Climate change

In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's

Climate change has an increasingly large impact on the environment. Deserts are expanding, while heat waves and wildfires are becoming more common.^[7][8] Amplified warming in the Arctic has contributed to thawing permafrost, retreat of glaciers and sea ice decline.^[9] Higher temperatures are also causing more intense storms, droughts, and other weather extremes.^[10] Rapid environmental change in mountains, coral reefs, and the Arctic is forcing many species to relocate or become extinct.^[11] Even if efforts to minimize future warming are successful, some effects will continue for centuries. These include ocean heating, ocean acidification and sea level rise.^[12]

Climate change

Examples of some effects of climate change: Wildfire intensified by heat and drought, bleaching of corals occurring more often due to marine heatwaves, and worsening droughts compromising water supplies.

Many climate change impacts have been felt in recent years, with 2023 the warmest on record at +1.48 °C (2.66 °F) since regular tracking began in 1850.

Before the 1980s it was unclear whether the warming effect of increased greenhouse gases was stronger than the cooling effect of airborne particulates in air pollution. Scientists used the term inadvertent climate modification to refer to human impacts on the climate at this time.^[33] In the 1980s, the terms global warming and climate change became more common, often being used interchangeably.^[34][35][36] Scientifically, global warming refers only to increased surface warming, while climate change describes both global warming and its effects on Earth's climate system, such as precipitation changes.^[33]

Climate change can also be used more broadly to include

Over the last few million years

Temperatures stabilized in the current interglacial period beginning 11,700 years ago.^[49] Historical patterns of warming and cooling, like the Medieval Warm Period and the Little Ice Age, did not occur at the same time across different regions. Temperatures may have reached as high as those of the late 20th century in a limited set of regions.^[50][51] Climate information for that period comes from climate proxies, such as trees and ice cores.^[52][53]

Around 1850 thermometer records began to provide global coverage.^[56] Between the 18th century and 1970 there was little net warming, as the warming impact of greenhouse gas emissions was offset by cooling from sulfur dioxide emissions. Sulfur dioxide causes acid rain, but it also produces sulfate aerosols in the atmosphere, which reflect sunlight and cause so-called global dimming. After 1970, the increasing accumulation of greenhouse gases and controls on sulfur pollution led to a marked increase in temperature.^[57][58][59]

Ongoing changes in climate have had no precedent for several thousand years.

Different regions of the world warm at different rates. The pattern is independent of where greenhouse gases are emitted, because the gases persist long enough to diffuse across the planet. Since the pre-industrial period, the average surface temperature over land regions has increased almost twice as fast as the global average surface temperature.^[77] This is because oceans lose more heat by evaporation and oceans can store a lot of heat.^[78] The thermal energy in the global climate system has grown with only brief pauses since at least 1970, and over 90% of this extra energy has been stored in the ocean.^[79][80] The rest has heated the atmosphere, melted ice, and warmed the continents.^[81]

The Northern Hemisphere and the North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more seasonal snow cover and sea ice. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat.^[82] Local black carbon deposits on snow and ice also contribute to Arctic warming.^[83] Arctic surface temperatures are increasing between three and four times faster than in the rest of the world.^[84][85][86] Melting of ice sheets near the poles weakens both the Atlantic and the Antarctic limb of thermohaline circulation, which further changes the distribution of heat and precipitation around the globe.^{[87][88][89][90]}

The World Meteorological Organization estimates a 66% chance of global temperatures exceeding 1.5 °C warming from the preindustrial baseline for at least one year between 2023 and 2027.^[93][94] Because the IPCC uses a 20-year average to define global temperature changes, a single year exceeding 1.5 °C does not break the limit.

The IPCC expects the 20-year average global temperature to exceed +1.5 °C in the early 2030s.^[95] The IPCC Sixth Assessment Report (2023) included projections that by 2100 global warming is very likely to reach 1.0-1.8 °C under a scenario with very low emissions of greenhouse gases, 2.1-3.5 °C under an intermediate emissions scenario, or 3.3-5.7 °C under a very high emissions scenario.^[96] The warming will continue past 2100 in the intermediate and high emission scenarios,^[97][98] with future projections of global surface temperatures by year 2300 being similar to millions of years ago.^[99]

The remaining carbon budget for staying beneath certain temperature increases is determined by modelling the carbon cycle and climate sensitivity to greenhouse gases.^[100] According to the IPCC, global warming can be kept below 1.5 °C with a two-thirds chance if emissions after 2018 do not exceed 420 or 570 gigatonnes of CO₂. This corresponds to 10 to 13 years of current emissions. There are high uncertainties about the budget. For instance, it may be 100 gigatonnes of CO₂ equivalent smaller due to CO₂ and methane release from permafrost and wetlands.^[101] However, it is clear that fossil fuel resources need to be proactively kept in the ground to prevent substantial warming. Otherwise, their shortages would not occur until the emissions have already locked in significant long-term impacts.^[102]

The climate system experiences various cycles on its own which can last for years, decades or even centuries. For example,

Greenhouse gases are transparent to sunlight, and thus allow it to pass through the atmosphere to heat the Earth's surface. The Earth radiates it as heat, and greenhouse gases absorb a portion of it. This absorption slows the rate at which heat escapes into space, trapping heat near the Earth's surface and warming it over time.^[115]

While

Before the Industrial Revolution, naturally-occurring amounts of greenhouse gases caused the air near the surface to be about 33 °C warmer than it would have been in their absence.^[118][119] Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (coal, oil, and natural gas),^[120] has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. In 2019, the concentrations of CO₂ and methane had increased by about 48% and 160%, respectively, since 1750.^[121] These CO₂ levels are higher than they have been at any time during the last 2 million years. Concentrations of methane are far higher than they were over the last 800,000 years.^[122]

Global anthropogenic greenhouse gas emissions in 2019 were

According to

Aerosols also have indirect effects on the Earth's energy budget. Sulfate aerosols act as cloud condensation nuclei and lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets.^[150] They also reduce the growth of raindrops, which makes clouds more reflective to incoming sunlight.^[151] Indirect effects of aerosols are the largest uncertainty in radiative forcing.^[152]

While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases melting and sea-level rise.^[153] Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.^[154] The effect of decreasing sulfur content of fuel oil for ships since 2020^[155] is estimated to cause an additional 0.05 °C increase in global mean temperature by 2050.^[156]

As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the

Explosive volcanic eruptions can release gases, dust and ash that partially block sunlight and reduce temperatures, or they can send water vapour into the atmosphere, which adds to greenhouse gases and increases temperatures.^[162] These impacts on temperature only last for several years, because both water vapour and volcanic material have low persistence in the atmosphere.^[163] volcanic CO₂ emissions are more persistent, but they are equivalent to less than 1% of current human-caused CO₂ emissions.^[164] Volcanic activity still represents the single largest natural impact (forcing) on temperature in the industrial era. Yet, like the other natural forcings, it has had negligible impacts on global temperature trends since the Industrial Revolution.^[163]

The response of the climate system to an initial forcing is modified by feedbacks: increased by

Around half of human-caused CO₂ emissions have been absorbed by land plants and by the oceans.[176] This fraction is not static and if future CO₂ emissions decrease, the Earth will be able to absorb up to around 70%. If they increase substantially, it'll still absorb more carbon than now, but the overall fraction will decrease to below 40%.^[177] This is because climate change increases droughts and heat waves that eventually inhibit plant growth on land, and soils will release more carbon from dead plants when they are warmer.^[178][179] The rate at which oceans absorb atmospheric carbon will be lowered as they become more acidic and experience changes in thermohaline circulation and phytoplankton distribution.^{[180][181][88]} Uncertainty over feedbacks, particularly cloud cover,^[182] is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.^[183]

A climate model is a representation of the physical, chemical and biological processes that affect the climate system.^[184] Models include natural processes like changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing.^[185] Models are used to estimate the degree of warming future emissions will cause when accounting for the strength of climate feedbacks.^[186][187] Models also predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere.^[188]

The physical realism of models is tested by examining their ability to simulate current or past climates.

A subset of climate models add societal factors to a physical climate model. These models simulate how population, economic growth, and energy use affect—and interact with—the physical climate. With this information, these models can produce scenarios of future greenhouse gas emissions. This is then used as input for physical climate models and carbon cycle models to predict how atmospheric concentrations of greenhouse gases might change.^[195][196] Depending on the socioeconomic scenario and the mitigation scenario, models produce atmospheric CO₂ concentrations that range widely between 380 and 1400 ppm.^[197]

The environmental effects of climate change are broad and far-reaching, affecting oceans, ice, and weather. Changes may occur gradually or rapidly. Evidence for these effects comes from studying climate change in the past, from modelling, and from modern observations.^[198] Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency.^[199] Extremely wet or dry events within the monsoon period have increased in India and East Asia.^[200] Monsoonal precipitation over the Northern Hemisphere has increased since 1980.^[201] The rainfall rate and intensity of hurricanes and typhoons is likely increasing,^[202] and the geographic range likely expanding poleward in response to climate warming.^[203] Frequency of tropical cyclones has not increased as a result of climate change.^[204]

Global sea level is rising as a consequence of

Climate change has led to decades of shrinking and thinning of the Arctic sea ice.^[210] While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur once every three to ten years at a warming level of 2 °C.^[211] Higher atmospheric CO₂ concentrations cause more CO₂ to dissolve in the oceans, which is making them more acidic.^[212] Because oxygen is less soluble in warmer water,^[213] its concentrations in the ocean are decreasing, and dead zones are expanding.^[214]

Greater degrees of global warming increase the risk of passing through '

Recent warming has driven many terrestrial and freshwater species poleward and towards higher

The effects of climate change are impacting humans everywhere in the world.^[245] Impacts can be observed on all continents and ocean regions,^[246] with low-latitude, less developed areas facing the greatest risk.^[247] Continued warming has potentially "severe, pervasive and irreversible impacts" for people and ecosystems.^[248] The risks are unevenly distributed, but are generally greater for disadvantaged people in developing and developed countries.^[249]

The

While total crop yields have been increasing in the past 50 years due to agricultural improvements, climate change has already decreased the rate of yield growth.^[253] Fisheries have been negatively affected in multiple regions.^[253] While agricultural productivity has been positively affected in some high latitude areas, mid- and low-latitude areas have been negatively affected.^[253] According to the World Economic Forum, an increase in drought in certain regions could cause 3.2 million deaths from malnutrition by 2050 and stunting in children.^[258] With 2 °C warming, global livestock headcounts could decline by 7–10% by 2050, as less animal feed will be available.^[259] If the emissions continue to increase for the rest of century, then over 9 million climate-related deaths would occur annually by 2100.^[260]

Economic damages due to climate change may be severe and there is a chance of disastrous consequences.

While women are not inherently more at risk from climate change and shocks, limits on women's resources and discriminatory gender norms constrain their adaptive capacity and resilience.[270] For example, women's work burdens, including hours worked in agriculture, tend to decline less than men's during climate shocks such as heat stress.^[270]

Low-lying islands and coastal communities are threatened by sea level rise, which makes urban flooding more common. Sometimes, land is permanently lost to the sea.^[271] This could lead to statelessness for people in island nations, such as the Maldives and Tuvalu.^[272] In some regions, the rise in temperature and humidity may be too severe for humans to adapt to.^[273] With worst-case climate change, models project that almost one-third of humanity might live in Sahara-like uninhabitable and extremely hot climates.^[274]

These factors can drive

Climate change can be mitigated by reducing the rate at which greenhouse gases are emitted into the atmosphere, and by increasing the rate at which carbon dioxide is removed from the atmosphere.

The United Nations Environment Programme estimates that countries need to triple their pledges under the Paris Agreement within the next decade to limit global warming to 2 °C. An even greater level of reduction is required to meet the 1.5 °C goal.^[283] With pledges made under the Paris Agreement as of October 2021, global warming would still have a 66% chance of reaching about 2.7 °C (range: 2.2–3.2 °C) by the end of the century.^[24] Globally, limiting warming to 2 °C may result in higher economic benefits than economic costs.^[284]

Although there is no single pathway to limit global warming to 1.5 or 2 °C,^[285] most scenarios and strategies see a major increase in the use of renewable energy in combination with increased energy efficiency measures to generate the needed greenhouse gas reductions.^[286] To reduce pressures on ecosystems and enhance their carbon sequestration capabilities, changes would also be necessary in agriculture and forestry,^[287] such as preventing deforestation and restoring natural ecosystems by reforestation.^[288]

Other approaches to mitigating climate change have a higher level of risk. Scenarios that limit global warming to 1.5 °C typically project the large-scale use of carbon dioxide removal methods over the 21st century.^[289] There are concerns, though, about over-reliance on these technologies, and environmental impacts.^[290] Solar radiation modification (SRM) is also a possible supplement to deep reductions in emissions. However, SRM raises significant ethical and legal concerns, and the risks are imperfectly understood.^[291]

Renewable energy is key to limiting climate change.^[293] For decades, fossil fuels have accounted for roughly 80% of the world's energy use.^[294] The remaining share has been split between nuclear power and renewables (including hydropower, bioenergy, wind and solar power and geothermal energy).^[295] Fossil fuel use is expected to peak in absolute terms prior to 2030 and then to decline, with coal use experiencing the sharpest reductions.^[296] Renewables represented 75% of all new electricity generation installed in 2019, nearly all solar and wind.^[297] Other forms of clean energy, such as nuclear and hydropower, currently have a larger share of the energy supply. However, their future growth forecasts appear limited in comparison.^[298]

While solar panels and onshore wind are now among the cheapest forms of adding new power generation capacity in many locations,^[299] green energy policies are needed to achieve a rapid transition from fossil fuels to renewables.^[300] To achieve carbon neutrality by 2050, renewable energy would become the dominant form of electricity generation, rising to 85% or more by 2050 in some scenarios. Investment in coal would be eliminated and coal use nearly phased out by 2050.^[301][302]

Electricity generated from renewable sources would also need to become the main energy source for heating and transport.^[303] Transport can switch away from internal combustion engine vehicles and towards electric vehicles, public transit, and active transport (cycling and walking).^[304][305] For shipping and flying, low-carbon fuels would reduce emissions.^[304] Heating could be increasingly decarbonized with technologies like heat pumps.^[306]

There are obstacles to the continued rapid growth of clean energy, including renewables. For wind and solar, there are environmental and land use concerns for new projects.

Low-carbon energy improves human health by minimizing climate change as well as reducing air pollution deaths,^[312] which were estimated at 7 million annually in 2016.^[313] Meeting the Paris Agreement goals that limit warming to a 2 °C increase could save about a million of those lives per year by 2050, whereas limiting global warming to 1.5 °C could save millions and simultaneously increase energy security and reduce poverty.^[314] Improving air quality also has economic benefits which may be larger than mitigation costs.^[315]

Reducing energy demand is another major aspect of reducing emissions.^[316] If less energy is needed, there is more flexibility for clean energy development. It also makes it easier to manage the electricity grid, and minimizes carbon-intensive infrastructure development.^[317] Major increases in energy efficiency investment will be required to achieve climate goals, comparable to the level of investment in renewable energy.^[318] Several COVID-19 related changes in energy use patterns, energy efficiency investments, and funding have made forecasts for this decade more difficult and uncertain.^[319]

Strategies to reduce energy demand vary by sector. In the transport sector, passengers and freight can switch to more efficient travel modes, such as buses and trains, or use electric vehicles.^[320] Industrial strategies to reduce energy demand include improving heating systems and motors, designing less energy-intensive products, and increasing product lifetimes.^[321] In the building sector the focus is on better design of new buildings, and higher levels of energy efficiency in retrofitting.^[322] The use of technologies like heat pumps can also increase building energy efficiency.^[323]

Agriculture and forestry face a triple challenge of limiting greenhouse gas emissions, preventing the further conversion of forests to agricultural land, and meeting increases in world food demand.^[324] A set of actions could reduce agriculture and forestry-based emissions by two thirds from 2010 levels. These include reducing growth in demand for food and other agricultural products, increasing land productivity, protecting and restoring forests, and reducing greenhouse gas emissions from agricultural production.^[325]

On the demand side, a key component of reducing emissions is shifting people towards

Steel and cement production are responsible for about 13% of industrial CO₂ emissions. In these industries, carbon-intensive materials such as coke and lime play an integral role in the production, so that reducing CO₂ emissions requires research into alternative chemistries.[329]

Natural carbon sinks can be enhanced to sequester significantly larger amounts of CO₂ beyond naturally occurring levels.

Where energy production or CO₂-intensive heavy industries continue to produce waste CO₂, the gas can be captured and stored instead of released to the atmosphere. Although its current use is limited in scale and expensive,^[336] carbon capture and storage (CCS) may be able to play a significant role in limiting CO₂ emissions by mid-century.^[337] This technique, in combination with bioenergy (BECCS) can result in net negative emissions as CO₂ is drawn from the atmosphere.^[338] It remains highly uncertain whether carbon dioxide removal techniques will be able to play a large role in limiting warming to 1.5 °C. Policy decisions that rely on carbon dioxide removal increase the risk of global warming rising beyond international goals.^[339]

Adaptation is "the process of adjustment to current or expected changes in climate and its effects".^[340]: 5 Without additional mitigation, adaptation cannot avert the risk of "severe, widespread and irreversible" impacts.^[341] More severe climate change requires more transformative adaptation, which can be prohibitively expensive.^[342] The capacity and potential for humans to adapt is unevenly distributed across different regions and populations, and developing countries generally have less.^[343] The first two decades of the 21st century saw an increase in adaptive capacity in most low- and middle-income countries with improved access to basic sanitation and electricity, but progress is slow. Many countries have implemented adaptation policies. However, there is a considerable gap between necessary and available finance.^[344]

Adaptation to sea level rise consists of avoiding at-risk areas, learning to live with increased flooding, and building

Ecosystems adapt to climate change, a process that can be supported by human intervention. By increasing connectivity between ecosystems, species can migrate to more favourable climate conditions. Species can also be introduced to areas acquiring a favourable climate. Protection and restoration of natural and semi-natural areas helps build resilience, making it easier for ecosystems to adapt. Many of the actions that promote adaptation in ecosystems, also help humans adapt via ecosystem-based adaptation. For instance, restoration of natural fire regimes makes catastrophic fires less likely, and reduces human exposure. Giving rivers more space allows for more water storage in the natural system, reducing flood risk. Restored forest acts as a carbon sink, but planting trees in unsuitable regions can exacerbate climate impacts.^[352]

There are synergies but also trade-offs between adaptation and mitigation.^[353] An example for synergy is increased food productivity, which has large benefits for both adaptation and mitigation.^[354] An example of a trade-off is that increased use of air conditioning allows people to better cope with heat, but increases energy demand. Another trade-off example is that more compact urban development may reduce emissions from transport and construction, but may also increase the urban heat island effect, exposing people to heat-related health risks.^[355]

Countries that are most vulnerable to climate change have typically been responsible for a small share of global emissions. This raises questions about justice and fairness.^[356] Limiting global warming makes it much easier to achieve the UN's Sustainable Development Goals, such as eradicating poverty and reducing inequalities. The connection is recognized in Sustainable Development Goal 13 which is to "take urgent action to combat climate change and its impacts".^[357] The goals on food, clean water and ecosystem protection have synergies with climate mitigation.^[358]

The

Policy designed through the lens of climate justice tries to address human rights issues and social inequality. According to proponents of climate justice, the costs of climate adaptation should be paid by those most responsible for climate change, while the beneficiaries of payments should be those suffering impacts. One way this can be addressed in practice is to have wealthy nations pay poorer countries to adapt.^[368]

Oxfam found that in 2023 the wealthiest 10% of people were responsible for 50% of global emissions, while the bottom 50% were responsible for just 8%.^[369] Production of emissions is another way to look at responsibility: under that approach, the top 21 fossil fuel companies would owe cumulative climate reparations of $5.4 trillion over the period 2025–2050.^[370] To achieve a just transition, people working in the fossil fuel sector would also need other jobs, and their communities would need investments.^[371]

Nearly all countries in the world are parties to the 1994

The 1997 Kyoto Protocol extended the UNFCCC and included legally binding commitments for most developed countries to limit their emissions.^[378] During the negotiations, the G77 (representing developing countries) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions,^[379] since developed countries contributed most to the accumulation of greenhouse gases in the atmosphere. Per-capita emissions were also still relatively low in developing countries and developing countries would need to emit more to meet their development needs.^[380]

The 2009 Copenhagen Accord has been widely portrayed as disappointing because of its low goals, and was rejected by poorer nations including the G77.^[381] Associated parties aimed to limit the global temperature rise to below 2 °C.^[382] The Accord set the goal of sending $100 billion per year to developing countries for mitigation and adaptation by 2020, and proposed the founding of the Green Climate Fund.^[383] As of 2020^[update], only 83.3 billion were delivered. Only in 2023 the target is expected to be achieved.^[384]

In 2015 all UN countries negotiated the Paris Agreement, which aims to keep global warming well below 2.0 °C and contains an aspirational goal of keeping warming under 1.5 °C.^[385] The agreement replaced the Kyoto Protocol. Unlike Kyoto, no binding emission targets were set in the Paris Agreement. Instead, a set of procedures was made binding. Countries have to regularly set ever more ambitious goals and reevaluate these goals every five years.^[386] The Paris Agreement restated that developing countries must be financially supported.^[387] As of October 2021^[update], 194 states and the European Union have signed the treaty and 191 states and the EU have ratified or acceded to the agreement.^[388]

The 1987 Montreal Protocol, an international agreement to stop emitting ozone-depleting gases, may have been more effective at curbing greenhouse gas emissions than the Kyoto Protocol specifically designed to do so.^[389] The 2016 Kigali Amendment to the Montreal Protocol aims to reduce the emissions of hydrofluorocarbons, a group of powerful greenhouse gases which served as a replacement for banned ozone-depleting gases. This made the Montreal Protocol a stronger agreement against climate change.^[390]

In 2019, the United Kingdom parliament became the first national government to declare a climate emergency.^[391] Other countries and jurisdictions followed suit.^[392] That same year, the European Parliament declared a "climate and environmental emergency".^[393] The European Commission presented its European Green Deal with the goal of making the EU carbon-neutral by 2050.^[394] In 2021, the European Commission released its "Fit for 55" legislation package, which contains guidelines for the car industry; all new cars on the European market must be zero-emission vehicles from 2035.^[395]

Major countries in Asia have made similar pledges: South Korea and Japan have committed to become carbon-neutral by 2050, and China by 2060.^[396] While India has strong incentives for renewables, it also plans a significant expansion of coal in the country.^[397] Vietnam is among very few coal-dependent, fast-developing countries that pledged to phase out unabated coal power by the 2040s or as soon as possible thereafter.^[398]

As of 2021, based on information from 48

Public debate about climate change has been strongly affected by climate change denial and

Climate change came to international public attention in the late 1980s.^[411] Due to media coverage in the early 1990s, people often confused climate change with other environmental issues like ozone depletion.^[412] In popular culture, the climate fiction movie The Day After Tomorrow (2004) and the Al Gore documentary An Inconvenient Truth (2006) focused on climate change.^[411]

Significant regional, gender, age and political differences exist in both public concern for, and understanding of, climate change. More highly educated people, and in some countries, women and younger people, were more likely to see climate change as a serious threat.

Climate protests demand that political leaders take action to prevent climate change. They can take the form of public demonstrations,

Litigation is increasingly used as a tool to strengthen climate action from public institutions and companies. Activists also initiate lawsuits which target governments and demand that they take ambitious action or enforce existing laws on climate change.^[423] Lawsuits against fossil-fuel companies generally seek compensation for loss and damage.^[424]

Scientists in the 19th century such as Alexander von Humboldt began to foresee the effects of climate change.^{[426][427][428][429]} In the 1820s, Joseph Fourier proposed the greenhouse effect to explain why Earth's temperature was higher than the Sun's energy alone could explain. Earth's atmosphere is transparent to sunlight, so sunlight reaches the surface where it is converted to heat. However, the atmosphere is not transparent to heat radiating from the surface, and captures some of that heat, which in turn warms the planet.^[430]

In 1856 Eunice Newton Foote demonstrated that the warming effect of the Sun is greater for air with water vapour than for dry air, and that the effect is even greater with carbon dioxide (CO₂). She concluded that "An atmosphere of that gas would give to our earth a high temperature..."^[431][432]

Starting in 1859,^[433] John Tyndall established that nitrogen and oxygen—together totalling 99% of dry air—are transparent to radiated heat. However, water vapour and gases such as methane and carbon dioxide absorb radiated heat and re-radiate that heat into the atmosphere. Tyndall proposed that changes in the concentrations of these gases may have caused climatic changes in the past, including ice ages.^[434]

Svante Arrhenius noted that water vapour in air continuously varied, but the CO₂ concentration in air was influenced by long-term geological processes. Warming from increased CO₂ levels would increase the amount of water vapour, amplifying warming in a positive feedback loop. In 1896, he published the first climate model of its kind, projecting that halving CO₂ levels could have produced a drop in temperature initiating an ice age. Arrhenius calculated the temperature increase expected from doubling CO₂ to be around 5–6 °C.^[435] Other scientists were initially sceptical and believed that the greenhouse effect was saturated so that adding more CO₂ would make no difference, and that the climate would be self-regulating.^[436] Beginning in 1938, Guy Stewart Callendar published evidence that climate was warming and CO₂ levels were rising,^[437] but his calculations met the same objections.^[436]

In the 1950s, Gilbert Plass created a detailed computer model that included different atmospheric layers and the infrared spectrum. This model predicted that increasing CO₂ levels would cause warming. Around the same time, Hans Suess found evidence that CO₂ levels had been rising, and Roger Revelle showed that the oceans would not absorb the increase. The two scientists subsequently helped Charles Keeling to begin a record of continued increase, which has been termed the "Keeling Curve".^[436] Scientists alerted the public,^[442] and the dangers were highlighted at James Hansen's 1988 Congressional testimony.^[39] The Intergovernmental Panel on Climate Change (IPCC), set up in 1988 to provide formal advice to the world's governments, spurred interdisciplinary research.^[443] As part of the IPCC reports, scientists assess the scientific discussion that takes place in peer-reviewed journal articles.^[444]

There is a near-complete scientific consensus that the climate is warming and that this is caused by human activities. As of 2019, agreement in recent literature reached over 99%.^[439][440] No scientific body of national or international standing disagrees with this view.^[445] Consensus has further developed that some form of action should be taken to protect people against the impacts of climate change. National science academies have called on world leaders to cut global emissions.^[446] The 2021 IPCC Assessment Report stated that it is "unequivocal" that climate change is caused by humans.^[440]

• Climate change portal
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• List of climate scientists