thesis of global warming

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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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• U.S. Department of Transportation - Global Warming: A Science Overview
• NOAA Climate.gov - Climate Change: Global Temperature
• Natural Resources Defense Council - Global Warming 101
• American Institute of Physics - The discovery of global warming
• LiveScience - Causes of Global Warming
• global warming - Children's Encyclopedia (Ages 8-11)
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Human activity affects global surface temperatures by changing Earth ’s radiative balance—the “give and take” between what comes in during the day and what Earth emits at night. Increases in greenhouse gases —i.e., trace gases such as carbon dioxide and methane that absorb heat energy emitted from Earth’s surface and reradiate it back—generated by industry and transportation cause the atmosphere to retain more heat, which increases temperatures and alters precipitation patterns.

Global warming, the phenomenon of increasing average air temperatures near Earth’s surface over the past one to two centuries, happens mostly in the troposphere , the lowest level of the atmosphere, which extends from Earth’s surface up to a height of 6–11 miles. This layer contains most of Earth’s clouds and is where living things and their habitats and weather primarily occur.

Continued global warming is expected to impact everything from energy use to water availability to crop productivity throughout the world. Poor countries and communities with limited abilities to adapt to these changes are expected to suffer disproportionately. Global warming is already being associated with increases in the incidence of severe and extreme weather, heavy flooding , and wildfires —phenomena that threaten homes, dams, transportation networks, and other facets of human infrastructure. Learn more about how the IPCC’s Sixth Assessment Report, released in 2021, describes the social impacts of global warming.

Polar bears live in the Arctic , where they use the region’s ice floes as they hunt seals and other marine mammals . Temperature increases related to global warming have been the most pronounced at the poles, where they often make the difference between frozen and melted ice. Polar bears rely on small gaps in the ice to hunt their prey. As these gaps widen because of continued melting, prey capture has become more challenging for these animals.

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global warming , the phenomenon of increasing average air temperatures near the surface of Earth over the past one to two centuries. Climate scientists have since the mid-20th century gathered detailed observations of various weather phenomena (such as temperatures, precipitation , and storms) and of related influences on climate (such as ocean currents and the atmosphere’s chemical composition). These data indicate that Earth’s climate has changed over almost every conceivable timescale since the beginning of geologic time and that human activities since at least the beginning of the Industrial Revolution have a growing influence over the pace and extent of present-day climate change .

Giving voice to a growing conviction of most of the scientific community , the Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP). The IPCC’s Sixth Assessment Report (AR6), published in 2021, noted that the best estimate of the increase in global average surface temperature between 1850 and 2019 was 1.07 °C (1.9 °F). An IPCC special report produced in 2018 noted that human beings and their activities have been responsible for a worldwide average temperature increase between 0.8 and 1.2 °C (1.4 and 2.2 °F) since preindustrial times, and most of the warming over the second half of the 20th century could be attributed to human activities.

AR6 produced a series of global climate predictions based on modeling five greenhouse gas emission scenarios that accounted for future emissions, mitigation (severity reduction) measures, and uncertainties in the model projections. Some of the main uncertainties include the precise role of feedback processes and the impacts of industrial pollutants known as aerosols , which may offset some warming. The lowest-emissions scenario, which assumed steep cuts in greenhouse gas emissions beginning in 2015, predicted that the global mean surface temperature would increase between 1.0 and 1.8 °C (1.8 and 3.2 °F) by 2100 relative to the 1850–1900 average. This range stood in stark contrast to the highest-emissions scenario, which predicted that the mean surface temperature would rise between 3.3 and 5.7 °C (5.9 and 10.2 °F) by 2100 based on the assumption that greenhouse gas emissions would continue to increase throughout the 21st century. The intermediate-emissions scenario, which assumed that emissions would stabilize by 2050 before declining gradually, projected an increase of between 2.1 and 3.5 °C (3.8 and 6.3 °F) by 2100.

Many climate scientists agree that significant societal, economic, and ecological damage would result if the global average temperature rose by more than 2 °C (3.6 °F) in such a short time. Such damage would include increased extinction of many plant and animal species, shifts in patterns of agriculture , and rising sea levels. By 2015 all but a few national governments had begun the process of instituting carbon reduction plans as part of the Paris Agreement , a treaty designed to help countries keep global warming to 1.5 °C (2.7 °F) above preindustrial levels in order to avoid the worst of the predicted effects. Whereas authors of the 2018 special report noted that should carbon emissions continue at their present rate, the increase in average near-surface air temperature would reach 1.5 °C sometime between 2030 and 2052, authors of the AR6 report suggested that this threshold would be reached by 2041 at the latest.

The AR6 report also noted that the global average sea level had risen by some 20 cm (7.9 inches) between 1901 and 2018 and that sea level rose faster in the second half of the 20th century than in the first half. It also predicted, again depending on a wide range of scenarios, that the global average sea level would rise by different amounts by 2100 relative to the 1995–2014 average. Under the report’s lowest-emission scenario, sea level would rise by 28–55 cm (11–21.7 inches), whereas, under the intermediate emissions scenario, sea level would rise by 44–76 cm (17.3–29.9 inches). The highest-emissions scenario suggested that sea level would rise by 63–101 cm (24.8–39.8 inches) by 2100.

The scenarios referred to above depend mainly on future concentrations of certain trace gases, called greenhouse gases , that have been injected into the lower atmosphere in increasing amounts through the burning of fossil fuels for industry, transportation , and residential uses. Modern global warming is the result of an increase in magnitude of the so-called greenhouse effect , a warming of Earth’s surface and lower atmosphere caused by the presence of water vapour , carbon dioxide , methane , nitrous oxides , and other greenhouse gases. In 2014 the IPCC first reported that concentrations of carbon dioxide, methane, and nitrous oxides in the atmosphere surpassed those found in ice cores dating back 800,000 years.

Of all these gases, carbon dioxide is the most important, both for its role in the greenhouse effect and for its role in the human economy. It has been estimated that, at the beginning of the industrial age in the mid-18th century, carbon dioxide concentrations in the atmosphere were roughly 280 parts per million (ppm). By the end of 2022 they had risen to 419 ppm, and, if fossil fuels continue to be burned at current rates, they are projected to reach 550 ppm by the mid-21st century—essentially, a doubling of carbon dioxide concentrations in 300 years.

A vigorous debate is in progress over the extent and seriousness of rising surface temperatures, the effects of past and future warming on human life, and the need for action to reduce future warming and deal with its consequences. This article provides an overview of the scientific background related to the subject of global warming. It considers the causes of rising near-surface air temperatures, the influencing factors, the process of climate research and forecasting, and the possible ecological and social impacts of rising temperatures. For an overview of the public policy developments related to global warming occurring since the mid-20th century, see global warming policy . For a detailed description of Earth’s climate, its processes, and the responses of living things to its changing nature, see climate . For additional background on how Earth’s climate has changed throughout geologic time , see climatic variation and change . For a full description of Earth’s gaseous envelope, within which climate change and global warming occur, see atmosphere .

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Essay on Global Warming

• Updated on  
• Apr 27, 2024

Being able to write an essay is an integral part of mastering any language. Essays form an integral part of many academic and scholastic exams like the SAT, and UPSC amongst many others. It is a crucial evaluative part of English proficiency tests as well like IELTS, TOEFL, etc. Major essays are meant to emphasize public issues of concern that can have significant consequences on the world. To understand the concept of Global Warming and its causes and effects, we must first examine the many factors that influence the planet’s temperature and what this implies for the world’s future. Here’s an unbiased look at the essay on Global Warming and other essential related topics.

Short Essay on Global Warming and Climate Change?

Since the industrial and scientific revolutions, Earth’s resources have been gradually depleted. Furthermore, the start of the world’s population’s exponential expansion is particularly hard on the environment. Simply put, as the population’s need for consumption grows, so does the use of natural resources , as well as the waste generated by that consumption.

Climate change has been one of the most significant long-term consequences of this. Climate change is more than just the rise or fall of global temperatures; it also affects rain cycles, wind patterns, cyclone frequencies, sea levels, and other factors. It has an impact on all major life groupings on the planet.

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What is Global Warming?

Global warming is the unusually rapid increase in Earth’s average surface temperature over the past century, primarily due to the greenhouse gases released by people burning fossil fuels . The greenhouse gases consist of methane, nitrous oxide, ozone, carbon dioxide, water vapour, and chlorofluorocarbons. The weather prediction has been becoming more complex with every passing year, with seasons more indistinguishable, and the general temperatures hotter.

The number of hurricanes, cyclones, droughts, floods, etc., has risen steadily since the onset of the 21st century. The supervillain behind all these changes is Global Warming. The name is quite self-explanatory; it means the rise in the temperature of the Earth.

Also Read: What is a Natural Disaster?

What are the Causes of Global Warming?

According to recent studies, many scientists believe the following are the primary four causes of global warming:

• Deforestation 
• Greenhouse emissions
• Carbon emissions per capita

Extreme global warming is causing natural disasters , which can be seen all around us. One of the causes of global warming is the extreme release of greenhouse gases that become trapped on the earth’s surface, causing the temperature to rise. Similarly, volcanoes contribute to global warming by spewing excessive CO2 into the atmosphere.

The increase in population is one of the major causes of Global Warming. This increase in population also leads to increased air pollution . Automobiles emit a lot of CO2, which remains in the atmosphere. This increase in population is also causing deforestation, which contributes to global warming.

The earth’s surface emits energy into the atmosphere in the form of heat, keeping the balance with the incoming energy. Global warming depletes the ozone layer, bringing about the end of the world. There is a clear indication that increased global warming will result in the extinction of all life on Earth’s surface.

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Solutions for Global Warming

Of course, industries and multinational conglomerates emit more carbon than the average citizen. Nonetheless, activism and community effort are the only viable ways to slow the worsening effects of global warming. Furthermore, at the state or government level, world leaders must develop concrete plans and step-by-step programmes to ensure that no further harm is done to the environment in general.

Although we are almost too late to slow the rate of global warming, finding the right solution is critical. Everyone, from individuals to governments, must work together to find a solution to Global Warming. Some of the factors to consider are pollution control, population growth, and the use of natural resources.

One very important contribution you can make is to reduce your use of plastic. Plastic is the primary cause of global warming, and recycling it takes years. Another factor to consider is deforestation, which will aid in the control of global warming. More tree planting should be encouraged to green the environment. Certain rules should also govern industrialization. Building industries in green zones that affect plants and species should be prohibited.

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Effects of Global Warming

Global warming is a real problem that many people want to disprove to gain political advantage. However, as global citizens, we must ensure that only the truth is presented in the media.

This decade has seen a significant impact from global warming. The two most common phenomena observed are glacier retreat and arctic shrinkage. Glaciers are rapidly melting. These are clear manifestations of climate change.

Another significant effect of global warming is the rise in sea level. Flooding is occurring in low-lying areas as a result of sea-level rise. Many countries have experienced extreme weather conditions. Every year, we have unusually heavy rain, extreme heat and cold, wildfires, and other natural disasters.

Similarly, as global warming continues, marine life is being severely impacted. This is causing the extinction of marine species as well as other problems. Furthermore, changes are expected in coral reefs, which will face extinction in the coming years. These effects will intensify in the coming years, effectively halting species expansion. Furthermore, humans will eventually feel the negative effects of Global Warming.

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Sample Essays on Global Warming

Here are some sample essays on Global Warming:

Essay on Global Warming Paragraph in 100 – 150 words

Global Warming is caused by the increase of carbon dioxide levels in the earth’s atmosphere and is a result of human activities that have been causing harm to our environment for the past few centuries now. Global Warming is something that can’t be ignored and steps have to be taken to tackle the situation globally. The average temperature is constantly rising by 1.5 degrees Celsius over the last few years.

The best method to prevent future damage to the earth, cutting down more forests should be banned and Afforestation should be encouraged. Start by planting trees near your homes and offices, participate in events, and teach the importance of planting trees. It is impossible to undo the damage but it is possible to stop further harm.

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Essay on Global Warming in 250 Words

Over a long period, it is observed that the temperature of the earth is increasing. This affected wildlife, animals, humans, and every living organism on earth. Glaciers have been melting, and many countries have started water shortages, flooding, and erosion and all this is because of global warming. 

No one can be blamed for global warming except for humans. Human activities such as gases released from power plants, transportation, and deforestation have increased gases such as carbon dioxide, CFCs, and other pollutants in the earth’s atmosphere.                                              The main question is how can we control the current situation and build a better world for future generations. It starts with little steps by every individual. 

Start using cloth bags made from sustainable materials for all shopping purposes, instead of using high-watt lights use energy-efficient bulbs, switch off the electricity, don’t waste water, abolish deforestation and encourage planting more trees. Shift the use of energy from petroleum or other fossil fuels to wind and solar energy. Instead of throwing out the old clothes donate them to someone so that it is recycled. 

Donate old books, don’t waste paper.  Above all, spread awareness about global warming. Every little thing a person does towards saving the earth will contribute in big or small amounts. We must learn that 1% effort is better than no effort. Pledge to take care of Mother Nature and speak up about global warming.

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Essay on Global Warming in 500 Words

Global warming isn’t a prediction, it is happening! A person denying it or unaware of it is in the most simple terms complicit. Do we have another planet to live on? Unfortunately, we have been bestowed with this one planet only that can sustain life yet over the years we have turned a blind eye to the plight it is in. Global warming is not an abstract concept but a global phenomenon occurring ever so slowly even at this moment. Global Warming is a phenomenon that is occurring every minute resulting in a gradual increase in the Earth’s overall climate. Brought about by greenhouse gases that trap the solar radiation in the atmosphere, global warming can change the entire map of the earth, displacing areas, flooding many countries, and destroying multiple lifeforms. Extreme weather is a direct consequence of global warming but it is not an exhaustive consequence. There are virtually limitless effects of global warming which are all harmful to life on earth. The sea level is increasing by 0.12 inches per year worldwide. This is happening because of the melting of polar ice caps because of global warming. This has increased the frequency of floods in many lowland areas and has caused damage to coral reefs. The Arctic is one of the worst-hit areas affected by global warming. Air quality has been adversely affected and the acidity of the seawater has also increased causing severe damage to marine life forms. Severe natural disasters are brought about by global warming which has had dire effects on life and property. As long as mankind produces greenhouse gases, global warming will continue to accelerate. The consequences are felt at a much smaller scale which will increase to become drastic shortly. The power to save the day lies in the hands of humans, the need is to seize the day. Energy consumption should be reduced on an individual basis. Fuel-efficient cars and other electronics should be encouraged to reduce the wastage of energy sources. This will also improve air quality and reduce the concentration of greenhouse gases in the atmosphere. Global warming is an evil that can only be defeated when fought together. It is better late than never. If we all take steps today, we will have a much brighter future tomorrow. Global warming is the bane of our existence and various policies have come up worldwide to fight it but that is not enough. The actual difference is made when we work at an individual level to fight it. Understanding its import now is crucial before it becomes an irrevocable mistake. Exterminating global warming is of utmost importance and each one of us is as responsible for it as the next.  

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Essay on Global Warming UPSC

Always hear about global warming everywhere, but do we know what it is? The evil of the worst form, global warming is a phenomenon that can affect life more fatally. Global warming refers to the increase in the earth’s temperature as a result of various human activities. The planet is gradually getting hotter and threatening the existence of lifeforms on it. Despite being relentlessly studied and researched, global warming for the majority of the population remains an abstract concept of science. It is this concept that over the years has culminated in making global warming a stark reality and not a concept covered in books. Global warming is not caused by one sole reason that can be curbed. Multifarious factors cause global warming most of which are a part of an individual’s daily existence. Burning of fuels for cooking, in vehicles, and for other conventional uses, a large amount of greenhouse gases like carbon dioxide, and methane amongst many others is produced which accelerates global warming. Rampant deforestation also results in global warming as lesser green cover results in an increased presence of carbon dioxide in the atmosphere which is a greenhouse gas.  Finding a solution to global warming is of immediate importance. Global warming is a phenomenon that has to be fought unitedly. Planting more trees can be the first step that can be taken toward warding off the severe consequences of global warming. Increasing the green cover will result in regulating the carbon cycle. There should be a shift from using nonrenewable energy to renewable energy such as wind or solar energy which causes less pollution and thereby hinder the acceleration of global warming. Reducing energy needs at an individual level and not wasting energy in any form is the most important step to be taken against global warming. The warning bells are tolling to awaken us from the deep slumber of complacency we have slipped into. Humans can fight against nature and it is high time we acknowledged that. With all our scientific progress and technological inventions, fighting off the negative effects of global warming is implausible. We have to remember that we do not inherit the earth from our ancestors but borrow it from our future generations and the responsibility lies on our shoulders to bequeath them a healthy planet for life to exist. 

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Climate Change and Global Warming Essay

Global Warming and Climate Change are two sides of the same coin. Both are interrelated with each other and are two issues of major concern worldwide. Greenhouse gases released such as carbon dioxide, CFCs, and other pollutants in the earth’s atmosphere cause Global Warming which leads to climate change. Black holes have started to form in the ozone layer that protects the earth from harmful ultraviolet rays. 

Human activities have created climate change and global warming. Industrial waste and fumes are the major contributors to global warming. 

Another factor affecting is the burning of fossil fuels, deforestation and also one of the reasons for climate change.  Global warming has resulted in shrinking mountain glaciers in Antarctica, Greenland, and the Arctic and causing climate change. Switching from the use of fossil fuels to energy sources like wind and solar. 

When buying any electronic appliance buy the best quality with energy savings stars. Don’t waste water and encourage rainwater harvesting in your community. 

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Tips to Write an Essay

Writing an effective essay needs skills that few people possess and even fewer know how to implement. While writing an essay can be an assiduous task that can be unnerving at times, some key pointers can be inculcated to draft a successful essay. These involve focusing on the structure of the essay, planning it out well, and emphasizing crucial details.

Mentioned below are some pointers that can help you write better structure and more thoughtful essays that will get across to your readers:

• Prepare an outline for the essay to ensure continuity and relevance and no break in the structure of the essay
• Decide on a thesis statement that will form the basis of your essay. It will be the point of your essay and help readers understand your contention
• Follow the structure of an introduction, a detailed body followed by a conclusion so that the readers can comprehend the essay in a particular manner without any dissonance.
• Make your beginning catchy and include solutions in your conclusion to make the essay insightful and lucrative to read
• Reread before putting it out and add your flair to the essay to make it more personal and thereby unique and intriguing for readers  

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Ans. Both natural and man-made factors contribute to global warming. The natural one also contains methane gas, volcanic eruptions, and greenhouse gases. Deforestation, mining, livestock raising, burning fossil fuels, and other man-made causes are next.

Ans. The government and the general public can work together to stop global warming. Trees must be planted more often, and deforestation must be prohibited. Auto usage needs to be curbed, and recycling needs to be promoted.

Ans. Switching to renewable energy sources , adopting sustainable farming, transportation, and energy methods, and conserving water and other natural resources.

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Having 2+ years of experience in educational content writing, withholding a Bachelor's in Physical Education and Sports Science and a strong interest in writing educational content for students enrolled in domestic and foreign study abroad programmes. I believe in offering a distinct viewpoint to the table, to help students deal with the complexities of both domestic and foreign educational systems. Through engaging storytelling and insightful analysis, I aim to inspire my readers to embark on their educational journeys, whether abroad or at home, and to make the most of every learning opportunity that comes their way.

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This was really a good essay on global warming… There has been used many unic words..and I really liked it!!!Seriously I had been looking for a essay about Global warming just like this…

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It is not good , to have global warming in our earth .So we all have to afforestation program on all the world.

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Global warming is the increase in 𝓽𝓱𝓮 ᴀᴠᴇʀᴀɢᴇ ᴛᴇᴍᴘᴇʀᴀᴛᴜʀᴇs ᴏғ ᴇᴀʀᴛʜ🌎 ᴀᴛᴍᴏsᴘʜᴇʀᴇ

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What evidence exists that Earth is warming and that humans are the main cause?

We know the world is warming because people have been recording daily high and low temperatures at thousands of weather stations worldwide, over land and ocean, for many decades and, in some locations, for more than a century. When different teams of climate scientists in different agencies (e.g., NOAA and NASA) and in other countries (e.g., the U.K.’s Hadley Centre) average these data together, they all find essentially the same result: Earth’s average surface temperature has risen by about 1.8°F (1.0°C) since 1880. 

( bar chart ) Yearly temperature compared to the twentieth-century average from 1850–2023. Red bars mean warmer-than-average years; blue bars mean colder-than-average years. (line graph) Atmospheric carbon dioxide amounts: 1850-1958 from IAC , 1959-2023 from NOAA Global Monitoring Lab . NOAA Climate.gov graph, adapted from original by Dr. Howard Diamond (NOAA ARL).

In addition to our surface station data, we have many different lines of evidence that Earth is warming ( learn more ). Birds are migrating earlier, and their migration patterns are changing.  Lobsters  and  other marine species  are moving north. Plants are blooming earlier in the spring. Mountain glaciers are melting worldwide, and snow cover is declining in the Northern Hemisphere (Learn more  here  and  here ). Greenland’s ice sheet—which holds about 8 percent of Earth’s fresh water—is melting at an accelerating rate ( learn more ). Mean global sea level is rising ( learn more ). Arctic sea ice is declining rapidly in both thickness and extent ( learn more ).

The Greenland Ice Sheet lost mass again in 2020, but not as much as it did 2019. Adapted from the 2020 Arctic Report Card, this graph tracks Greenland mass loss measured by NASA's GRACE satellite missions since 2002. The background photo shows a glacier calving front in western Greenland, captured from an airplane during a NASA Operation IceBridge field campaign. Full story.

We know this warming is largely caused by human activities because the key role that carbon dioxide plays in maintaining Earth’s natural greenhouse effect has been understood since the mid-1800s. Unless it is offset by some equally large cooling influence, more atmospheric carbon dioxide will lead to warmer surface temperatures. Since 1800, the amount of carbon dioxide in the atmosphere  has increased  from about 280 parts per million to 410 ppm in 2019. We know from both its rapid increase and its isotopic “fingerprint” that the source of this new carbon dioxide is fossil fuels, and not natural sources like forest fires, volcanoes, or outgassing from the ocean.

Philip James de Loutherbourg's 1801 painting, Coalbrookdale by Night , came to symbolize the start of the Industrial Revolution, when humans began to harness the power of fossil fuels—and to contribute significantly to Earth's atmospheric greenhouse gas composition. Image from Wikipedia .

Finally, no other known climate influences have changed enough to account for the observed warming trend. Taken together, these and other lines of evidence point squarely to human activities as the cause of recent global warming.

USGCRP (2017). Climate Science Special Report: Fourth National Climate Assessment, Volume 1 [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 470 pp, doi:  10.7930/J0J964J6 .

National Fish, Wildlife, and Plants Climate Adaptation Partnership (2012):  National Fish, Wildlife, and Plants Climate Adaptation Strategy . Association of Fish and Wildlife Agencies, Council on Environmental Quality, Great Lakes Indian Fish and Wildlife Commission, National Oceanic and Atmospheric Administration, and U.S. Fish and Wildlife Service. Washington, D.C. DOI: 10.3996/082012-FWSReport-1

IPCC (2019). Summary for Policymakers. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. In press.

NASA JPL: "Consensus: 97% of climate scientists agree."  Global Climate Change . A website at NASA's Jet Propulsion Laboratory (climate.nasa.gov/scientific-consensus). (Accessed July 2013.)

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Global warming.

The causes, effects, and complexities of global warming are important to understand so that we can fight for the health of our planet.

Earth Science, Climatology

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Global warming is the long-term warming of the planet’s overall temperature. Though this warming trend has been going on for a long time, its pace has significantly increased in the last hundred years due to the burning of fossil fuels . As the human population has increased, so has the volume of fossil fuels burned. Fossil fuels include coal, oil, and natural gas, and burning them causes what is known as the “greenhouse effect” in Earth’s atmosphere.

The greenhouse effect is when the sun’s rays penetrate the atmosphere, but when that heat is reflected off the surface cannot escape back into space. Gases produced by the burning of fossil fuels prevent the heat from leaving the atmosphere. These greenhouse gasses are carbon dioxide , chlorofluorocarbons, water vapor , methane , and nitrous oxide . The excess heat in the atmosphere has caused the average global temperature to rise overtime, otherwise known as global warming.

Global warming has presented another issue called climate change. Sometimes these phrases are used interchangeably, however, they are different. Climate change refers to changes in weather patterns and growing seasons around the world. It also refers to sea level rise caused by the expansion of warmer seas and melting ice sheets and glaciers . Global warming causes climate change, which poses a serious threat to life on Earth in the forms of widespread flooding and extreme weather. Scientists continue to study global warming and its impact on Earth.

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A problem built into our relationship with energy itself. Photo by Ferdinando Scianna/Magnum

Deep warming

Even if we ‘solve’ global warming, we face an older, slower problem. waste heat could radically alter earth’s future.

by Mark Buchanan   + BIO

The world will be transformed. By 2050, we will be driving electric cars and flying in aircraft running on synthetic fuels produced through solar and wind energy. New energy-efficient technologies, most likely harnessing artificial intelligence, will dominate nearly all human activities from farming to heavy industry. The fossil fuel industry will be in the final stages of a terminal decline. Nuclear fusion and other new energy sources may have become widespread. Perhaps our planet will even be orbited by massive solar arrays capturing cosmic energy from sunlight and generating seemingly endless energy for all our needs.

That is one possible future for humanity. It’s an optimistic view of how radical changes to energy production might help us slow or avoid the worst outcomes of global warming. In a report from 1965, scientists from the US government warned that our ongoing use of fossil fuels would cause global warming with potentially disastrous consequences for Earth’s climate. The report, one of the first government-produced documents to predict a major crisis caused by humanity’s large-scale activities, noted that the likely consequences would include higher global temperatures, the melting of the ice caps and rising sea levels. ‘Through his worldwide industrial civilisation,’ the report concluded, ‘Man is unwittingly conducting a vast geophysical experiment’ – an experiment with a highly uncertain outcome, but clear and important risks for life on Earth.

Since then, we’ve dithered and doubted and argued about what to do, but still have not managed to take serious action to reduce greenhouse gas emissions, which continue to rise. Governments around the planet have promised to phase out emissions in the coming decades and transition to ‘green energy’. But global temperatures may be rising faster than we expected: some climate scientists worry that rapid rises could create new problems and positive feedback loops that may accelerate climate destabilisation and make parts of the world uninhabitable long before a hoped-for transition is possible.

Despite this bleak vision of the future, there are reasons for optimists to hope due to progress on cleaner sources of renewable energy, especially solar power. Around 2010, solar energy generation accounted for less than 1 per cent of the electricity generated by humanity. But experts believe that, by 2027, due to falling costs, better technology and exponential growth in new installations, solar power will become the largest global energy source for producing electricity. If progress on renewables continues, we might find a way to resolve the warming problem linked to greenhouse gas emissions. By 2050, large-scale societal and ecological changes might have helped us avoid the worst consequences of our extensive use of fossil fuels.

It’s a momentous challenge. And it won’t be easy. But this story of transformation only hints at the true depth of the future problems humanity will confront in managing our energy use and its influence over our climate.

As scientists are gradually learning, even if we solve the immediate warming problem linked to the greenhouse effect, there’s another warming problem steadily growing beneath it. Let’s call it the ‘deep warming’ problem. This deeper problem also raises Earth’s surface temperature but, unlike global warming, it has nothing to do with greenhouse gases and our use of fossil fuels. It stems directly from our use of energy in all forms and our tendency to use more energy over time – a problem created by the inevitable waste heat that is generated whenever we use energy to do something. Yes, the world may well be transformed by 2050. Carbon dioxide levels may stabilise or fall thanks to advanced AI-assisted technologies that run on energy harvested from the sun and wind. And the fossil fuel industry may be taking its last breaths. But we will still face a deeper problem. That’s because ‘deep warming’ is not created by the release of greenhouse gases into the atmosphere. It’s a problem built into our relationship with energy itself.

F inding new ways to harness more energy has been a constant theme of human development. The evolution of humanity – from early modes of hunter-gathering to farming and industry – has involved large systematic increases in our per-capita energy use. The British historian and archaeologist Ian Morris estimates, in his book Foragers, Farmers, and Fossil Fuels: How Human Values Evolve (2015), that early human hunter-gatherers, living more than 10,000 years ago, ‘captured’ around 5,000 kcal per person per day by consuming food, burning fuel, making clothing, building shelter, or through other activities. Later, after we turned to farming and enlisted the energies of domesticated animals, we were able to harness as much as 30,000 kcal per day. In the late 17th century , the exploitation of coal and steam power marked another leap: by 1970, the use of fossil fuels allowed humans to consume some 230,000 kcal per person per day. (When we think about humanity writ large as ‘humans’, it’s important to acknowledge that the average person in the wealthiest nations consumes up to 100 times more energy than the average person in the poorest nations.) As the global population has risen and people have invented new energy-dependent technologies, our global energy use has continued to climb.

In many respects, this is great. We can now do more with less effort and achieve things that were unimaginable to the 17th-century inventors of steam engines, let alone to our hominin ancestors. We’ve made powerful mining machines, superfast trains, lasers for use in telecommunications and brain-imaging equipment. But these creations, while helping us, are also subtly heating the planet.

All the energy we humans use – to heat our homes, run our factories, propel our automobiles and aircraft, or to run our electronics – eventually ends up as heat in the environment. In the shorter term, most of the energy we use flows directly into the environment. It gets there through hot exhaust gases, friction between tires and roads, the noises generated by powerful engines, which spread out, dissipate, and eventually end up as heat. However, a small portion of the energy we use gets stored in physical changes, such as in new steel, plastic or concrete. It’s stored in our cities and technologies. In the longer term, as these materials break down, the energy stored inside also finds its way into the environment as heat. This is a direct consequence of the well-tested principles of thermodynamics.

Waste heat will pose a problem that is every bit as serious as global warming from greenhouse gases

In the early decades of the 21st century , this heat created by simply using energy, known as ‘waste heat’, is not so serious. It’s equivalent to roughly 2 per cent of the planetary heating imbalance caused by greenhouse gases – for now. But, with the passing of time, the problem is likely to get much more serious. That’s because humans have a historical tendency to consistently discover and produce things, creating entirely new technologies and industries in the process: domesticated animals for farming; railways and automobiles; global air travel and shipping; personal computers, the internet and mobile phones. The result of such activities is that we end up using more and more energy, despite improved energy efficiency in nearly every area of technology.

During the past two centuries at least (and likely for much longer), our yearly energy use has doubled roughly every 30 to 50 years . Our energy use seems to be growing exponentially, a trend that shows every sign of continuing. We keep finding new things to do and almost everything we invent requires more and more energy: consider the enormous energy demands of cryptocurrency mining or the accelerating energy requirements of AI.

If this historical trend continues, scientists estimate waste heat will pose a problem in roughly 150-200 years that is every bit as serious as the current problem of global warming from greenhouse gases. However, deep heating will be more pernicious as we won’t be able to avoid it by merely shifting from one kind energy to another. A profound problem will loom before us: can we set strict limits on all the energy we use? Can we reign in the seemingly inexorable expansion of our activities to avoid destroying our own environment?

Deep warming is a problem hiding beneath global warming, but one that will become prominent if and when we manage to solve the more pressing issue of greenhouse gases. It remains just out of sight, which might explain why scientists only became concerned about the ‘waste heat’ problem around 15 years ago.

O ne of the first people to describe the problem is the Harvard astrophysicist Eric Chaisson, who discussed the issue of waste heat in a paper titled ‘Long-Term Global Heating from Energy Usage’ (2008). He concluded that our technological society may be facing a fundamental limit to growth due to ‘unavoidable global heating … dictated solely by the second law of thermodynamics, a biogeophysical effect often ignored when estimating future planetary warming scenarios’. When I emailed Chaisson to learn more, he told me the history of his thinking on the problem:

It was on a night flight, Paris-Boston [circa] 2006, after a UNESCO meeting on the environment when it dawned on me that the IPCC were overlooking something. While others on the plane slept, I crunched some numbers literally on the back of an envelope … and then hoped I was wrong, that is, hoped that I was incorrect in thinking that the very act of using energy heats the air, however slightly now.

The transformation of energy into heat is among the most ubiquitous processes of physics

Chaisson drafted the idea up as a paper and sent it to an academic journal. Two anonymous reviewers were eager for it to be published. ‘A third tried his damnedest to kill it,’ Chaisson said, the reviewer claiming the findings were ‘irrelevant and distracting’. After it was finally published, the paper got some traction when it was covered by a journalist and ran as a feature story on the front page of The Boston Globe . The numbers Chaisson crunched, predictions of our mounting waste heat, were even run on a supercomputer at the US National Center for Atmospheric Research, by Mark Flanner, a professor of earth system science. Flanner, Chaisson suspected at the time, was likely ‘out to prove it wrong’. But, ‘after his machine crunched for many hours’, he saw the same results that Chaisson had written on the back of an envelope that night in the plane.

Around the same time, also in 2008, two engineers, Nick Cowern and Chihak Ahn, wrote a research paper entirely independent of Chaisson’s work, but with similar conclusions. This was how I first came across the problem. Cowern and Ahn’s study estimated the total amount of waste heat we’re currently releasing to the environment, and found that it is, right now, quite small. But, like Chaisson, they acknowledged that the problem would eventually become serious unless steps were taken to avoid it.

That’s some of the early history of thinking in this area. But these two papers, and a few other analyses since, point to the same unsettling conclusion: what I am calling ‘deep warming’ will be a big problem for humanity at some point in the not-too-distant future. The precise date is far from certain. It might be 150 years , or 400, or 800, but it’s in the relatively near future, not the distant future of, say, thousands or millions of years. This is our future.

T he transformation of energy into heat is among the most ubiquitous processes of physics. As cars drive down roads, trains roar along railways, planes cross the skies and industrial plants turn raw materials into refined products, energy gets turned into heat, which is the scientific word for energy stored in the disorganised motions of molecules at the microscopic level. As a plane flies from Paris to Boston, it burns fuel and thrusts hot gases into the air, generates lots of sound and stirs up contrails. These swirls of air give rise to swirls on smaller scales which in turn make smaller ones until the energy ultimately ends up lost in heat – the air is a little warmer than before, the molecules making it up moving about a little more vigorously. A similar process takes place when energy is used by the tiny electrical currents inside the microchips of computers, silently carrying out computations. Energy used always ends up as heat. Decades ago, research by the IBM physicist Rolf Landauer showed that a computation involving even a single computing bit will release a certain minimum amount of heat to the environment.

How this happens is described by the laws of thermodynamics, which were described in the mid-19th century by scientists including Sadi Carnot in France and Rudolf Clausius in Germany. Two key ‘laws’ summarise its main principles.

The first law of thermodynamics simply states that the total quantity of energy never changes but is conserved. Energy, in other words, never disappears, but only changes form. The energy initially stored in an aircraft’s fuel, for example, can be changed into the energetic motion of the plane. Turn on an electric heater, and energy initially held in electric currents gets turned into heat, which spreads into the air, walls and fabric of your house. The total energy remains the same, but it markedly changes form.

We’re generating waste heat all the time with everything we do

The second law of thermodynamics, equally important, is more subtle and states that, in natural processes, the transformation of energy always moves from more organised and useful forms to less organised and less useful forms. For an aircraft, the energy initially concentrated in jet fuel ends up dissipated in stirred-up winds, sounds and heat spread over vast areas of the atmosphere in a largely invisible way. It’s the same with the electric heater: the organised useful energy in the electric currents gets dissipated and spread into the low-grade warmth of the walls, then leaks into the outside air. Although the amount of energy remains the same, it gradually turns into less organised, less usable forms. The end point of the energy process produces waste heat. And we’re generating it all the time with everything we do.

Data on world energy consumption shows that, collectively, all humans on Earth are currently using about 170,000 terawatt-hours (TWh), which is a lot of energy in absolute terms – a terawatt-hour is the total energy consumed in one hour by any process using energy at a rate of 1 trillion watts. This huge number isn’t surprising, as it represents all the energy being used every day by the billions of cars and homes around the world, as well as by industry, farming, construction, air traffic and so on. But, in the early 21st century , the warming from this energy is still much less than the planetary heating due to greenhouse gases.

Concentrations of greenhouse gases such as CO 2 and methane are quite small, and only make a fractional difference to how much of the Sun’s energy gets trapped in the atmosphere, rather than making it back out to space. Even so, this fractional difference has a huge effect because the stream of energy arriving from the Sun to Earth is so large. Current estimates of this greenhouse energy imbalance come to around 0.87 W per square meter, which translates into a total energy figure about 50 times larger than our waste heat. That’s reassuring. But as Cowern and Ahn wrote in their 2008 paper, things aren’t likely to stay this way over time because our energy usage keeps rising. Unless, that is, we can find some radical way to break the trend of using ever more energy.

O ne common objection to the idea of the deep warming is to claim that the problem won’t really arise. ‘Don’t worry,’ someone might say, ‘with efficient technology, we’re going to find ways to stop using more energy; though we’ll end up doing more things in the future, we’ll use less energy.’ This may sound plausible at first, because we are indeed getting more efficient at using energy in most areas of technology. Our cars, appliances and laptops are all doing more with less energy. If efficiency keeps improving, perhaps we can learn to run these things with almost no energy at all? Not likely, because there are limits to energy efficiency.

Over the past few decades, the efficiency of heating in homes – including oil and gas furnaces, and boilers used to heat water – has increased from less than 50 per cent to well above 90 per cent of what is theoretically possible. That’s good news, but there’s not much more efficiency to be realised in basic heating. The efficiency of lighting has also vastly improved, with modern LED lighting turning something like 70 per cent of the applied electrical energy into light. We will gain some efficiencies as older lighting gets completely replaced by LEDs, but there’s not a lot of room left for future efficiency improvements. Similar efficiency limits arise in the growing or cooking of food; in the manufacturing of cars, bikes and electronic devices; in transportation, as we’re taken from place to place; in the running of search engines, translation software, GPT-4 or other large-language models.

Even if we made significant improvements in the efficiencies of these technologies, we will only have bought a little time. These changes won’t delay by much the date when deep warming becomes a problem we must reckon with.

Optimising efficiencies is just a temporary reprieve, not a radical change in our human future

As a thought experiment, suppose we could immediately improve the energy efficiency of everything we do by a factor of 10 – a fantastically optimistic proposal. That is, imagine the energy output of humans on Earth has been reduced 10 times , from 170,000 TWh to 17,000 TWh . If our energy use keeps expanding, doubling every 30-50 years or so (as it has for centuries), then a 10-fold increase in waste heat will happen in just over three doubling times, which is about 130 years : 17,000 TWh doubles to 34,000 TWh , which doubles to 68,000 TWh , which doubles to 136,000 TWh , and so on. All those improvements in energy efficiency would quickly evaporate. The date when deep warming hits would recede by 130 years or so, but not much more. Optimising efficiencies is just a temporary reprieve, not a radical change in our human future.

Improvements in energy efficiency can also have an inverse effect on our overall energy use. It’s easy to think that if we make a technology more efficient, we’ll then use less energy through the technology. But economists are deeply aware of a paradoxical effect known as ‘rebound’, whereby improved energy efficiency, by making the use of a technology cheaper, actually leads to more widespread use of that technology – and more energy use too. The classic example, as noted by the British economist William Stanley Jevons in his book The Coal Question (1865), is the invention of the steam engine. This new technology could extract energy from burning coal more efficiently, but it also made possible so many new applications that the use of coal increased. A recent study by economists suggests that, across the economy, such rebound effects might easily swallow at least 50 per cent of any efficiency gains in energy use. Something similar has already happened with LED lights, for which people have found thousands of new uses.

If gains in efficiency won’t buy us lots of time, how about other factors, such as a reduction of the global population? Scientists generally believe that the current human population of more than 8 billion people is well beyond the limits of our finite planet, especially if a large fraction of this population aspires to the resource-intensive lifestyles of wealthy nations. Some estimates suggest that a more sustainable population might be more like 2 billion , which could reduce energy use significantly, potentially by a factor of three or four. However, this isn’t a real solution: again, as with the example of improved energy efficiency, a one-time reduction of our energy consumption by a factor of three will quickly be swallowed up by an inexorable rise in energy use. If Earth’s population were suddenly reduced to 2 billion – about a quarter of the current population – our energy gains would initially be enormous. But those gains would be erased in two doubling times, or roughly 60-100 years , as our energy demands would grow fourfold.

S o, why aren’t more people talking about this? The deep warming problem is starting to get more attention. It was recently mentioned on Twitter by the German climate scientist Stefan Rahmstorf, who cautioned that nuclear fusion, despite excitement over recent advances, won’t arrive in time to save us from our waste heat, and might make the problem worse. By providing another cheap source of energy, fusion energy could accelerate both the growth of our energy use and the reckoning of deep warming. A student of Rahmstorf’s, Peter Steiglechner, wrote his master’s thesis on the problem in 2018. Recognition of deep warming and its long-term implications for humanity is spreading. But what can we do about the problem?

Avoiding or delaying deep warming will involve slowing the rise of our waste heat, which means restricting the amount of energy we use and also choosing energy sources that exacerbate the problem as little as possible. Unlike the energy from fossil fuels or nuclear power, which add to our waste energy burden, renewable energy sources intercept energy that is already on its way to Earth, rather than producing additional waste heat. In this sense, the deep warming problem is another reason to pursue renewable energy sources such as solar or wind rather than alternatives such as nuclear fusion, fission or even geothermal power. If we derive energy from any of these sources, we’re unleashing new flows of energy into the Earth system without making a compensating reduction. As a result, all such sources will add to the waste heat problem. However, if renewable sources of energy are deployed correctly, they need not add to our deposition of waste heat in the environment. By using this energy, we produce no more waste heat than would have been created by sunlight in the first place.

Take the example of wind energy. Sunlight first stirs winds into motion by heating parts of the planet unequally, causing vast cells of convection. As wind churns through the atmosphere, blows through trees and over mountains and waves, most of its energy gets turned into heat, ending up in the microscopic motions of molecules. If we harvest some of this wind energy through turbines, it will also be turned into heat in the form of stored energy. But, crucially, no more heat is generated than if there had been no turbines to capture the wind.

The same can hold true for solar energy. In an array of solar cells, if each cell only collects the sunlight falling on it – which would ordinarily have been absorbed by Earth’s surface – then the cells don’t alter how much waste heat gets produced as they generate energy. The light that would have warmed Earth’s surface instead goes into the solar cells, gets used by people for some purpose, and then later ends up as heat. In this way we reduce the amount of heat being absorbed by Earth by precisely the same amount as the energy we are extracting for human use. We are not adding to overall planetary heating. This keeps the waste energy burden unchanged, at least in the relatively near future, even if we go on extracting and using ever larger amounts of energy.

Covering deserts in dark panels would absorb a lot more energy than the desert floor

Chaisson summarised the problem quite clearly in 2008:

I’m now of the opinion … that any energy that’s dug up on Earth – including all fossil fuels of course, but also nuclear and ground-sourced geothermal – will inevitably produce waste heat as a byproduct of humankind’s use of energy. The only exception to that is energy arriving from beyond Earth, this is energy here and now and not dug up, namely the many solar energies (plural) caused by the Sun’s rays landing here daily … The need to avoid waste heat is indeed the single, strongest, scientific argument to embrace solar energies of all types.

But not just any method of gathering solar energy will avoid the deep warming problem. Doing so requires careful engineering. For example, covering deserts with solar panels would add to planetary heating because deserts reflect a lot of incident light back out to space, so it is never absorbed by Earth (and therefore doesn’t produce waste heat). Covering deserts in dark panels would absorb a lot more energy than the desert floor and would heat the planet further.

We’ll also face serious problems in the long run if our energy appetite keeps increasing. Futurists dream of technologies deployed in space where huge panels would absorb sunlight that would otherwise have passed by Earth and never entered our atmosphere. Ultimately, they believe, this energy could be beamed down to Earth. Like nuclear energy, such technologies would add an additional energy source to the planet without any compensating removal of heating from the sunlight currently striking our planet’s surface. Any effort to produce more energy than is normally available from sunlight at Earth’s surface will only make our heating problems worse.

D eep warming is simply a consequence of the laws of physics and our inquisitive nature. It seems to be in our nature to constantly learn and develop new things, changing our environment in the process. For thousands of years, we have harvested and exploited ever greater quantities of energy in this pursuit, and we appear poised to continue along this path with the rapidly expanding use of renewable energy sources – and perhaps even more novel sources such as nuclear fusion. But this path cannot proceed indefinitely without consequences.

The logic that more energy equals more warming sets up a profound dilemma for our future. The laws of physics and the habits ingrained in us from our long evolutionary history are steering us toward trouble. We may have a technological fix for greenhouse gas warming – just shift from fossil fuels to cleaner energy sources – but there is no technical trick to get us out of the deep warming problem. That won’t stop some scientists from trying.

Perhaps, believing that humanity is incapable of reducing its energy usage, we’ll adopt a fantastic scheme to cool the planet, such as planetary-scale refrigeration or using artificially engineered tornadoes to transport heat from Earth’s surface to the upper atmosphere where it can be radiated away to space. As far-fetched as such approaches sound, scientists have given some serious thought to these and other equally bizarre ideas, which seem wholly in the realm of science fiction. They’re schemes that will likely make the problem worse not better.

We will need to transform the human story. It must become a story of doing less, not more

I see several possibilities for how we might ultimately respond. As with greenhouse gas warming, there will probably be an initial period of disbelief, denial and inaction, as we continue with unconstrained technological advance and growing energy use. Our planet will continue warming. Sooner or later, however, such warming will lead to serious disruptions of the Earth environment and its ecosystems. We won’t be able to ignore this for long, and it may provide a natural counterbalance to our energy use, as our technical and social capacity to generate and use ever more energy will be eroded. We may eventually come to some uncomfortable balance in which we just scrabble out a life on a hot, compromised planet because we lack the moral and organisational ability to restrict our energy use enough to maintain a sound environment.

An alternative would require a radical break with our past: using less energy. Finding a way to use less energy would represent a truly fundamental rupture with all of human history, something entirely novel. A rupture of this magnitude won’t come easily. However, if we could learn to view restrictions on our energy use as a non-negotiable element of life on Earth, we may still be able to do many of the things that make us essentially human: learning, discovering, inventing, creating. In this scenario, any helpful new technology that comes into use and begins using lots of energy would require a balancing reduction in energy use elsewhere. In such a way, we might go on with the future being perpetually new, and possibly better.

None of this is easily achieved and will likely mirror our current struggles to come to agreements on greenhouse gas heating. There will be vicious squabbles, arguments and profound polarisation, quite possibly major wars. Humanity will never have faced a challenge of this magnitude, and we won’t face up to it quickly or easily, I expect. But we must. Planetary heating is in our future – the very near future and further out as well. Many people will find this conclusion surprisingly hard to swallow, perhaps because it implies fundamental restrictions on our future here on Earth: we can’t go on forever using more and more energy, and, at the same time, expecting the planet’s climate to remain stable.

The world will likely be transformed by 2050. And, sometime after that, we will need to transform the human story. The narrative arc of humanity must become a tale of continuing innovation and learning, but also one of careful management. It must become a story, in energy terms, of doing less, not more. There’s no technology for entirely escaping waste heat, only techniques.

This is important to remember as we face up to the extremely urgent challenge of heating linked to fossil-fuel use and greenhouse gases. Global warming is just the beginning of our problems. It’s a testing ground to see if we can manage an intelligent and coordinated response. If we can handle this challenge, we might be better prepared, more capable and resilient as a species to tackle an even harder one.

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Roz Pidcock

Which of the many thousands of papers on climate change published each year in scientific journals are the most successful? Which ones have done the most to advance scientists’ understanding, alter the course of climate change research, or inspire future generations?

On Wednesday, Carbon Brief will reveal the results of our analysis into which scientific papers on the topic of climate change are the most “cited”. That means, how many times other scientists have mentioned them in their own published research. It’s a pretty good measure of how much impact a paper has had in the science world.

But there are other ways to measure influence. Before we reveal the figures on the most-cited research, Carbon Brief has asked climate experts what they think are the most influential papers.

We asked all the coordinating lead authors, lead authors and review editors on the last Intergovernmental Panel on Climate Change (IPCC) report to nominate three papers from any time in history. This is the exact question we posed:

What do you consider to be the three most influential papers in the field of climate change?

As you might expect from a broad mix of physical scientists, economists, social scientists and policy experts, the nominations spanned a range of topics and historical periods, capturing some of the great climate pioneers and the very latest climate economics research.

Here’s a link to our summary of who said what . But one paper clearly takes the top spot.

Winner: Manabe & Wetherald ( 1967 )

With eight nominations, a seminal paper by Syukuro Manabe and Richard. T. Wetherald published in the Journal of the Atmospheric Sciences in 1967 tops the Carbon Brief poll as the IPCC scientists’ top choice for the most influential climate change paper of all time.

Entitled, “Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity”, the work was the first to represent the fundamental elements of the Earth’s climate in a computer model, and to explore what doubling carbon dioxide (CO2) would do to global temperature.

Manabe & Wetherald (1967), Journal of the Atmospheric Sciences

The Manabe & Wetherald paper is considered by many as a pioneering effort in the field of climate modelling, one that effectively opened the door to projecting future climate change. And the value of climate sensitivity is something climate scientists are still grappling with today .

Prof Piers Forster , a physical climate scientist at Leeds University and lead author of the chapter on clouds and aerosols in working group one of the last IPCC report, tells Carbon Brief:

This was really the first physically sound climate model allowing accurate predictions of climate change.

The paper’s findings have stood the test of time amazingly well, Forster says.

Its results are still valid today. Often when I’ve think I’ve done a new bit of work, I found that it had already been included in this paper.

Prof Steve Sherwood , expert in atmospheric climate dynamics at the University of New South Wales and another lead author on the clouds and aerosols chapter, says it’s a tough choice, but Manabe & Wetherald (1967) gets his vote, too. Sherwood tells Carbon Brief:

[The paper was] the first proper computation of global warming and stratospheric cooling from enhanced greenhouse gas concentrations, including atmospheric emission and water-vapour feedback.

Prof Danny Harvey , professor of climate modelling at the University of Toronto and lead author on the buildings chapter in the IPCC’s working group three report on mitigation, emphasises the Manabe & Wetherald paper’s impact on future generations of scientists. He says:

[The paper was] the first to assess the magnitude of the water vapour feedback, and was frequently cited for a good 20 years after it was published.

Tomorrow, Carbon Brief will be publishing an interview with Syukuro Manabe, alongside a special summary by Prof John Mitchell , the Met Office Hadley Centre’s chief scientist from 2002 to 2008 and director of climate science from 2008 to 2010, on why the paper still holds such significance today.

Joint second: Keeling, C.D et al. ( 1976 )

Jumping forward a decade, a classic paper by Charles Keeling and colleagues in 1976 came in joint second place in the Carbon Brief survey.

Published in the journal Tellus under the title, “Atmospheric carbon dioxide variations at Mauna Loa observatory,” the paper documented for the first time the stark rise of carbon dioxide in the atmosphere at the Mauna Loa observatory in Hawaii.

A photocopy of Keeling et al., (1976) Source: University of California, Santa Cruz

Dr Jorge Carrasco , Antarctic climate change researcher at the University of Magallanes  in Chile and lead author on the cryosphere chapter in the last IPCC report, tells Carbon Brief why the research underpinning the “Keeling Curve’ was so important.

This paper revealed for the first time the observing increased of the atmospheric CO2 as the result of the combustion of carbon, petroleum and natural gas.

Prof David Stern , energy and environmental economist at the Australian National University and lead author on the Drivers, Trends and Mitigation chapter of the IPCC’s working group three report, also chooses the 1976 Keeling paper, though he notes:

This is a really tough question as there are so many dimensions to the climate problem – natural science, social science, policy etc.

With the Mauna Loa measurements continuing today , the so-called “Keeling curve” is the longest continuous record of carbon dioxide concentration in the world. Its historical significance and striking simplicity has made it one of the most iconic visualisations of climate change.

Source: US National Oceanic and Atmospheric Administration (NOAA)

Also in joint second place: Held, I.M. & Soden, B.J. ( 2006 )

Fast forwarding a few decades, in joint second place comes a paper by Isaac Held and Brian Soden published in the journal Science in 2006.

The paper, “Robust Responses of the Hydrological Cycle to Global Warming”, identified how rainfall from one place to another would be affected by climate change. Prof Sherwood, who nominated this paper as well as the winning one from Manabe and Wetherald, tells Carbon Brief why it represented an important step forward. He says:

[This paper] advanced what is known as the “wet-get-wetter, dry-get-drier” paradigm for precipitation in global warming. This mantra has been widely misunderstood and misapplied, but was the first and perhaps still the only systematic conclusion about regional precipitation and global warming based on robust physical understanding of the atmosphere.

Held & Soden (2006), Journal of Climate

Honourable mentions

Rather than choosing a single paper, quite a few academics in our survey nominated one or more of the Working Group contributions to the last IPCC report. A couple even suggested the Fifth Assessment Report in its entirety, running to several thousands of pages. The original IPCC report , published in 1990, also got mentioned.

It was clear from the results that scientists tended to pick papers related to their own field. For example, Prof Ottmar Edenhofer , chief economist at the Potsdam Institute for Climate Impact Research and co-chair of the IPCC’s Working Group Three report on mitigation, selected four papers from the last 20 years on the economics of climate change costs versus risks, recent emissions trends, the technological feasibility of strong emissions reductions and the nature of international climate cooperation.

Taking a historical perspective, a few more of the early pioneers of climate science featured in our results, too. For example, Svante Arrhenius’ famous 1896 paper  on the Greenhouse Effect, entitled “On the influence of carbonic acid in the air upon the temperature of the ground”, received a couple of votes.

Prof Jonathan Wiener , environmental policy expert at Duke University in the US and lead author on the International Cooperation chapter in the IPCC’s working group three report, explains why this paper should be remembered as one of the most influential in climate policy. He says:

[This is the] classic paper showing that rising greenhouse gas concentrations lead to increasing global average surface temperature.

Svante Arrhenius (1896), Philosophical Magazine

A few decades later, a paper by Guy Callendar in 1938  linked the increase in carbon dioxide concentration over the previous 50 years to rising temperatures. Entitled, “The artificial production of carbon dioxide and its influence on temperature,” the paper marked an important step forward in climate change research, says Andrew Solow , director of the Woods Hole Marine Policy centre and lead author on the detection and attribution of climate impacts chapter in the IPCC’s working group two report. He says:

There is earlier work on the greenhouse effect, but not (to my knowledge) on the connection between increasing levels of CO2 and temperature.

Though it may feature in the climate change literature hall of fame, this paper raises a question about how to define a paper’s influence, says Forster. Rather than being celebrated among his contemporaries, Callendar’s work achieved recognition a long time after it was published. Forster says:

I would loved to have chosen Callendar (1938) as the first attribution paper that changed the world. Unfortunately, the 1938 effort of Callendar was only really recognised afterwards as being a founding publication of the field … The same comment applies to earlier Arrhenius and Tyndall efforts. They were only influential in hindsight.

Guy Callendar and his 1938 paper in Quarterly Journal of the Royal Meteorological Society

Other honourable mentions in the Carbon Brief survey of most influential climate papers go to Norman Phillips, whose 1956 paper described the first general circulation model, William Nordhaus’s 1991 paper on the economics of the greenhouse effect, and a paper by Camile Parmesan and Gary Yohe in 2003 , considered by many to provide the first formal attribution of climate change impacts on animal and plant species.

Finally, James Hansen’s 2012 paper , “Public perception of climate change and the new climate dice”, was important in highlighting the real-world impacts of climate change, says Prof Andy Challinor , expert in climate change impacts at the University of Leeds and lead author on the food security chapter in the working group two report. He says:

[It] helped with demonstrating the strong links between extreme events this century and climate change. Result: more clarity and less hedging.

Marc Levi , a political scientist at Columbia University and lead author on the IPCC’s human security chapter, makes a wider point, telling Carbon Brief:

The importance is in showing that climate change is observable in the present.

Indeed, attribution of extreme weather continues to be at the forefront of climate science, pushing scientists’ understanding of the climate system and modern technology to their limits.

Look out for more on the latest in attribution research as Carbon Brief reports on the Our Common Futures Under Climate Change conference taking place in Paris this week.

Pinning down which climate science papers most changed the world is difficult, and we suspect climate scientists could argue about this all day. But while the question elicits a range of very personal preferences, stories and characters, one paper has clearly stood the test of time and emerged as the popular choice among today’s climate experts – Manabe and Wetherald, 1967.

Main image: Satellite image of Hurricane Katrina.

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A Systematic Literature Review of the Impact of Climate Change on the Global Demand for Psychiatric Services

Julia feriato corvetto.

1 Heidelberg Institute of Global Health (HIGH), Heidelberg University Hospital, Heidelberg University, 69120 Heidelberg, Germany

Ammir Yacoub Helou

2 Department of Anatomy, Institute of Biomedical Sciences, University of São Paulo, São Paulo 05508-900, Brazil

Peter Dambach

Thomas müller.

3 Private Clinic Meiringen, 3860 Meiringen, Switzerland

4 Translational Research Center, University Hospital of Psychiatry and Psychotherapy, University of Bern, 3000 Bern, Switzerland

Rainer Sauerborn

Associated data.

Not applicable.

Climate Change (CC) imposes important global health risks, including on mental health (MH). They are related mostly to psychological suffering caused by climate-related events and to the heat-vulnerability caused by psychiatric disorders. This growing burden may press MH services worldwide, increasing demand on public and private systems in low-, middle-, and high-income countries. According to PRISMA, two independent reviewers searched four databases for papers published before May 2022 that associated climate-related events with healthcare demand for psychiatric conditions. Of the 7432 papers retrieved, we included 105. Only 29 were carried out in low- and middle-income countries. Twelve related the admission numbers to (i) extreme events, while 93 to (ii) meteorological factors—mostly heat. Emergency visits and hospitalizations were significantly higher during hot periods for MH disorders, especially until lag 5–7. Extreme events also caused more consultations. Suicide (completed or attempted), substance misuse, schizophrenia, mood, organic and neurotic disorders, and mortality were strongly affected by CC. This high healthcare demand is evidence of the burden patients may undergo. In addition, public and private services may face a shortage of financial and human resources. Finally, the increased use of healthcare facilities, in turn, intensifies greenhouse gas emissions, representing a self-enforcing cycle for CC. Further research is needed to better clarify how extreme events affect MH services and, in addition, if services in low- and middle-income countries are more intensely demanded by CC, as compared to richer countries.

1. Introduction

Climate is changing drastically and unprecedentedly [ 1 , 2 , 3 ]. Recently, the eight consecutive years from 2015 until 2022 were considered the warmest years since the beginning of temperature measurement [ 4 , 5 ]. Concomitantly, extreme events have become more visible and deadly [ 6 ]. Still, this threatening scenario tends to be aggravated, considering that only few countries have met their Nationally Determined Contributions (NDCs) so far, and that greenhouse gas emissions (GHG) continue to grow every year [ 7 , 8 ].

This wide range of climate change-sensitive variables—weather components potentially worsened by climate change (CC)—imposes consequences on human health. They were extensively explored during the past decades by the intersection between environmental and health research, but in contrast to the somatic medical conditions, much less is known about the mental health (MH) impacts of CC. Based on Verner et al. (2016), the Figure S1 and Table S1 in Supplementary Materials displays the publication numbers from the main health domains and compares them to how mental health received late scientific attention, in the context of CC [ 9 ].

The available literature could already indicate how CC can interfere on MH, as well [ 10 ]. In this scenario, there are specific reasons why MH patients are in a vulnerable condition. The main contributors are the heat intolerance created by the disorders, pre-existing social and economic vulnerability, and psychotropic drugs used as treatment [ 10 , 11 , 12 , 13 ].

This burden may be still intensified, due to the high rates of psychiatric disorders worldwide. Since 2005, there has been a growth of approximately 15% of the prevalence of common mental diseases, and, among youth, they occupy the leading cause of health afflictions in the United States [ 14 , 15 ]. The incidence during lifetime can reach levels up to 50% of the population, depending on the country [ 16 ]. Finally, once it is diagnosed, the condition imposes an important disability on the patient [ 17 ].

This health disturbance can be reflected in the overload of health systems worldwide, increasing costs for both public and private sources [ 18 , 19 ]. This acute effect can be measured through different methods. In a country where healthcare is effectively delivered, service utilization—emergency department visits, hospitalization, scheduled outpatients—is usually considered the most accurate method for acute needs, as it has the potential sensitivity for fluctuation according to external situations [ 20 ].

Contrarily, what happens in low- and middle-income countries is that the funding for mental illness care does not reflect the real need and corresponds to a median spending of less than 1% of their health budget [ 19 , 21 , 22 , 23 ]. Human resources are also scarce, and, commonly, there is only one psychiatrist responsible for thousands of patients [ 24 ]. In this case, the admission numbers, despite still being the best possible measurement, are probably fewer than anticipated, when compared to an ideal scenario. In both described cases, high- or low- and middle-income countries, the psychiatric health demand will gradually continue to pressure the systems, considering that CC tends to intensify and create new climate-related events [ 2 , 10 ].

Therefore, from a public health perspective, it is important to differentiate and focus on conditions that are potentially long-lasting or overwhelming for the collective. In addition, the recent terms ‘climate change distress’, ‘eco-anxiety’, and ‘solastalgia’ are terms used to describe the uncertainty and fear produced by CC, and they were found by Charlson et al. (2021) [ 25 ] to be among the most prevalent outcomes measured by the studies, even though they are often considered limited conditions [ 26 ]. By using health services as the final measure, we tend to delineate these conditions and then be more objective about the long-term burden.

One similar review, which also analyses mental health services demand, was recently conducted in Australia, with specific focus on heatwaves. The authors—Mason and Colleagues, 2022 [ 27 ]—indicate that such evidence is important to increase preparedness of health systems across the country, in order to supply and balance the demand of warmer-than-average periods.

Therefore, the purpose of this review is:

• To investigate the demand for mental health services caused by CC worldwide—hospital admissions (HA), emergency department visits (EDV), outpatient consultations, and ambulance dispatch—due to psychiatric symptoms, disorders, or mortality.
• To identify which of these disorders are responsible for the consultations, in the aftermath of CC variables.
• To compare health services use in low- and middle-income countries to that in high-income countries and examine how these populations are undertreated for having less access to healthcare services.

To the best of our knowledge, this is the first systematic literature review to focus on the effective health system burden correlated to CC and psychiatry.

2. Materials and Methods

This systematic literature review was conducted in accordance with the PRISMA guidelines (Preferred Reporting Items for Systematic reviews and Meta-Analyses) [ 28 ] and registered in PROSPERO (2022, CRD42022353023) [ 29 ].

Papers were retrieved using the applied search terms on the databases PubMed, Embase, Web of Science, and PsycInfo, which were published until 10 May 2022, without date limit.

The keywords from two categories were combined in the following search terms, using Boolean operators (detailed search terms and Boolean operators can be found in Supplementary Materials—Table S2 ): climate change, global warming, heatwave, hot temperature, natural disaster, psychiatric disorder, mental disorder, suicide, suicide attempt, depression, mania, schizophrenia, dementia, substance abuse, bipolar disorder, psychosis, organic disorder, anxiety disorder, neurotic disorder, neurocognitive disorder, and, lastly, post-traumatic stress disorder.

These psychiatric disorders were previously related to being susceptible to CC [ 10 , 25 , 30 ]. The search words are either MeSH terms or specific disease names listed on the International Classification of Diseases—10th revision (ICD-10th) ( Supplementary Materials—Figure S2 ) [ 31 ].

The CC variables were considered to be all the possible components that can be potentially influenced or worsened by CC: meteorological factors (mostly heat) and extreme events [ 2 ]. In this context, air pollution was not included in this review, since it is considered to be one of the causes of CC, instead of a consequence of it. Still, ‘hot temperature’, ‘heat’, or ‘warmer than average’ were temperature values above a certain threshold specified by the author ( Supplementary Materials—Table S3 ).

2.1. Studies Selection and Quality Assessment

In the first phase, during the title and abstract selection, the papers were kept if selected by at least one of the two independent readers (J.C. and A.H.), through the online selecting tool for systematic reviews Rayyan [ 32 ]. Then, the selected papers were full text reviewed according to the inclusion and exclusion criteria. In the full text selection, both readers agreed on its inclusion, otherwise a third independent reader was required to decide. Detailed inclusion and exclusion criteria can be found in Table 1 .

Eligibility criteria: inclusion and exclusion criteria for the selection of papers.

Studies that did not explicitly analyze the impact of CC variables on health services demand due to psychiatric causes (symptoms, disorders, or mortality) were excluded.

Before data appraisal, the ‘quality assessment tool for Observational Cohort and Cross-Sectional Studies’ or ‘for Case-Control studies’, depending on the design of the analyzed study, of the National Institute of Health (NIH) was used by the two reviewers, also independently, to determine the final eligibility for the included papers ( Supplementary Materials—Table S4 ) [ 33 ]. Studies were assessed as either ‘poor’, ‘fair’, or ‘good’. The ‘poor’ quality studies were excluded. In case only one of the reviewers rated the study as ‘poor’, a third independent reader was required again. Otherwise, if the two assessments were considered at least ‘fair’, the study was included. Each question from the NIH questionnaires received 1 score. In case a study achieved less than 50% of the maximum score, it was considered ‘poor’. Between 50% and 74% were placed in the ‘fair’ category, and, finally, the ‘good’ were the ones that scored 75% or more.

2.2. Data Appraisal

Given the two different mechanisms by which CC may affect MH, the studies were distributed in the following major groups for data appraisal: (i) extreme events and (ii) meteorological variables. Even though a heatwave is considered an extreme event, it was placed in the last group (ii), together with hot temperature, since both act through the same mechanism on MH patients. In each of the groups, the corresponding diseases’ classification followed the ICD—10th revision.

First, the subgroup called ‘MH in general’ comprises the unclassified outcomes, in which the authors represented all of the psychiatric conditions. Secondly, the specific outcomes were grouped as follows: organic disorders (F00–F09), substance misuse (F10–F19), schizophrenia (F20–F29), mood disorders (F30–F39), neurotic disorders (F40–F48), behavioral disorders (F50–F59), personality disorders (F60–F69), intellectual disabilities (F70–F79), specific developmental disorders (F80–F89), and behavioral and emotional disorders with onset usually in childhood (F90–F99) (ICD–10th detailed in Supplementary Materials—Figure S2 ). Still, suicide attempts or completed suicides were also listed separately. The last classification was called ‘mortality’, and corresponded the cases of death linked, by the authors, to a MH cause.

In case papers addressed multiple human disorders, the only retrieved data was the one corresponding to MH.

We performed an extensive literature review and could rely on a relatively large number of studies ( n = 105). The vast majority of the studies here comprised were ‘fair’ in quality ( n = 91), and only 1 was graded as ‘poor’ and, therefore, excluded from the analysis. Still, 14 papers were rated as ‘good’.

From the 7432 papers searched via four databases (PubMed = 1183, PsycInfo = 752, Embase = 2286, and Web of Science n = 3211), we included 105 for data extraction ( Figure 1 ). The general information from included studies can be found in Table 2 , including the number of each type of health service that was approached. A more detailed table is included in the Supplementary Material (Table S4) . The CC variables were found to be predominantly heat and heatwaves ( n = 92; 87.6%). Only one study reported other meteorological exposures, snow and rain, which they reported to be associated with CC. Few articles ( n = 12; 11.4%) studied the influences of extreme events on psychiatry.

PRISMA diagram for Systematic Literature Reviews [ 28 ].

General characteristics of all the included studies. * Weather variables included: snow and rain, temperature variability, temperature change between neighboring days, and daily excess hourly heat. The order studies were placed in the table was, as follows: MH in general or multiple types of disorders and, then, the sequence of specific ICD-10th, from F00 to F99. Mortality and suicide were placed next. Lastly, the extreme event-related outcomes.

The Figure 2 displays the number of included articles according to the year of publication. This review found, as expected and previously discussed, an increase in publication numbers, especially after 2016. Until May 2022, a total of 12 studies had already been published and are included here, following the positive trend.

Included studies according to the year of publication.

Twenty-nine studies (27.5%) were carried out in low- and middle-income countries. China contributed the most, with 19 publications. Figure 3 displays how the studies are spread in the world, evidencing the central role of high-income countries.

Global distribution of studies. Note: white color corresponds to missing studies in that country. Created with: paintmaps.com, accessed on 10 October 2022.

Hospital admission ( n = 34) and completed suicide ( n = 34) were found to be the most studied types of services. Even though suicide is not a service per se, it certainly requires specific sectors of the health system to be addressed. Emergency department visit was, as well, often studied ( n = 20), followed by mortality ( n = 15). Together, phone calls, ambulance dispatch, outpatient visits, and other types of provided healthcare were poorly researched ( n = 10) and could, in the future, receive more focus from science. In addition, no significant difference among them was found, and it seems that healthcare services are equally demanded. Figure 4 lists the measure of effect and displays them according to the used service.

Forest plot of significant results (relative risk), divided by type of demanded service. The numbers on the y -axis represent the study references. Abbreviations: AD: ambulance dispatch; EDV: emergency department visit. RR * represents the respective RR upper limits.

3.1. Extreme Events (n = 12)

Twelve papers ( n = 12; 11.4%) correlate CC-related extreme events with psychiatric consultations. The significant results are presented in the following Table 3 , considering a confidence interval of 95%. They are divided by type of service used and by the MH condition. In addition, the disasters responsible for the results can be found after each confidence interval. It is important to point out that the authors often presented a great range of results, given the different lags approached. Therefore, the results displayed in Table 3 represent the highest impact found by each of them, positive or negative.

Extreme events.

Main results, divided by type of service used and, still, by the MH condition that was demanded. Results presented in red are the significant negative associations. * Statistically significant result (CI 95%). Abbreviations: EDV—emergency department visit, HA—hospital admission, RR—relative risk, IRR—incidence rate ratio.

The disasters were hurricane ( n = 4), drought or dry weather ( n = 3), floods ( n = 4), extreme precipitation or extreme wet weather ( n = 2), bushfire ( n = 1), earthquake ( n = 1), severe winter weather or ice storm ( n = 2), dust storm ( n = 1), tornado ( n = 1), and severe storms ( n = 1).

In the aftermath of these extreme events, the most strongly affected MH conditions were suicide, mood—which includes Major Depressive Disorder—and neurotic disorders (anxiety, post-traumatic stress disorder, etc.). Therefore, these are the main responsible causes for the acute health service demand, showed by the studies. Substance misuse, schizophrenia and behavioral disorder were approached by fewer studies, but also showed to demand medical care.

The detailed results are described below (i–vi). They were divided in ICD-10th groups and placed according to the strength of evidence.

• (i) Suicide behavior ( n = 9)

Suicide behavior (attempt or completed suicide) showed the most consistent increase after CC-related extreme events among all studies. From the nine articles, only one showed no statistically significant increment on suicide [ 136 ].

Also, when compared to other MH conditions, suicide showed the highest risk ratio for EDV immediately after the disaster (1.68 *, 1.54–1.82), and the risk continued to rise during the whole period of study, until three years later [ 126 ].

One study found a relative increase of suicide behavior only during extreme wet weather (18.7% *, 6.2–31.2) and not during the dry [ 133 ]. In addition, farmers were particularly searched: one paper showed relative risk of 1.15 * (1.08–1.22) in male farmers when drought intensified [ 135 ]. In contrast to males, the risk for female farmers was significantly reduced [ 135 ]. In another study, the general population showed a RR of 4.23 * (1.28–13.93) due to moderate drought [ 134 ].

Lastly, it was reported that extreme large events increased the incidence of suicide, but in case of less damaging ones, the incidence decreased in the aftermath [ 131 ].

• (ii) Mood disorders ( n = 4)

The four papers found significant increase on mental health services demand for mood disorders. One showed a risk ratio for EDV of 1.59 * (1.39–1.80) right after the disaster, but it rapidly declined. On the other hand, HA appeared significantly higher one year later and, after three years, the risk ratio of hospitalization was still 1.67 * (1.47–1.87) [ 126 ].

Also, Major Depressive Disorder (MDD) was the only subtype analyzed. One study showed an incidence rate ratio of 2.57 * (1.60–4.14) of MDD in psychological services [ 126 ]. The two others pointed an increase in service use varying from 44% [ 15 ] to 245% [ 129 ]. MDD imposed a greater chance of medical need when compared also to anxiety disorders (see next section).

• (iii) Neurotic disorders ( n = 4)

All of the studies that analyzed anxiety disorders ( n = 4) found a positive correlation, showing an important increase in health services demand. One found 267 and 307% increase in incidence for elderly and non-elderly, respectively [ 129 ], while another one found an incidence rate ratio of psychological services of 2.06 * (1.21–3.49) [ 126 ]. One paper showed that EDV were not significant or even reduced until three years after a disaster, but contrarily, HA increased during most of the period, and two years later the relative risk was 1.15 (1.11–1.20) [ 126 ].

Adjustment disorder was not consistent. One study did not find any increase both for EDV and HA [ 126 ]. In opposition, two others pointed 62% more Medicaid use [ 15 ] and 160–200% incidence in health centres [ 129 ]. Only one paper studied post-traumatic stress disorder (PTSD) specifically, and the incidence for non-elderly in health services was 350% higher [ 129 ].

• (iv) Substance misuse ( n = 2)

For elderly, the risk ratio for EDV due to substance misuse was the second highest, only lower than suicide behavior. Immediately after the disaster, the risk ratio was 1.44 * (1.23–1.65), reduced after three months, but increased again, maintaining high levels until three years after the extreme event (1.19 *, 1.11–1.26). Hospitalizations appeared three months 1.12 * (1.04–1.21) and were significant until one year, then, reducing again [ 126 ]. One studied that analyzed Medicaid services in the USA showed 66% increase in services use, when comparing the numbers before and after an important flood [ 15 ].

• (v) Schizophrenia ( n = 1)

Schizophrenia (HA) was correlated with extreme precipitation. The result was positive for both urban and rural areas. The relative risk increased already at lag1 (1.056 *, 1.003–1.110), peaked at lag8 (1.072 *, 1.033–1.113), and remained significant until lag17 (1.039 *, 1.004–1.075). The fraction of HA attributable to extreme precipitation was 3.42% * (2.40–4.06). Still, this number was higher for men aged less than 39 years old [ 127 ].

• (vi) Behavioral disorders ( n = 1)

Health services in general received a higher incidence of 318 and 356% for non-elderly and elderly, respectively, due to insomnia (behavioral disorder) [ 127 ].

3.2. Meteorological Factors (n = 93)

Temperature was by far the most approached measure ( n = 92; 86.6%) and, therefore, it will be our focus on this section ( Figure 5 ). The ‘warmer than average’ factor was usually reported as high percentiles in relation to the threshold ( Supplementary Materials—Table S3 ). Heatwave (HW) was also commonly analyzed ( n = 18; 17.1%). Different definitions of HW were used, varying in terms of intensity (temperature) and duration. Five studies opted for the ‘diurnal temperature range’, whose definition is the numerical difference between the highest and lowest temperature values in 24 h. Other temperature-related factors were temperature variability, temperature change between neighboring days, daily excess hourly heat, and irregular daily variation. Finally, one study analyzed only snow and rain—among the CC sensitive variables [ 81 ], and another one included rainfall in the analysis [ 84 ].

Proportion of meteorological factors used by the authors. ‘Warmer than average’ was usually reported by the authors as high percentiles in relation to a threshold. * Other temperature variables: temperature variability (TV), temperature change between neighboring days (TCN), daily excess hourly heat (DEHH), irregular daily variation.

The most strongly affected specific MH conditions were suicide behavior; schizophrenia; substance misuse; organic, mood and neurotic disorders; and, lastly, mortality. The Figure 6 shows the proportion of significant findings, divided by disease subgroups, considering a confidence interval of 95%.

Heat and MH according to the percentage of studies found to be statistically significant (CI 95%).

The majority of studies found significant and positive evidence that MH diseases in general increased the risk for health service need, due to hot weather. The most studied services were HA, EDV, suicide, and mortality. One study [ 34 ] found that approximately 14% of all the emergency admission numbers for mental disorders were attributable to hot temperatures. The delayed or lagged effects showed an increase, according to different studies, until 5 days (RR 1.43 *) [ 46 ], 7 days (RR 1.28 *) [ 51 ], 11 days (RR 1.84 *) [ 54 ] and 14 days (Percent change in risk of 22% *) [ 60 ]. Likewise, the same-day effect was significant: RR 1.158 * for EDV [ 34 ] and 1.229 * for HA [ 52 ].

A longer HW (7 days) imposed a higher risk (RR 1.36 *) of HA than a 3-day (RR 1.15 *) and a 1-day HW (RR 1.04 *) [ 38 ]. According to one other study [ 59 ], HW intensity seemed to cause an even higher risk—62% (RR 1.62 *)—of hospitalization.

A study in China, during heatwave periods, pointed to important subgroups that were at higher risk of MH care need: residents of urban areas (OR 1.523 *, 1.233–2.349), outdoor workers (OR 1.714 *, 1.198–2.398), and single patients (OR 1.709 *, 1.233–2.349) [ 47 ].

Mortality was approached in thirteen studies, and eleven of them presented evidence on how MH is a risk factor for dying on hot days. A 1 °C increment in temperature increased the odds of dying in the general population and in MH patients by 1.9% (OR 1.019 *) and 5.5% (OR 1.055 *), respectively [ 88 ]. HW were often explored, and the percent change in cause was 29.7% * [ 95 ]. Four studies pointed out that elderly patients were more vulnerable [ 34 , 35 , 38 , 92 ]. One study found that poverty enhanced the risk of mortality during hot days [ 86 ].

Finally, Table 4 displays the main results. The significant numbers can be linked to which type of service was demanded and due to in which disorder group this demand took place.

Meteorological factors.

Main results, divided by type of service used and, still, by the MH condition that was demanded. * Statistically significant result (CI 95%). Abbreviations: EDV—emergency department visit, HA—hospital admission, RR—relative risk, IRR—incidence risk ratio, OR—odds ratio, RMR—relative mortality ratio.

Despite being included in the analysis, disorders of adult personality ( n = 5) were not found to be related to heat. Behavioral disorders ( n = 6) showed no consistent result, and five out of the six studies found no relationship between those variables, while one could point a positive correlation between temperature (PET) and admission numbers [ 49 ]. Additionally, only one study found that mental retardation ( n = 4) was affected by HW [ 38 ].

• (i) Suicide behavior ( n = 33)

Suicide was the most studied outcome, and approximately 94% of the papers ( n = 31) found a statistically significant relationship between temperature and suicide behavior: attempted or completed. Exceptionally, two of them evidenced a reduced risk of suicide during extreme hot temperatures, with a RR of 0.97 * (0.95—0.99) [ 90 ] and average decrease (according to the authors) of IRR 0.94 * between male and females [ 97 ], while the others pointed that heat increased suicide behavior. Studies showed a large range of significance: from IRR 1.06 * (1.01–1.12) [ 37 ] to RR 1.35 * [ 111 ]. Females had a higher risk, according to one study [ 102 ].

Heat increased violent suicide attempts and completed suicide, but not the non-violent behaviors in three studies [ 104 , 116 , 117 ]. One other study showed evidence that both methods were significantly higher, but still pointed out that the temperature threshold associated with increasing violent methods was lower (30.3 °C) than the temperature needed to significantly increase non-violent suicides (32.7 °C) [ 103 ].

Ambulance dispatch was measured by one study, during different intensities of heatwave, and the RR was found to be relatively high, varying from 3.70 * (1.00–13.66) to 4.53 * (1.23–16.68) depending on the HW intensity [ 115 ]. Another ambulance dispatch-based study showed a lower, but still positive, number: RR 1.11 * (1.07–1.15) [ 98 ].

Still, one study showed the indirect effect of temperature (increment of 1 °C) on a 3.6% decline of agricultural productivity, leading to a 4.8% increase in risk of suicide among farmers [ 108 ].

• (ii) Schizophrenia ( n = 24)

Seventeen studies (65.38%) found positive and significant results, showing these patients are more vulnerable to heat and, therefore, have a higher chance of needing healthcare services. Two studies presented both same-day and cumulative risk assessments. They found a higher risk when the heat was sustained up to seven or eight days (RR 2.49 *, 1.69–3.69 and 1.37 *, 1.168–1.614), while one-day heat distress brought relatively less risk: RR 1.10 * (1.03–1.17) and 1.06 * (1.019–1.106), respectively [ 45 , 71 ]. To mortality, schizophrenia was not significantly related.

In addition, it seems, from another study, that the heat intensity of a HW brings more risk than its duration, with a RR of 1.50 * (1.20–1.86) and RR:1.14 * (1.01–1.30), respectively [ 59 ].

Finally, there is also evidence that large temperature variation within the day (maximum minus minimum T) may increase hospital admission up to 22% (RR 1.22 *, 1.08–1.37) [ 76 ].

• (iii) Mood disorders ( n = 23)

Sixty-two percent of the papers ( n = 15) presented evidence of the influence of temperature on outpatient appointments (OR 1.32 *, 1.08–1.62) [ 36 ], emergency department visits (IRR 1.07 *, 1.05–1.09; RR 1.05 *, 1.01–1.09; and RR 1.33 *, 1.03–1.71) [ 37 , 39 , 51 ], and hospitalization (RR 1.34 *, 1.05–1.71) [ 35 ]. One showed that rainfall has negative effect on mania [ 84 ].

From the six studies that specifically researched bipolar disorder or mania, four of them found positive correlations with an increase in healthcare demand [ 80 , 81 , 82 , 83 ], while two did not present significant results [ 84 , 85 ].

Depression, another type of mood disorder, was not found to be at significantly higher risk from HA, according to one paper [ 83 ]. Contrarily, long-term exposure to higher temperatures (residing in areas with average temperatures above 23 °C) increased the risk by 7% of developing MDD per 1 °C of increment (hazard ratio 1.07 *, 1.02–1.12) [ 79 ].

• (iv) Organic disorders ( n = 17)

Thirteen of the studies (76.47%) that analyzed organic disorders found a positive association between temperature and medical needs or even death. Patients with organic disorders had 31% more risk of being admitted to an emergency department [ 51 ] and an odds ratio of 8.33 * of being hospitalized [ 68 ]. The risk of mortality was 3% higher [ 11 ].

Alzheimer’s disease was specifically studied and showed a significant RR of 1.30 * (1.12–1.52) [ 65 ] for hospital admission and an increase in RR of 269% * (76–665%) [ 66 ] for mortality.

• (v) Substance misuse ( n = 16)

Twelve papers (70.58%) presented statistically significant association between temperature and HA or EDV, where the RR varied from 1.13 * (1.00, 1.27) for hospitalization [ 35 ] and 1.30 * (1.18–1.42) for emergency visits [ 51 ]. For intense HW, the RR was even higher, displaying 221% * higher risk of hospitalization [ 59 ].

Phone calls requiring healthcare for substance misuse were also measured: the RR was 1.08 * (1.03–1.14) [ 63 ], and the risk of death increased 8% in patients diagnosed with alcohol misuse. In those who had used other substances (excluding alcohol), the risk increased to 20% [ 11 ].

• (vi) Neurotic and anxiety disorders ( n = 14)

Eleven out of fourteen studies (78.57%) pointed out that neurotic disorders influenced healthcare demand due to hot temperatures. HA did not appear to be influenced [ 35 ], but for EDV, the RR varied from 1.05 * (1.002–1.099) [ 46 ] to 1.27 * (1.19–1.36) [ 51 ].

The odds that patients scheduled a medical appointment after a cumulative heat exposure was 30% higher (OR 1.30 *, 1.08–1.58) even at lag 9, suggesting that not-emergency services, such as scheduled appointments, may receive a delayed demand [ 36 ].

• (vii) Behavioral and emotional disorders with onset during childhood/adolescence ( n = 4)

Two of the studies pointed a higher risk of healthcare need, with an IRR of 1.11 * (1.05–1.18) for EDV [ 37 ] and RR 1.29 * (1.09–1.54) [ 55 ] for HA. A third study, in contrast, reported an important reduction of the number of emergency room visits after a heatwave: IRR 0.578 * (0.349–0.955) [ 42 ].

• (viii) Disorders of psychological development ( n = 2)

The autism spectrum disorders were found to cause additional susceptibility to heat and, therefore, demand emergency department visits (IRR 1.64 *, 1.086–2.480) [ 42 ] and hospital admissions (RR 1.29 *, 1.09–1.54) [ 55 ], with relatively elevated risk.

4. Discussion

The findings from the 105 studies pointed to a high probability, according to strong evidence, that more consultations will be necessary in public and private systems. We could not find any evidence supporting that the distinct types of health services are differently affected. Still, ambulance dispatch, telephone calls, and outpatient visits were seldomly measured and need to be better addressed in the future, but were, likewise, positively associated with CC variables. Completed suicide and mortality were strongly affected. Even though these two conditions do not require common consultations at health facilities, they do demand other sources of assistance to be carried out.

We identified that elderly people—above 60 years of age—were among the most vulnerable group for both extreme events and meteorological conditions [ 35 , 38 ]. Poverty was found to enhance the risk of heat-related death, while male farmers were particularly at risk due to extreme events. Residents of urban areas, outdoor workers, and single patients were also reported to be at higher risk [ 47 ]. The increase in health service demand was most significant until a lag 5–7 [ 45 , 46 ], but some studies show the effect up to 14 [ 60 ], 21 [ 41 ] and even 30 days [ 45 ].

We also found that a great range of psychiatric disorders, mostly from F50 to F99, were not properly approached, and that the scientific evidence for them is still poor. Even though these conditions are less prevalent at healthcare facilities, future attention and research on the topic are needed.

One specific disease that received less attention than expected was post-traumatic stress disorder (PTSD). Despite being broadly discussed, only one study focused objectively on if PTSD demands healthcare after an extreme event (129). This important gap of knowledge should be addressed in the future, as well.

The lack of extreme event-related studies that addressed specifically health services use was another identified gap in the scientific evidence. Most of the studies about extreme events measured the distress over the affected population through questionnaires in the communities, instead of through health assistance needs. These methodologies were removed from this review, because even though they corroborate the important mental distress these people go through, they do not necessarily reflect long-lasting suffering and more consultations in health systems. As found by Lai et al. (2021) [ 26 ], most of the symptoms in the direct aftermath will not become chronic and burdening diseases. Therefore, it is likely they do not demand intense healthcare after all.

When comparing the consultation rates in low-, middle- and high-income countries, we did not identify any significant difference, meaning that the increase rate was similar among them. We hypothesized that, given the higher vulnerabilities present in poorer countries, the numbers found would possibly be larger if their health systems were adequately prepared to receive more patients. For a proper comparison, more studies addressing healthcare use in poorer countries are needed.

Few studies found negative associations linking CC to healthcare use [ 90 , 97 , 126 ]. Begum et al. (2022) linked Hurricane Sandy to less EDV for anxiety. Contrarily though, for HA, the same article reported a significant increase. We hypothesize that the so called ‘harvesting effect’ may be responsible for a forward shift in one category of healthcare visits, inducing a consequent reduction in the others.

This increase in health demand will likely overwhelm health systems and pressure costs of healthcare. As an example, one article using Medicaid data pointed out 8–10% more costs after a flood [ 19 ], especially as extreme events become more frequent and temperatures rise even more. According to the United Nations Office for Disaster Risk Reduction, there were 389 climate-related events in 2020, a number above the previous annual average (368). In addition, one of the latest available reports shows a probable increase in temperatures of 2.4 °C by 2100, provided that all countries implement their nationally determined contributions (NCD) [ 137 , 138 ].

In addition to the financial costs, this acute increase in demand can create an emergency situation in health facilities, with shortage of drugs, beds, human services, and other basic resources. In a much bigger proportion, this emergency picture happened during the coronavirus pandemic, mostly in less prepared regions.

Furthermore, the carbon footprint released from the healthcare services is, already nowadays, one of the biggest contributors of CC [ 139 ]. The data we report here corroborate that these services will be more intensively utilized, leading to a self-reinforcement cycle that promotes proportionally greater greenhouse gas emissions into the atmosphere.

Some characteristics of mental health patients—directly or externally related to the disorder—may lead to this higher healthcare demand. The responsible mechanisms for that are important to be delineated. The aim is to point out a pathway to policymakers and medical professionals where policies could effectively act, avoiding consultations for development of a new disorder, for disease exacerbation, heat-related disorders, suicide attempts, and, still, prevent mortality.

Firstly, climate-related events frequently cause mental suffering. These circumstances may lead to punctual and self-limited mental distress or to a chronic disorder [ 10 , 19 ]. In general, the two possible scenarios are determined by a community’s pre-existing vulnerability and by the adaptation and recovery power delivered through public policies [ 140 ].

Furthermore, heat-related vulnerability is promoted by some psychiatric disorders through inadequate heat dissipation, which facilitates a dangerous state of hyperthermia in the body [ 141 ]. Still, additional heat intolerance is caused by the use of certain psychotropic medications—antipsychotics, antiparkinsonians, hypnotics, anxiolytics, and opiates [ 12 , 13 , 117 ]. A further mechanism is related to disease exacerbation (acute phase of a disorder). It seems that neurotransmitters’ imbalance, induced by different weather variables—temperature, humidity, and luminosity—leads to the exacerbation of some disorders or to the triggering of violent and suicidal behavior [ 10 , 12 , 22 ]. As an example, an observed condition was suicide attempt, whose association with heat was reported to be of statistical significance. It seems that patients who previously attempted suicide have indeed a higher susceptibility to heat. In one study, differently from the first-attempt patients, who were vulnerable during the warm season only, the patients who had tried suicide multiple times were at higher risk during the entire year, in cases in which the temperature rose 5 °C [ 121 ].

Additionally, the cognition impairment caused by some mental disorders may also increase heat-susceptibility. There may be a lack of perception about the need to go to a cooler area or to take off extra layers of clothes to cool off the body [ 141 ].

At advanced ages, the delay and impairment to control heat maintenance is physiologically explained, since the receptor system and also the areas in the brain responsible for this function lose their efficiency over time, facilitating a higher body temperature [ 141 ].

Mentally ill patients as well as the elderly population have both a higher chance of being social isolated or living in nursing homes, which has also proved to increase risk of heat-related deaths or heat stress [ 11 ].

Similarly, poverty was found to further enhance the risk of heat-related death [ 86 ]. A possible reason for this susceptibility could be poor housing conditions. As previously discussed, low- and middle-income countries accounted for less than one-third of the studies. In addition, these regions rely on a less prepared healthcare system, which, nowadays, cannot accommodate the demand. In the future, in the absence of effective policies, these patients might become undertreated.

Finally, addressing not only the mechanisms surrounding the disorders, but also the socioeconomic vulnerabilities in the background may avoid the acute need for healthcare services.

5. Conclusions

This review is the first one to systematically assess and analyze the impact of CC on psychiatric health systems worldwide.

We found, based on strong evidence, that warmer temperatures, including heatwaves, can increase the demand uniformly for all types of medical care. Based on fewer number of studies, we also found that extreme events increase demand for all types of services. We support the hypothesis that, if public policies act to increase the recovery power of susceptible communities, fewer of these aftermath symptoms—anxiety, fear, and sadness—will become burdening disorders, and, therefore, lower numbers of EDV, HA, etc., will be required.

In this context, we found and listed three main gaps in the literature, which need to be properly addressed by scientific research in the future: (i) the low number of studies from low- and middle-income countries measuring health services utilization due to CC; (ii) the lack of studies carried out in the aftermath of extreme events using objective measures, as HA or EDV; and (iii) the poor approach of less common mental disorders. They are listed from F50–F99, in ICD-10th.

The lack of studies from low- and middle-income countries prevented us from measuring uniformly across the globe how health systems are affected, since the evidence is derived mostly from richer countries. Even though we found no difference of service utilization between low- and middle- when compared to high income-countries, we raise the concern that these assistance numbers from poorer countries are likely to be underrated. This could be explained by the fact that they do not have the same budget allocated to healthcare and that the population may face difficulties—financial, geographical, or transportation—in accessing health facilities.

Considering the continuous increase in numbers of psychiatric patients, individuals and health systems may witness more significant overwhelm than the one presented here, as temperatures rise and extreme events become more frequent. Therefore, additional public policies are the most effective way to help populations adapt, reducing the health impacts and, finally, avoiding the extra demand on public and private healthcare systems worldwide.

6. Strengths and Limitations

The use of mental health services, in terms of extra demand caused by CC, is an important strength. Still, we acknowledge that the use of services requires, firstly, the existence of functional and effective services, and these might not exist in some studied countries. Additionally, cognitive and educational access is needed for a patient’s clarity regarding when to seek healthcare. Therefore, this SLR may not reflect the magnitude of the impact of CC on MH disorders.

It has been pointed out that the lack of studies about extreme events and MH care use did not permit a vast analysis, as was the case for the ‘meteorological section’. There is a need to better elucidate this issue and the difference between the punctual distress after extreme events measured by community visits and, effectively, long-lasting and burdening disease.

Given the extensive number of studies and results we had access to, the forest plot ( Figure 5 ) is based on significant results and on relative risk-based studies only. Despite representing the biggest part of studies, we acknowledge that it may have prevented the reader from having another perspective.

Finally, the use of the ‘NIH Quality Assessment Tool’ is considered, by some authors, as not being the most appropriate tool. We acknowledge that, by having chosen this tool, we might have incurred potential bias. Nonetheless, all efforts were made to avoid this issue.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ijerph20021190/s1 , Figure S1. Mental and other health impacts studied with climate change in PubMed (until 2022).; Table S1. Search terms used on PubMed for the elaboration of Figure S1; Table S2. Detailed search terms and its sources used for this review; Figure S2. International Classification of Diseases 10th revision (ICD-10) with detailed information from the subgroup of the psychiatric chapter. Source; Figure S3. With the detailed information from the 105 retrieved studies; Table S3. Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies.

Funding Statement

This research was supported by a scholarship of the German Catholic Academic Exchange Service (Katholischer Akademischer Ausländer-Dienst).

Author Contributions

J.F.C. developed the research idea with input from R.S., T.M. and P.D.; J.F.C. and A.Y.H. conducted the literature review. J.F.C. wrote the manuscript, with input from A.Y.H. and revisions from R.S., T.M. and P.D. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Informed consent statement, data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

• ENVIRONMENT

What is global warming, explained

The planet is heating up—and fast.

Glaciers are melting , sea levels are rising, cloud forests are dying , and wildlife is scrambling to keep pace. It has become clear that humans have caused most of the past century's warming by releasing heat-trapping gases as we power our modern lives. Called greenhouse gases, their levels are higher now than at any time in the last 800,000 years .

We often call the result global warming, but it is causing a set of changes to the Earth's climate, or long-term weather patterns, that varies from place to place. While many people think of global warming and climate change as synonyms , scientists use “climate change” when describing the complex shifts now affecting our planet’s weather and climate systems—in part because some areas actually get cooler in the short term.

Climate change encompasses not only rising average temperatures but also extreme weather events , shifting wildlife populations and habitats, rising seas , and a range of other impacts. All of those changes are emerging as humans continue to add heat-trapping greenhouse gases to the atmosphere, changing the rhythms of climate that all living things have come to rely on.

What will we do—what can we do—to slow this human-caused warming? How will we cope with the changes we've already set into motion? While we struggle to figure it all out, the fate of the Earth as we know it—coasts, forests, farms, and snow-capped mountains—hangs in the balance.

Understanding the greenhouse effect

The "greenhouse effect" is the warming that happens when certain gases in Earth's atmosphere trap heat . These gases let in light but keep heat from escaping, like the glass walls of a greenhouse, hence the name.

Sunlight shines onto the Earth's surface, where the energy is absorbed and then radiate back into the atmosphere as heat. In the atmosphere, greenhouse gas molecules trap some of the heat, and the rest escapes into space. The more greenhouse gases concentrate in the atmosphere, the more heat gets locked up in the molecules.

Scientists have known about the greenhouse effect since 1824, when Joseph Fourier calculated that the Earth would be much colder if it had no atmosphere. This natural greenhouse effect is what keeps the Earth's climate livable. Without it, the Earth's surface would be an average of about 60 degrees Fahrenheit (33 degrees Celsius) cooler.

A polar bear stands sentinel on Rudolf Island in Russia’s Franz Josef Land archipelago, where the perennial ice is melting.

In 1895, the Swedish chemist Svante Arrhenius discovered that humans could enhance the greenhouse effect by making carbon dioxide , a greenhouse gas. He kicked off 100 years of climate research that has given us a sophisticated understanding of global warming.

Levels of greenhouse gases have gone up and down over the Earth's history, but they had been fairly constant for the past few thousand years. Global average temperatures had also stayed fairly constant over that time— until the past 150 years . Through the burning of fossil fuels and other activities that have emitted large amounts of greenhouse gases, particularly over the past few decades, humans are now enhancing the greenhouse effect and warming Earth significantly, and in ways that promise many effects , scientists warn.

Aren't temperature changes natural?

Human activity isn't the only factor that affects Earth's climate. Volcanic eruptions and variations in solar radiation from sunspots, solar wind, and the Earth's position relative to the sun also play a role. So do large-scale weather patterns such as El Niño .

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But climate models that scientists use to monitor Earth’s temperatures take those factors into account. Changes in solar radiation levels as well as minute particles suspended in the atmosphere from volcanic eruptions , for example, have contributed only about two percent to the recent warming effect. The balance comes from greenhouse gases and other human-caused factors, such as land use change .

The short timescale of this recent warming is singular as well. Volcanic eruptions , for example, emit particles that temporarily cool the Earth's surface. But their effect lasts just a few years. Events like El Niño also work on fairly short and predictable cycles. On the other hand, the types of global temperature fluctuations that have contributed to ice ages occur on a cycle of hundreds of thousands of years.

For thousands of years now, emissions of greenhouse gases to the atmosphere have been balanced out by greenhouse gases that are naturally absorbed. As a result, greenhouse gas concentrations and temperatures have been fairly stable, which has allowed human civilization to flourish within a consistent climate.

Greenland is covered with a vast amount of ice—but the ice is melting four times faster than thought, suggesting that Greenland may be approaching a dangerous tipping point, with implications for global sea-level rise.

Now, humans have increased the amount of carbon dioxide in the atmosphere by more than a third since the Industrial Revolution. Changes that have historically taken thousands of years are now happening over the course of decades .

Why does this matter?

The rapid rise in greenhouse gases is a problem because it’s changing the climate faster than some living things can adapt to. Also, a new and more unpredictable climate poses unique challenges to all life.

Historically, Earth's climate has regularly shifted between temperatures like those we see today and temperatures cold enough to cover much of North America and Europe with ice. The difference between average global temperatures today and during those ice ages is only about 9 degrees Fahrenheit (5 degrees Celsius), and the swings have tended to happen slowly, over hundreds of thousands of years.

But with concentrations of greenhouse gases rising, Earth's remaining ice sheets such as Greenland and Antarctica are starting to melt too . That extra water could raise sea levels significantly, and quickly. By 2050, sea levels are predicted to rise between one and 2.3 feet as glaciers melt.

As the mercury rises, the climate can change in unexpected ways. In addition to sea levels rising, weather can become more extreme . This means more intense major storms, more rain followed by longer and drier droughts—a challenge for growing crops—changes in the ranges in which plants and animals can live, and loss of water supplies that have historically come from glaciers.

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The Science of Climate Change Explained: Facts, Evidence and Proof

Definitive answers to the big questions.

Credit... Photo Illustration by Andrea D'Aquino

Supported by

By Julia Rosen

Ms. Rosen is a journalist with a Ph.D. in geology. Her research involved studying ice cores from Greenland and Antarctica to understand past climate changes.

• Published April 19, 2021 Updated Nov. 6, 2021

The science of climate change is more solid and widely agreed upon than you might think. But the scope of the topic, as well as rampant disinformation, can make it hard to separate fact from fiction. Here, we’ve done our best to present you with not only the most accurate scientific information, but also an explanation of how we know it.

How do we know climate change is really happening?

• How much agreement is there among scientists about climate change?
• Do we really only have 150 years of climate data? How is that enough to tell us about centuries of change?
• How do we know climate change is caused by humans?
• Since greenhouse gases occur naturally, how do we know they’re causing Earth’s temperature to rise?
• Why should we be worried that the planet has warmed 2°F since the 1800s?
• Is climate change a part of the planet’s natural warming and cooling cycles?
• How do we know global warming is not because of the sun or volcanoes?
• How can winters and certain places be getting colder if the planet is warming?
• Wildfires and bad weather have always happened. How do we know there’s a connection to climate change?
• How bad are the effects of climate change going to be?
• What will it cost to do something about climate change, versus doing nothing?

Climate change is often cast as a prediction made by complicated computer models. But the scientific basis for climate change is much broader, and models are actually only one part of it (and, for what it’s worth, they’re surprisingly accurate ).

For more than a century , scientists have understood the basic physics behind why greenhouse gases like carbon dioxide cause warming. These gases make up just a small fraction of the atmosphere but exert outsized control on Earth’s climate by trapping some of the planet’s heat before it escapes into space. This greenhouse effect is important: It’s why a planet so far from the sun has liquid water and life!

However, during the Industrial Revolution, people started burning coal and other fossil fuels to power factories, smelters and steam engines, which added more greenhouse gases to the atmosphere. Ever since, human activities have been heating the planet.

Where it was cooler or warmer in 2020 compared with the middle of the 20th century

Global average temperature compared with the middle of the 20th century

+0.75°C

–0.25°

30 billion metric tons

Carbon dioxide emitted worldwide 1850-2017

Rest of world

Other developed

European Union

Developed economies

Other countries

United States

E.U. and U.K.

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There is unequivocal evidence that Earth is warming at an unprecedented rate. Human activity is the principal cause.

• While Earth’s climate has changed throughout its history , the current warming is happening at a rate not seen in the past 10,000 years.
• According to the Intergovernmental Panel on Climate Change ( IPCC ), "Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact." 1
• Scientific information taken from natural sources (such as ice cores, rocks, and tree rings) and from modern equipment (like satellites and instruments) all show the signs of a changing climate.
• From global temperature rise to melting ice sheets, the evidence of a warming planet abounds.

The rate of change since the mid-20th century is unprecedented over millennia.

Earth's climate has changed throughout history. Just in the last 800,000 years, there have been eight cycles of ice ages and warmer periods, with the end of the last ice age about 11,700 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives.

The current warming trend is different because it is clearly the result of human activities since the mid-1800s, and is proceeding at a rate not seen over many recent millennia. 1 It is undeniable that human activities have produced the atmospheric gases that have trapped more of the Sun’s energy in the Earth system. This extra energy has warmed the atmosphere, ocean, and land, and widespread and rapid changes in the atmosphere, ocean, cryosphere, and biosphere have occurred.

Earth-orbiting satellites and new technologies have helped scientists see the big picture, collecting many different types of information about our planet and its climate all over the world. These data, collected over many years, reveal the signs and patterns of a changing climate.

Scientists demonstrated the heat-trapping nature of carbon dioxide and other gases in the mid-19th century. 2 Many of the science instruments NASA uses to study our climate focus on how these gases affect the movement of infrared radiation through the atmosphere. From the measured impacts of increases in these gases, there is no question that increased greenhouse gas levels warm Earth in response.

Scientific evidence for warming of the climate system is unequivocal.

Intergovernmental Panel on Climate Change

Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that Earth’s climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly 10 times faster than the average rate of warming after an ice age. Carbon dioxide from human activities is increasing about 250 times faster than it did from natural sources after the last Ice Age. 3

The Evidence for Rapid Climate Change Is Compelling:

Global Temperature Is Rising

The planet's average surface temperature has risen about 2 degrees Fahrenheit (1 degrees Celsius) since the late 19th century, a change driven largely by increased carbon dioxide emissions into the atmosphere and other human activities. 4 Most of the warming occurred in the past 40 years, with the seven most recent years being the warmest. The years 2016 and 2020 are tied for the warmest year on record. 5 Image credit: Ashwin Kumar, Creative Commons Attribution-Share Alike 2.0 Generic.

The Ocean Is Getting Warmer

The ocean has absorbed much of this increased heat, with the top 100 meters (about 328 feet) of ocean showing warming of 0.67 degrees Fahrenheit (0.33 degrees Celsius) since 1969. 6 Earth stores 90% of the extra energy in the ocean. Image credit: Kelsey Roberts/USGS

The Ice Sheets Are Shrinking

The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA's Gravity Recovery and Climate Experiment show Greenland lost an average of 279 billion tons of ice per year between 1993 and 2019, while Antarctica lost about 148 billion tons of ice per year. 7 Image: The Antarctic Peninsula, Credit: NASA

Glaciers Are Retreating

Glaciers are retreating almost everywhere around the world — including in the Alps, Himalayas, Andes, Rockies, Alaska, and Africa. 8 Image: Miles Glacier, Alaska Image credit: NASA

Snow Cover Is Decreasing

Satellite observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and the snow is melting earlier. 9 Image credit: NASA/JPL-Caltech

Sea Level Is Rising

Global sea level rose about 8 inches (20 centimeters) in the last century. The rate in the last two decades, however, is nearly double that of the last century and accelerating slightly every year. 10 Image credit: U.S. Army Corps of Engineers Norfolk District

Arctic Sea Ice Is Declining

Both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades. 11 Credit: NASA's Scientific Visualization Studio

Extreme Events Are Increasing in Frequency

The number of record high temperature events in the United States has been increasing, while the number of record low temperature events has been decreasing, since 1950. The U.S. has also witnessed increasing numbers of intense rainfall events. 12 Image credit: Régine Fabri,  CC BY-SA 4.0 , via Wikimedia Commons

Ocean Acidification Is Increasing

Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30%. 13 , 14 This increase is due to humans emitting more carbon dioxide into the atmosphere and hence more being absorbed into the ocean. The ocean has absorbed between 20% and 30% of total anthropogenic carbon dioxide emissions in recent decades (7.2 to 10.8 billion metric tons per year). 1 5 , 16 Image credit: NOAA

1. IPCC Sixth Assessment Report, WGI, Technical Summary . B.D. Santer et.al., “A search for human influences on the thermal structure of the atmosphere.” Nature 382 (04 July 1996): 39-46. https://doi.org/10.1038/382039a0. Gabriele C. Hegerl et al., “Detecting Greenhouse-Gas-Induced Climate Change with an Optimal Fingerprint Method.” Journal of Climate 9 (October 1996): 2281-2306. https://doi.org/10.1175/1520-0442(1996)009<2281:DGGICC>2.0.CO;2. V. Ramaswamy, et al., “Anthropogenic and Natural Influences in the Evolution of Lower Stratospheric Cooling.” Science 311 (24 February 2006): 1138-1141. https://doi.org/10.1126/science.1122587. B.D. Santer et al., “Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes.” Science 301 (25 July 2003): 479-483. https://doi.org/10.1126/science.1084123. T. Westerhold et al., "An astronomically dated record of Earth’s climate and its predictability over the last 66 million years." Science 369 (11 Sept. 2020): 1383-1387. https://doi.org/10.1126/science.1094123

2. In 1824, Joseph Fourier calculated that an Earth-sized planet, at our distance from the Sun, ought to be much colder. He suggested something in the atmosphere must be acting like an insulating blanket. In 1856, Eunice Foote discovered that blanket, showing that carbon dioxide and water vapor in Earth's atmosphere trap escaping infrared (heat) radiation. In the 1860s, physicist John Tyndall recognized Earth's natural greenhouse effect and suggested that slight changes in the atmospheric composition could bring about climatic variations. In 1896, a seminal paper by Swedish scientist Svante Arrhenius first predicted that changes in atmospheric carbon dioxide levels could substantially alter the surface temperature through the greenhouse effect. In 1938, Guy Callendar connected carbon dioxide increases in Earth’s atmosphere to global warming. In 1941, Milutin Milankovic linked ice ages to Earth’s orbital characteristics. Gilbert Plass formulated the Carbon Dioxide Theory of Climate Change in 1956.

3. IPCC Sixth Assessment Report, WG1, Chapter 2 Vostok ice core data; NOAA Mauna Loa CO2 record O. Gaffney, W. Steffen, "The Anthropocene Equation." The Anthropocene Review 4, issue 1 (April 2017): 53-61. https://doi.org/abs/10.1177/2053019616688022.

4. https://www.ncei.noaa.gov/monitoring https://crudata.uea.ac.uk/cru/data/temperature/ http://data.giss.nasa.gov/gistemp

5. https://www.giss.nasa.gov/research/news/20170118/

6. S. Levitus, J. Antonov, T. Boyer, O Baranova, H. Garcia, R. Locarnini, A. Mishonov, J. Reagan, D. Seidov, E. Yarosh, M. Zweng, " NCEI ocean heat content, temperature anomalies, salinity anomalies, thermosteric sea level anomalies, halosteric sea level anomalies, and total steric sea level anomalies from 1955 to present calculated from in situ oceanographic subsurface profile data (NCEI Accession 0164586), Version 4.4. (2017) NOAA National Centers for Environmental Information. https://www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/index3.html K. von Schuckmann, L. Cheng, L,. D. Palmer, J. Hansen, C. Tassone, V. Aich, S. Adusumilli, H. Beltrami, H., T. Boyer, F. Cuesta-Valero, D. Desbruyeres, C. Domingues, A. Garcia-Garcia, P. Gentine, J. Gilson, M. Gorfer, L. Haimberger, M. Ishii, M., G. Johnson, R. Killick, B. King, G. Kirchengast, N. Kolodziejczyk, J. Lyman, B. Marzeion, M. Mayer, M. Monier, D. Monselesan, S. Purkey, D. Roemmich, A. Schweiger, S. Seneviratne, A. Shepherd, D. Slater, A. Steiner, F. Straneo, M.L. Timmermans, S. Wijffels. "Heat stored in the Earth system: where does the energy go?" Earth System Science Data 12, Issue 3 (07 September 2020): 2013-2041. https://doi.org/10.5194/essd-12-2013-2020.

7. I. Velicogna, Yara Mohajerani, A. Geruo, F. Landerer, J. Mouginot, B. Noel, E. Rignot, T. Sutterly, M. van den Broeke, M. Wessem, D. Wiese, "Continuity of Ice Sheet Mass Loss in Greenland and Antarctica From the GRACE and GRACE Follow-On Missions." Geophysical Research Letters 47, Issue 8 (28 April 2020): e2020GL087291. https://doi.org/10.1029/2020GL087291.

8. National Snow and Ice Data Center World Glacier Monitoring Service

9. National Snow and Ice Data Center D.A. Robinson, D. K. Hall, and T. L. Mote, "MEaSUREs Northern Hemisphere Terrestrial Snow Cover Extent Daily 25km EASE-Grid 2.0, Version 1 (2017). Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/MEASURES/CRYOSPHERE/nsidc-0530.001 . http://nsidc.org/cryosphere/sotc/snow_extent.html Rutgers University Global Snow Lab. Data History

10. R.S. Nerem, B.D. Beckley, J. T. Fasullo, B.D. Hamlington, D. Masters, and G.T. Mitchum, "Climate-change–driven accelerated sea-level rise detected in the altimeter era." PNAS 15, no. 9 (12 Feb. 2018): 2022-2025. https://doi.org/10.1073/pnas.1717312115.

11. https://nsidc.org/cryosphere/sotc/sea_ice.html Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS, Zhang and Rothrock, 2003) http://psc.apl.washington.edu/research/projects/arctic-sea-ice-volume-anomaly/ http://psc.apl.uw.edu/research/projects/projections-of-an-ice-diminished-arctic-ocean/

12. USGCRP, 2017: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 470 pp, https://doi.org/10.7930/j0j964j6 .

13. http://www.pmel.noaa.gov/co2/story/What+is+Ocean+Acidification%3F

14. http://www.pmel.noaa.gov/co2/story/Ocean+Acidification

15. C.L. Sabine, et al., “The Oceanic Sink for Anthropogenic CO2.” Science 305 (16 July 2004): 367-371. https://doi.org/10.1126/science.1097403.

16. Special Report on the Ocean and Cryosphere in a Changing Climate , Technical Summary, Chapter TS.5, Changing Ocean, Marine Ecosystems, and Dependent Communities, Section 5.2.2.3. https://www.ipcc.ch/srocc/chapter/technical-summary/

Header image shows clouds imitating mountains as the sun sets after midnight as seen from Denali's backcountry Unit 13 on June 14, 2019. Credit: NPS/Emily Mesner Image credit in list of evidence: Ashwin Kumar, Creative Commons Attribution-Share Alike 2.0 Generic.

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