Climate change

Temperatures prior to present-day global warming

Over the last few million years the climate cycled through ice ages. One of the hotter periods was the Last Interglacial, around 125,000 years ago, where temperatures were between 0.5 °C and 1.5 °C warmer than before the start of global warming.^[40] This period saw sea levels 5 to 10 metres higher than today. The most recent glacial maximum 20,000 years ago was some 5–7 °C colder. This period has sea levels that were over 125 metres (410 ft) lower than today.^[41]

Temperatures stabilized in the current interglacial period beginning 11,700 years ago.^[42] This period also saw the start of agriculture.^[43] 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.^[44][45] Climate information for that period comes from climate proxies, such as trees and ice cores.^[46][47]

Warming since the Industrial Revolution

Around 1850 thermometer records began to provide global coverage.^[50] 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 global dimming. After 1970, the increasing accumulation of greenhouse gases and controls on sulfur pollution led to a marked increase in temperature.^[51][52][53]

Ongoing changes in climate have had no precedent for several thousand years.^[54] Multiple independent datasets all show worldwide increases in surface temperature,^[55] at a rate of around 0.2 °C per decade.^[56] The 2014–2023 decade warmed to an average 1.19 °C [1.06–1.30 °C] compared to the pre-industrial baseline (1850–1900).^[57] Not every single year was warmer than the last: internal climate variability processes can make any year 0.2 °C warmer or colder than the average.^[58] From 1998 to 2013, negative phases of two such processes, Pacific Decadal Oscillation (PDO)^[59] and Atlantic Multidecadal Oscillation (AMO)^[60] caused a short slower period of warming called the "global warming hiatus".^[61] After the "hiatus", the opposite occurred, with years like 2023 exhibiting temperatures well above even the recent average.^[62] This is why the temperature change is defined in terms of a 20-year average, which reduces the noise of hot and cold years and decadal climate patterns, and detects the long-term signal.^{[63]: 5 [64]}

A wide range of other observations reinforce the evidence of warming.^[65][66] The upper atmosphere is cooling, because greenhouse gases are trapping heat near the Earth's surface, and so less heat is radiating into space.^[67] Warming reduces average snow cover and forces the retreat of glaciers. At the same time, warming also causes greater evaporation from the oceans, leading to more atmospheric humidity, more and heavier precipitation.^[68][69] Plants are flowering earlier in spring, and thousands of animal species have been permanently moving to cooler areas.^[70]

Future global temperatures

The World Meteorological Organization estimates there is an 80% chance that global temperatures will exceed 1.5 °C warming for at least one year between 2024 and 2028. The chance of the 5-year average being above 1.5 °C is almost half.^[87]

The IPCC expects the 20-year average global temperature to exceed +1.5 °C in the early 2030s.^[88] The IPCC Sixth Assessment Report (2021) 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.^[89] The warming will continue past 2100 in the intermediate and high emission scenarios,^[90][91] with future projections of global surface temperatures by year 2300 being similar to millions of years ago.^[92]

The remaining carbon budget for staying beneath certain temperature increases is determined by modelling the carbon cycle and climate sensitivity to greenhouse gases.^[93] According to UNEP, global warming can be kept below 1.5 °C with a 50% chance if emissions after 2023 do not exceed 200 gigatonnes of CO₂. This corresponds to around 4 years of current emissions. To stay under 2.0 °C, the carbon budget is 900 gigatonnes of CO₂, or 16 years of current emissions.^[94]

Greenhouse gases

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.^[102]

While water vapour (≈50%) and clouds (≈25%) are the biggest contributors to the greenhouse effect, they primarily change as a function of temperature and are therefore mostly considered to be feedbacks that change climate sensitivity. On the other hand, concentrations of gases such as CO₂ (≈20%), tropospheric ozone,^[103] CFCs and nitrous oxide are added or removed independently from temperature, and are therefore considered to be external forcings that change global temperatures.^[104]

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.^[105][106] Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (coal, oil, and natural gas),^[107] has increased the amount of greenhouse gases in the atmosphere. In 2022, the concentrations of CO₂ and methane had increased by about 50% and 164%, respectively, since 1750.^[108] These CO₂ levels are higher than they have been at any time during the last 14 million years.^[109] Concentrations of methane are far higher than they were over the last 800,000 years.^[110]

Global human-caused greenhouse gas emissions in 2019 were equivalent to 59 billion tonnes of CO₂. Of these emissions, 75% was CO₂, 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases.^[111] CO₂ emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity.^[5] Additional CO₂ emissions come from deforestation and industrial processes, which include the CO₂ released by the chemical reactions for making cement, steel, aluminum, and fertilizer.^{[112][113][114][115]} Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, and coal mining, as well as oil and gas extraction.^[116][117] Nitrous oxide emissions largely come from the microbial decomposition of fertilizer.^[118][119]

While methane only lasts in the atmosphere for an average of 12 years,^[120] CO₂ lasts much longer. The Earth's surface absorbs CO₂ as part of the carbon cycle. While plants on land and in the ocean absorb most excess emissions of CO₂ every year, that CO₂ is returned to the atmosphere when biological matter is digested, burns, or decays.^[121] Land-surface carbon sink processes, such as carbon fixation in the soil and photosynthesis, remove about 29% of annual global CO₂ emissions.^[122] The ocean has absorbed 20 to 30% of emitted CO₂ over the last two decades.^[123] CO₂ is only removed from the atmosphere for the long term when it is stored in the Earth's crust, which is a process that can take millions of years to complete.^[121]

Land surface changes

Around 30% of Earth's land area is largely unusable for humans (glaciers, deserts, etc.), 26% is forests, 10% is shrubland and 34% is agricultural land.^[125] Deforestation is the main land use change contributor to global warming,^[126] as the destroyed trees release CO₂, and are not replaced by new trees, removing that carbon sink.^[127] Between 2001 and 2018, 27% of deforestation was from permanent clearing to enable agricultural expansion for crops and livestock. Another 24% has been lost to temporary clearing under the shifting cultivation agricultural systems. 26% was due to logging for wood and derived products, and wildfires have accounted for the remaining 23%.^[128] Some forests have not been fully cleared, but were already degraded by these impacts. Restoring these forests also recovers their potential as a carbon sink.^[129]

Local vegetation cover impacts how much of the sunlight gets reflected back into space (albedo), and how much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, causing it to reflect more sunlight. Deforestation can also modify the release of chemical compounds that influence clouds, and by changing wind patterns.^[130] In tropic and temperate areas the net effect is to produce significant warming, and forest restoration can make local temperatures cooler.^[129] At latitudes closer to the poles, there is a cooling effect as forest is replaced by snow-covered (and more reflective) plains.^[130] Globally, these increases in surface albedo have been the dominant direct influence on temperature from land use change. Thus, land use change to date is estimated to have a slight cooling effect.^[131]

Other factors

Solar and volcanic activity

Further information: Solar activity and climate

As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system.^[140] Solar irradiance has been measured directly by satellites,^[147] and indirect measurements are available from the early 1600s onwards.^[140] Since 1880, there has been no upward trend in the amount of the Sun's energy reaching the Earth, in contrast to the warming of the lower atmosphere (the troposphere).^[148] The upper atmosphere (the stratosphere) would also be warming if the Sun was sending more energy to Earth, but instead, it has been cooling.^[100] This is consistent with greenhouse gases preventing heat from leaving the Earth's atmosphere.^[149]

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.^[150] These impacts on temperature only last for several years, because both water vapour and volcanic material have low persistence in the atmosphere.^[151] volcanic CO₂ emissions are more persistent, but they are equivalent to less than 1% of current human-caused CO₂ emissions.^[152] 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.^[151]

Climate change feedbacks

Main articles: Climate change feedbacks and Climate sensitivity

The climate system's response to an initial forcing is shaped by feedbacks, which either amplify or dampen the change. Self-reinforcing or positive feedbacks increase the response, while balancing or negative feedbacks reduce it.^[154] The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and the net effect of clouds.^[155][156] The primary balancing mechanism is radiative cooling, as Earth's surface gives off more heat to space in response to rising temperature.^[157] In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of CO₂ on plant growth.^[158] Feedbacks are expected to trend in a positive direction as greenhouse gas emissions continue, raising climate sensitivity.^[159]

These feedback processes alter the pace of global warming. For instance, warmer air can hold more moisture in the form of water vapour, which is itself a potent greenhouse gas.^[155] Warmer air can also make clouds higher and thinner, and therefore more insulating, increasing climate warming.^[160] The reduction of snow cover and sea ice in the Arctic is another major feedback, this reduces the reflectivity of the Earth's surface in the region and accelerates Arctic warming.^[161][162] This additional warming also contributes to permafrost thawing, which releases methane and CO₂ into the atmosphere.^[163]

Around half of human-caused CO₂ emissions have been absorbed by land plants and by the oceans.^[164] 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%.^[165] 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.^[166][167] 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.^{[168][169][82]} Uncertainty over feedbacks, particularly cloud cover,^[170] is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.^[171]

Environmental effects

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.^[186] Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency.^[187] Extremely wet or dry events within the monsoon period have increased in India and East Asia.^[188] Monsoonal precipitation over the Northern Hemisphere has increased since 1980.^[189] The rainfall rate and intensity of hurricanes and typhoons is likely increasing,^[190] and the geographic range likely expanding poleward in response to climate warming.^[191] Frequency of tropical cyclones has not increased as a result of climate change.^[192]

Global sea level is rising as a consequence of thermal expansion and the melting of glaciers and ice sheets. Sea level rise has increased over time, reaching 4.8 cm per decade between 2014 and 2023.^[194] Over the 21st century, the IPCC projects 32–62 cm of sea level rise under a low emission scenario, 44–76 cm under an intermediate one and 65–101 cm under a very high emission scenario.^[195] Marine ice sheet instability processes in Antarctica may add substantially to these values,^[196] including the possibility of a 2-meter sea level rise by 2100 under high emissions.^[197]

Climate change has led to decades of shrinking and thinning of the Arctic sea ice.^[198] 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.^[199] Higher atmospheric CO₂ concentrations cause more CO₂ to dissolve in the oceans, which is making them more acidic.^[200] Because oxygen is less soluble in warmer water,^[201] its concentrations in the ocean are decreasing, and dead zones are expanding.^[202]

Tipping points and long-term impacts

Greater degrees of global warming increase the risk of passing through 'tipping points'—thresholds beyond which certain major impacts can no longer be avoided even if temperatures return to their previous state.^[205][206] For instance, the Greenland ice sheet is already melting, but if global warming reaches levels between 1.7 °C and 2.3 °C, its melting will continue until it fully disappears. If the warming is later reduced to 1.5 °C or less, it will still lose a lot more ice than if the warming was never allowed to reach the threshold in the first place.^[207] While the ice sheets would melt over millennia, other tipping points would occur faster and give societies less time to respond. The collapse of major ocean currents like the Atlantic meridional overturning circulation (AMOC), and irreversible damage to key ecosystems like the Amazon rainforest and coral reefs can unfold in a matter of decades.^[204]

The long-term effects of climate change on oceans include further ice melt, ocean warming, sea level rise, ocean acidification and ocean deoxygenation.^[208] The timescale of long-term impacts are centuries to millennia due to CO₂'s long atmospheric lifetime.^[209] The result is an estimated total sea level rise of 2.3 metres per degree Celsius (4.2 ft/°F) after 2000 years.^[210] Oceanic CO₂ uptake is slow enough that ocean acidification will also continue for hundreds to thousands of years.^[211] Deep oceans (below 2,000 metres (6,600 ft)) are also already committed to losing over 10% of their dissolved oxygen by the warming which occurred to date.^[212] Further, the West Antarctic ice sheet appears committed to practically irreversible melting, which would increase the sea levels by at least 3.3 m (10 ft 10 in) over approximately 2000 years.^{[204][213][214]}

Nature and wildlife

Recent warming has driven many terrestrial and freshwater species poleward and towards higher altitudes.^[215] For instance, the range of hundreds of North American birds has shifted northward at an average rate of 1.5 km/year over the past 55 years.^[216] Higher atmospheric CO₂ levels and an extended growing season have resulted in global greening. However, heatwaves and drought have reduced ecosystem productivity in some regions. The future balance of these opposing effects is unclear.^[217] A related phenomenon driven by climate change is woody plant encroachment, affecting up to 500 million hectares globally.^[218] Climate change has contributed to the expansion of drier climate zones, such as the expansion of deserts in the subtropics.^[219] The size and speed of global warming is making abrupt changes in ecosystems more likely.^[220] Overall, it is expected that climate change will result in the extinction of many species.^[221]

The oceans have heated more slowly than the land, but plants and animals in the ocean have migrated towards the colder poles faster than species on land.^[222] Just as on land, heat waves in the ocean occur more frequently due to climate change, harming a wide range of organisms such as corals, kelp, and seabirds.^[223] Ocean acidification makes it harder for marine calcifying organisms such as mussels, barnacles and corals to produce shells and skeletons; and heatwaves have bleached coral reefs.^[224] Harmful algal blooms enhanced by climate change and eutrophication lower oxygen levels, disrupt food webs and cause great loss of marine life.^[225] Coastal ecosystems are under particular stress. Almost half of global wetlands have disappeared due to climate change and other human impacts.^[226] Plants have come under increased stress from damage by insects.^[227]

Humans

The effects of climate change are impacting humans everywhere in the world.^[233] Impacts can be observed on all continents and ocean regions,^[234] with low-latitude, less developed areas facing the greatest risk.^[235] Continued warming has potentially "severe, pervasive and irreversible impacts" for people and ecosystems.^[236] The risks are unevenly distributed, but are generally greater for disadvantaged people in developing and developed countries.^[237]

Clean energy

Renewable energy is key to limiting climate change.^[284] For decades, fossil fuels have accounted for roughly 80% of the world's energy use.^[285] The remaining share has been split between nuclear power and renewables (including hydropower, bioenergy, wind and solar power and geothermal energy).^[286] Fossil fuel use is expected to peak in absolute terms prior to 2030 and then to decline, with coal use experiencing the sharpest reductions.^[287] Renewables represented 86% of all new electricity generation installed in 2023.^[288] 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.^[289]

While solar panels and onshore wind are now among the cheapest forms of adding new power generation capacity in many locations,^[290] green energy policies are needed to achieve a rapid transition from fossil fuels to renewables.^[291] 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.^[292][293]

Electricity generated from renewable sources would also need to become the main energy source for heating and transport.^[294] Transport can switch away from internal combustion engine vehicles and towards electric vehicles, public transit, and active transport (cycling and walking).^[295][296] For shipping and flying, low-carbon fuels would reduce emissions.^[295] Heating could be increasingly decarbonized with technologies like heat pumps.^[297]

There are obstacles to the continued rapid growth of clean energy, including renewables.^[298] Wind and solar produce energy intermittently and with seasonal variability. Traditionally, hydro dams with reservoirs and fossil fuel power plants have been used when variable energy production is low. Going forward, battery storage can be expanded, energy demand and supply can be matched, and long-distance transmission can smooth variability of renewable outputs.^[284] Bioenergy is often not carbon-neutral and may have negative consequences for food security.^[299] The growth of nuclear power is constrained by controversy around radioactive waste, nuclear weapon proliferation, and accidents.^[300][301] Hydropower growth is limited by the fact that the best sites have been developed, and new projects are confronting increased social and environmental concerns.^[302]

Low-carbon energy improves human health by minimizing climate change as well as reducing air pollution deaths,^[303] which were estimated at 7 million annually in 2016.^[304] 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.^[305] Improving air quality also has economic benefits which may be larger than mitigation costs.^[306]

Energy conservation

Reducing energy demand is another major aspect of reducing emissions.^[307] 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.^[308] Major increases in energy efficiency investment will be required to achieve climate goals, comparable to the level of investment in renewable energy.^[309] Several COVID-19 related changes in energy use patterns, energy efficiency investments, and funding have made forecasts for this decade more difficult and uncertain.^[310]

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.^[311] Industrial strategies to reduce energy demand include improving heating systems and motors, designing less energy-intensive products, and increasing product lifetimes.^[312] In the building sector the focus is on better design of new buildings, and higher levels of energy efficiency in retrofitting.^[313] The use of technologies like heat pumps can also increase building energy efficiency.^[314]

Agriculture and industry

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.^[315] 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.^[316]

On the demand side, a key component of reducing emissions is shifting people towards plant-based diets.^[317] Eliminating the production of livestock for meat and dairy would eliminate about 3/4ths of all emissions from agriculture and other land use.^[318] Livestock also occupy 37% of ice-free land area on Earth and consume feed from the 12% of land area used for crops, driving deforestation and land degradation.^[319]

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.^[320] Where energy production or CO₂-intensive heavy industries continue to produce waste CO₂, technology can sometimes be used to capture and store most of the gas instead of releasing it to the atmosphere.^[321] This technology, carbon capture and storage (CCS), could have a critical but limited role in reducing emissions.^[321] It is relatively expensive^[322] and has been deployed only to an extent that removes around 0.1% of annual greenhouse gas emissions.^[321]

Carbon dioxide removal

Natural carbon sinks can be enhanced to sequester significantly larger amounts of CO₂ beyond naturally occurring levels.^[323] Reforestation and afforestation (planting forests where there were none before) are among the most mature sequestration techniques, although the latter raises food security concerns.^[324] Farmers can promote sequestration of carbon in soils through practices such as use of winter cover crops, reducing the intensity and frequency of tillage, and using compost and manure as soil amendments.^[325] Forest and landscape restoration yields many benefits for the climate, including greenhouse gas emissions sequestration and reduction.^[129] Restoration/recreation of coastal wetlands, prairie plots and seagrass meadows increases the uptake of carbon into organic matter.^[326][327] When carbon is sequestered in soils and in organic matter such as trees, there is a risk of the carbon being re-released into the atmosphere later through changes in land use, fire, or other changes in ecosystems.^[328]

The use of bioenergy in conjunction with carbon capture and storage (BECCS) can result in net negative emissions as CO₂ is drawn from the atmosphere.^[329] 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.^[330]

Policy options

A wide range of policies, regulations, and laws are being used to reduce emissions. As of 2019, carbon pricing covers about 20% of global greenhouse gas emissions.^[352] Carbon can be priced with carbon taxes and emissions trading systems.^[353] Direct global fossil fuel subsidies reached $319 billion in 2017, and $5.2 trillion when indirect costs such as air pollution are priced in.^[354] Ending these can cause a 28% reduction in global carbon emissions and a 46% reduction in air pollution deaths.^[355] Money saved on fossil subsidies could be used to support the transition to clean energy instead.^[356] More direct methods to reduce greenhouse gases include vehicle efficiency standards, renewable fuel standards, and air pollution regulations on heavy industry.^[357] Several countries require utilities to increase the share of renewables in power production.^[358]

International climate agreements

Nearly all countries in the world are parties to the 1994 United Nations Framework Convention on Climate Change (UNFCCC).^[364] The goal of the UNFCCC is to prevent dangerous human interference with the climate system.^[365] As stated in the convention, this requires that greenhouse gas concentrations are stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and economic development can be sustained.^[366] The UNFCCC does not itself restrict emissions but rather provides a framework for protocols that do. Global emissions have risen since the UNFCCC was signed.^[367] Its yearly conferences are the stage of global negotiations.^[368]

The 1997 Kyoto Protocol extended the UNFCCC and included legally binding commitments for most developed countries to limit their emissions.^[369] During the negotiations, the G77 (representing developing countries) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions,^[370] 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.^[371]

The 2009 Copenhagen Accord has been widely portrayed as disappointing because of its low goals, and was rejected by poorer nations including the G77.^[372] Associated parties aimed to limit the global temperature rise to below 2 °C.^[373] 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.^[374] As of 2020^[update], only 83.3 billion were delivered. Only in 2023 the target is expected to be achieved.^[375]

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.^[376] 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.^[377] The Paris Agreement restated that developing countries must be financially supported.^[378] 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.^[379]

The 1987 Montreal Protocol, an international agreement to phase out production of ozone-depleting gases, has had benefits for climate change mitigation.^[380] Several ozone-depleting gases like chlorofluorocarbons are powerful greenhouse gases, so banning their production and usage may have avoided a temperature rise of 0.5 °C–1.0 °C,^[381] as well as additional warming by preventing damage to vegetation from ultraviolet radiation.^[382] It is estimated that the agreement has been more effective at curbing greenhouse gas emissions than the Kyoto Protocol specifically designed to do so.^[383] The most recent amendment to the Montreal Protocol, the 2016 Kigali Amendment, committed to reducing the emissions of hydrofluorocarbons, which served as a replacement for banned ozone-depleting gases and are also potent greenhouse gases.^[384] Should countries comply with the amendment, a warming of 0.3 °C–0.5 °C is estimated to be avoided.^[385]

National responses

In 2019, the United Kingdom parliament became the first national government to declare a climate emergency.^[387] Other countries and jurisdictions followed suit.^[388] That same year, the European Parliament declared a "climate and environmental emergency".^[389] The European Commission presented its European Green Deal with the goal of making the EU carbon-neutral by 2050.^[390] 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.^[391]

Major countries in Asia have made similar pledges: South Korea and Japan have committed to become carbon-neutral by 2050, and China by 2060.^[392] While India has strong incentives for renewables, it also plans a significant expansion of coal in the country.^[393] 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.^[394]

As of 2021, based on information from 48 national climate plans, which represent 40% of the parties to the Paris Agreement, estimated total greenhouse gas emissions will be 0.5% lower compared to 2010 levels, below the 45% or 25% reduction goals to limit global warming to 1.5 °C or 2 °C, respectively.^[395]

Denial and misinformation

Public debate about climate change has been strongly affected by climate change denial and misinformation, which originated in the United States and has since spread to other countries, particularly Canada and Australia. Climate change denial has originated from fossil fuel companies, industry groups, conservative think tanks, and contrarian scientists.^[397] Like the tobacco industry, the main strategy of these groups has been to manufacture doubt about climate-change related scientific data and results.^[398] People who hold unwarranted doubt about climate change are called climate change "skeptics", although "contrarians" or "deniers" are more appropriate terms.^[399]

There are different variants of climate denial: some deny that warming takes place at all, some acknowledge warming but attribute it to natural influences, and some minimize the negative impacts of climate change.^[400] Manufacturing uncertainty about the science later developed into a manufactured controversy: creating the belief that there is significant uncertainty about climate change within the scientific community to delay policy changes.^[401] Strategies to promote these ideas include criticism of scientific institutions,^[402] and questioning the motives of individual scientists.^[400] An echo chamber of climate-denying blogs and media has further fomented misunderstanding of climate change.^[403]

Public awareness and opinion

Climate change came to international public attention in the late 1980s.^[407] Due to media coverage in the early 1990s, people often confused climate change with other environmental issues like ozone depletion.^[408] 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.^[407]

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.^[409] College biology textbooks from the 2010s featured less content on climate change compared to those from the preceding decade, with decreasing emphasis on solutions.^[410] Partisan gaps also exist in many countries,^[411] and countries with high CO₂ emissions tend to be less concerned.^[412] Views on causes of climate change vary widely between countries.^[413] Concern has increased over time,^[414] and a majority of citizens in many countries now express a high level of worry about climate change, or view it as a global emergency.^[415] Higher levels of worry are associated with stronger public support for policies that address climate change.^[416]

Early discoveries

Scientists in the 19th century such as Alexander von Humboldt began to foresee the effects of climate change.^{[423][424][425][426]} 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.^[427]

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..."^[428][429]

Starting in 1859,^[430] 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.^[431]

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.^[432] 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.^[433] Beginning in 1938, Guy Stewart Callendar published evidence that climate was warming and CO₂ levels were rising,^[434] but his calculations met the same objections.^[433]

Development of a scientific consensus

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".^[433] Scientists alerted the public,^[439] and the dangers were highlighted at James Hansen's 1988 Congressional testimony.^[34] The Intergovernmental Panel on Climate Change (IPCC), set up in 1988 to provide formal advice to the world's governments, spurred interdisciplinary research.^[440] As part of the IPCC reports, scientists assess the scientific discussion that takes place in peer-reviewed journal articles.^[441]

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%.^[436][437] No scientific body of national or international standing disagrees with this view.^[442] 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.^[443] The 2021 IPCC Assessment Report stated that it is "unequivocal" that climate change is caused by humans.^[437]