna Haigh FRS, Professor of Atmospheric Physics, Imperial College London
• Isaac Held (NAS), NOAA Geophysical Fluid Dynamics Laboratory
• John Kutzbach (NAS), Center for Climatic Research, University of Wisconsin
• Jerry Meehl, Senior Scientist, National Center for Atmospheric Research
• John Pendry FRS, Imperial College London
• John Pyle FRS, Department of Chemistry, University of Cambridge
• Gavin Schmidt, NASA Goddard Space Flight Center
• Emily Shuckburgh, British Antarctic Survey
• Gabrielle Walker, Journalist
• Andrew Watson FRS, University of East Anglia

The Support for the 2014 Edition was provided by NAS Endowment Funds. We offer sincere thanks to the Ralph J. and Carol M. Cicerone Endowment for NAS Missions for supporting the production of this 2020 Edition.

F OR FURTHER READING

For more detailed discussion of the topics addressed in this document (including references to the underlying original research), see:

• Intergovernmental Panel on Climate Change (IPCC), 2019: Special Report on the Ocean and Cryosphere in a Changing Climate [ https://www.ipcc.ch/srocc ]
• National Academies of Sciences, Engineering, and Medicine (NASEM), 2019: Negative Emissions Technologies and Reliable Sequestration: A Research Agenda [ https://www.nap.edu/catalog/25259 ]
• Royal Society, 2018: Greenhouse gas removal [ https://raeng.org.uk/greenhousegasremoval ]
• U.S. Global Change Research Program (USGCRP), 2018: Fourth National Climate Assessment Volume II: Impacts, Risks, and Adaptation in the United States [ https://nca2018.globalchange.gov ]
• IPCC, 2018: Global Warming of 1.5°C [ https://www.ipcc.ch/sr15 ]
• USGCRP, 2017: Fourth National Climate Assessment Volume I: Climate Science Special Reports [ https://science2017.globalchange.gov ]
• NASEM, 2016: Attribution of Extreme Weather Events in the Context of Climate Change [ https://www.nap.edu/catalog/21852 ]
• IPCC, 2013: Fifth Assessment Report (AR5) Working Group 1. Climate Change 2013: The Physical Science Basis [ https://www.ipcc.ch/report/ar5/wg1 ]
• NRC, 2013: Abrupt Impacts of Climate Change: Anticipating Surprises [ https://www.nap.edu/catalog/18373 ]
• NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia [ https://www.nap.edu/catalog/12877 ]
• Royal Society 2010: Climate Change: A Summary of the Science [ https://royalsociety.org/topics-policy/publications/2010/climate-change-summary-science ]
• NRC, 2010: America’s Climate Choices: Advancing the Science of Climate Change [ https://www.nap.edu/catalog/12782 ]

Much of the original data underlying the scientific findings discussed here are available at:

• https://data.ucar.edu/
• https://climatedataguide.ucar.edu
• https://iridl.ldeo.columbia.edu
• https://ess-dive.lbl.gov/
• https://www.ncdc.noaa.gov/
• https://www.esrl.noaa.gov/gmd/ccgg/trends/
• http://scrippsco2.ucsd.edu
• http://hahana.soest.hawaii.edu/hot/

Climate change is one of the defining issues of our time. It is now more certain than ever, based on many lines of evidence, that humans are changing Earth's climate. The Royal Society and the US National Academy of Sciences, with their similar missions to promote the use of science to benefit society and to inform critical policy debates, produced the original Climate Change: Evidence and Causes in 2014. It was written and reviewed by a UK-US team of leading climate scientists. This new edition, prepared by the same author team, has been updated with the most recent climate data and scientific analyses, all of which reinforce our understanding of human-caused climate change.

Scientific information is a vital component for society to make informed decisions about how to reduce the magnitude of climate change and how to adapt to its impacts. This booklet serves as a key reference document for decision makers, policy makers, educators, and others seeking authoritative answers about the current state of climate-change science.

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