Global warming is the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of climate change and has been demonstrated by direct temperature measurements and by measurements of various effects of the warming.Global warming and climate change are often used interchangeably. But more accurately, global warming is the mainly human-caused increase in global surface temperatures and its projected continuation, while climate change includes both global warming and its effects, such as changes in precipitation. While there have been prehistoric periods of global warming, many observed changes since the mid-20th century have been unprecedented over decades to millennia.
Societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possibly climate engineering. Countries work together on climate change under the umbrella of the United Nations Framework Convention on Climate Change (UNFCCC), which entered into force in 1994 and has near-universal membership. The ultimate goal of the convention is to "prevent dangerous anthropogenic interference with the climate system". Although the parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2 °C (3.6 °F) in the Paris Agreement of 2016, the Earth's average surface temperature has already increased by about half this threshold and current pledges by countries to cut emissions are inadequate to limit future warming.
Global average temperatures declined for thousands of years, until fossil fuel-based industrialization beginning roughly 200 years ago reversed the decline. Global warming has intensified in recent decades.
Scientists have investigated many possible causes of global warming, and have found that accumulation in the atmosphere of greenhouse gases, especially those resulting from humans burning fossil fuels, is the predominant cause.
Climate proxy records show that natural variations offset the early effects of the Industrial Revolution, so there was little net warming between the 18th century and the mid-19th century, when thermometer records began to provide global coverage. The IPCC has adopted the baseline reference period 1850–1900 as an approximation of pre-industrial global mean surface temperature.
Multiple independently produced instrumental datasets confirm that the 2009–2018 decade was 0.93 ± 0.07 °C warmer than the pre-industrial baseline (1850–1900). Currently, surface temperatures are rising by about 0.2 °C per decade. Since 1950, the number of cold days and nights have decreased, and the number of warm days and night have increased. Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the Little Ice Age, were not as synchronous as current warming, but may have reached temperatures as high as those of the late-20th century in a limited set of regions. Past examples of climate change provide insight into modern climate change.
Although the most common measure of global warming is the increase in the near-surface atmospheric temperature, over 90% of the additional energy stored in the climate system over the last 50 years has warmed ocean water. The remainder of the additional energy has melted ice and warmed the continents and the atmosphere.
The warming evident in the instrumental temperature record is consistent with a wide range of observations, documented by many independent scientific groups; for example, in most continental regions the frequency and intensity of heavy precipitation has increased. Further examples include sea level rise, widespread melting of snow and land ice, increased heat content of the oceans, increased humidity, and the earlier timing of spring events, such as the flowering of plants.
Average annual temperature has risen faster on land than on the ice-free surface of the sea.
NASA animation of annual average temperature maps dating from the late 1800's to modern times.
Global warming refers to global averages, with the amount of warming varying by region. Since the pre-industrial period, global average land temperatures have increased almost twice as fast as global average temperatures. This is due to the larger heat capacity of oceans and because oceans lose more heat by evaporation. Patterns of warming are independent of the locations of greenhouse gas emissions because the gases persist long enough to diffuse across the planet; however, localized black carbon deposits on snow and ice do contribute to Arctic warming.
The Northern Hemisphere and North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but the arrangement of land masses around the Arctic Ocean has resulted in the maximum surface area flipping from reflective snow and ice cover to ocean and land surfaces that absorb more sunlight and thus more heat.Arctic temperatures have increased and are predicted to continue to increase during this century at over twice the rate of the rest of the world. As the temperature difference between the Arctic and the equator decreases, ocean currents that are driven by that temperature difference, like the Gulf Stream, are weakening.
Short-term slowdowns and surges
Because the climate system has large thermal inertia, it can take centuries for the climate to fully adjust. While record-breaking years attract considerable public interest, individual years are less significant than the overall trend. Global surface temperature is subject to short-term fluctuations that overlie long-term trends, and can temporarily mask or magnify them. An example of such an episode is the slower rate of surface temperature increase from 1998 to 2012, which was dubbed the global warming hiatus. Throughout this period ocean heat storage continued to progress steadily upwards, and in subsequent years surface temperatures have spiked upwards. The slower pace of warming can be attributed to a combination of natural fluctuations, reduced solar activity, and increased volcanic activity.
Attributing detected temperature changes and extreme events to human-caused increases in greenhouse gases requires scientists to rule out known internal climate variability and natural external forcings. Therefore, a key approach is to use physically or statistically based computer modelling of the climate system to determine unique fingerprints for all potential causes. By comparing these fingerprints with observed patterns and evolution of climate change, and the observed evolution of the forcings, the causes of the observed changes can be determined.Scientists have determined that the major factors causing the current climate change are greenhouse gases, land use changes, and aerosols and soot.
Greenhouse effect schematic showing energy flows between space, the atmosphere, and the Earth's surface. Energy exchanges are expressed in watts per square meter (W/m2).
CO 2 concentrations over the last 800,000 years as measured from ice cores (blue/green) and directly (black)
Greenhouse gases trap heat radiating from the Earth to space. This heat, in the form of infrared radiation, gets absorbed and emitted by these gases in the atmosphere, thus warming the lower atmosphere and the surface. Before the Industrial Revolution, naturally occurring amounts of greenhouse gases caused the air near the surface to be warmer by about 33 °C (59 °F) than it would be in their absence.Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing temperature of water. While water vapour (~50%) and clouds (~25%) are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore considered feedbacks. Increased concentrations of gases such as CO 2 (~20%), ozone and N 2O are external forcing on the other hand.
Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs, and nitrous oxide. As of 2011, the concentrations of CO2 and methane had increased by about 40% and 150%, respectively, since pre-industrial times. In 2013, CO2 readings taken at the world's primary benchmark site in Mauna Loa surpassing 400 ppm for the first time. These levels are much higher than at any time during the last 800,000 years, the period for which reliable data have been collected from ice cores. Less direct geological evidence indicates that CO2 values have not been this high for millions of years.
Global anthropogenic greenhouse gas emissions in 2010 were equivalent to 49 billion tonnes of carbon dioxide (using the most recent global warming potentials over 100 years from the AR5 report). Of these emissions, 65% was carbon dioxide from fossil fuel burning and industry, 11% was carbon dioxide from land use change, which is primarily due to deforestation, 16% was from methane, 6.2% was from nitrous oxide, and 2.0% was from fluorinated gases. Using life-cycle assessment to estimate emissions relating to final consumption, the dominant sources of 2010 emissions were: food (26–30% of emissions); washing, heating, and lighting (26%); personal transport and freight (20%); and building construction (15%).
The 10th Emissions Gap Report issued by the United Nations Environment Programme (UNEP) predicts that if emissions continue to increase at the same rate as they have in 2010–2020, global temperatures would rise by as much as 4° by 2100.
Land use change
Humans change the land surface mainly to create more agricultural land. Today agriculture takes up 50% of the world's habitable land, while 37% is forests, and that latter figure continues to decrease, largely due to continued forest loss in the tropics. This deforestation is the most significant aspect of land use change affecting global warming. The main causes are: deforestation through permanent land use change for agricultural products such as beef and palm oil (27%), forestry/forest products (26%), short term agricultural cultivation (24%), and wildfires (23%).
Current patterns of land use affect global warming in a variety of ways. While some aspects cause significant GHG emissions, other land use processes act as a significant carbon sink for CO2, more than offsetting these GHG sources. The net result is an estimated removal (sink) of about 6 billion tonnes annually, or about 12% of global GHG emissions. The land acts as a carbon sink via carbon fixation in the soil and increased photosynthesis.
Land use changes also affect global warming through a variety of other chemical and physical dynamics. Changing the type of vegetation in a region impacts the local temperature by changing how much sunlight gets reflected back into space, called 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 contribute to changing temperatures by affecting the release of aerosols and other chemical compounds that affect clouds; and by changing wind patterns when the land surface has different obstacles. Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo. But there is significant geographic variation in how this works. In the tropics the net effect is to produce a significant warming, while at latitudes closer to the poles a loss of albedo leads to an overall cooling effect.
In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets. This effect also causes droplets to be of more uniform size, which reduces the growth of raindrops and makes clouds more reflective to incoming sunlight. Indirect effects of aerosols are the largest uncertainty in radiative forcing.
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. Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050. When soot is suspended in the atmosphere, it directly absorbs solar radiation, heating the atmosphere and cooling the surface. In areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.
Minor forcings: the Sun and short-lived greenhouse gases
As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system.Solar irradiance has been measured directly by satellites, and indirect measurements are available beginning in the early 1600s. There has been no upward trend in the amount of the Sun's energy reaching the Earth, so it cannot be responsible for the current warming. Physical climate models are also unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity. Another line of evidence for the warming not being due to the Sun is how temperature changes differ at different levels in the Earth's atmosphere. According to basic physical principles, the greenhouse effect produces warming of the lower atmosphere (the troposphere), but cooling of the upper atmosphere (the stratosphere). If solar variations were responsible for the observed warming, warming of both the troposphere and the stratosphere would be expected, but that has not been the case.
Ozone in the lowest layer of the atmosphere, the troposphere, is itself a greenhouse gas. Furthermore, it is highly reactive and interacts with other greenhouse gases and aerosols.
The dark ocean surface reflects only 6 percent of incoming solar radiation, whereas sea ice reflects 50 to 70 percent.
The response of the climate system to an initial forcing is increased by self-reinforcing feedbacks and reduced by balancing feedbacks. The main balancing feedback to global temperature change is radiative cooling to space as infrared radiation, which increases strongly with increasing temperature. The main reinforcing feedbacks are the water vapour feedback, the ice–albedo feedback, and probably the net effect of clouds. Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.
As air gets warmer, it can hold more moisture. After an initial warming due to emissions of greenhouse gases, the atmosphere will hold more water. As water is a potent greenhouse gas, this further heats the climate: the water vapour feedback. The reduction of snow cover and sea ice in the Arctic reduces the albedo of the Earth's surface. More of the Sun's energy is now absorbed in these regions, contributing to Arctic amplification, which has caused Arctic temperatures to increase at more than twice the rate of the rest of the world. Arctic amplification also causes methane to be released as permafrost melts, which is expected to surpass land use changes as the second strongest anthropogenic source of greenhouse gases by the end of the century.
Cloud cover may change in the future. If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. Simultaneously, the clouds enhance the greenhouse effect, warming the planet. The opposite is true if cloud cover decreases. It depends on the cloud type and location which process is more important. Overall, the net feedback over the industrial era has probably been self-reinforcing.
Roughly half of each year's CO2 emissions have been absorbed by plants on land and in oceans. Carbon dioxide and an extended growing season have stimulated plant growth making the land carbon cycle a balancing feedback. Climate change also increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this balancing feedback will persist in the future. Soils contain large quantities of carbon and may release some when they heat up. As more CO2 and heat are absorbed by the ocean, it is acidifying and ocean circulation can change, changing the rate at which the ocean can absorb atmospheric carbon.
A concern is that self-reinforcing feedbacks will lead to a tipping point, where global temperatures transition to a hothouse climate state even if greenhouse gas emissions are reduced or eliminated. A 2018 study tried to identify such a planetary threshold for self-reinforcing feedbacks and found that even a 2 °C (3.6 °F) increase in temperature over pre-industrial levels may be enough to trigger such a hothouse Earth scenario.
Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model mean projections (i.e., the average of the model projections available) for the 2081–2100 period under the RCP 2.6 and RCP 8.5 scenarios for change in annual mean surface temperature. Changes are shown relative to the 1986–2005 period. Data from the IPCC Fifth Assessment Report.
A climate model is a representation of the physical, chemical, and biological processes that affect the climate system. Computer models are run on supercomputers to reproduce and predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere. There are more than two dozen scientific institutions that develop climate models. Models not only project different future temperature with different emissions of greenhouse gases, but also do not fully agree on the strength of different feedbacks on climate sensitivity and the amount of inertia of the system.
A subset of climate models add societal factors to a simple physical climate model. These models simulate how population, economic growth, and energy use affect – and interact with – the physical climate. With this information, scientists can produce scenarios of how greenhouse gas emissions may vary in the future. Scientists can then run these scenarios through physical climate models to generate climate change projections.
Climate models include different external forcings for their models. For different greenhouse gas inputs four RCPs (Representative Concentration Pathways) are used: "a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0) and one scenario with very high GHG [greenhouse gas] emissions (RCP8.5)". Models also include changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing. RCPs only look at concentrations of greenhouse gases, factoring out uncertainty as to whether the carbon cycle will continue to remove about half of the carbon dioxide from the atmosphere each year.
The physical realism of models is tested by examining their ability to simulate contemporary or past climates. Past models have underestimated the rate of Arctic shrinkage and underestimated the rate of precipitation increase. Sea level rise since 1990 was underestimated in older models, but now agrees well with observations. The 2017 United States-published National Climate Assessment notes that "climate models may still be underestimating or missing relevant feedback processes".
The environmental effects of global warming are broad and far-reaching. They include effects on the oceans, ice, and weather and may occur gradually or rapidly.
Between 1993 and 2017, the global mean sea level rose on average by 3.1 ± 0.3 mm per year, with an acceleration detected as well. Over the 21st century, the IPCC projects that in a very high emissions scenario the sea level could rise by 61–110 cm. The rate of ice loss from glaciers and ice sheets in the Antarctic is a key area of uncertainty since this source could account for 90% of the potential sea level rise: increased ocean warmth is undermining and threatening to unplug Antarctic glacier outlets, potentially resulting in more rapid sea level rise. The retreat of non-polar glaciers also contributes to sea level rise.
Long-term effects of global warming: On the timescale of centuries to millennia, the magnitude of global warming will be determined primarily by anthropogenic CO2 emissions. This is due to carbon dioxide's very long lifetime in the atmosphere. The emissions are estimated to have prolonged the current interglacial period by at least 100,000 years. Because the great mass of glaciers and ice caps depressed the Earth's crust, another long-term effect of ice melt and deglaciation is the gradual rising of landmasses, a process called post-glacial rebound. This could be facilitating seismic and volcanic activity in places like Iceland.Tsunamis could be generated by submarine landslides caused by warmer ocean water thawing ocean-floor permafrost or releasing gas hydrates. Sea level rise will continue over many centuries.
In terrestrial ecosystems, the earlier timing of spring events, as well as poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. It is expected that most ecosystems will be affected by higher atmospheric CO2 levels and higher global temperatures. Global warming has contributed to the expansion of drier climatic zones, such as, probably, the expansion of deserts in the subtropics. Without substantial actions to reduce the rate of global warming, land-based ecosystems risk major shifts in their composition and structure. Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems. Rising temperatures push bees to their physiological limits, and could cause the extinction of bee populations.
The ocean has heated more slowly than the land, but plants and animals in the ocean have migrated towards the colder poles as fast as or faster than species on land. Just as on land, heat waves in the ocean occur more due to climate change, with harmful effects found on a wide range of organisms such as corals, kelp, and seabirds. Ocean acidification threatens damage to coral reefs, fisheries, protected species, and other natural resources of value to society. Higher oceanic CO2 may affect the brain and central nervous system of certain fish species, which reduces their ability to hear, smell, and evade predators.
Crop production will probably be negatively affected in low-latitude countries, while effects at northern latitudes may be positive or negative. Global warming of around 4 °C relative to late 20th century levels could pose a large risk to global and regional food security. The impact of climate change on crop productivity for the four major crops was negative for wheat and maize, and neutral for soy and rice, in the years 1960–2013. Up to an additional 182 million people worldwide, particularly those with lower incomes, are at risk of hunger as a consequence of warming. While increased CO 2 levels help crop growth at lower temperature increases, those crops do become less nutritious. Based on local and indigenous knowledge, climate change is already affecting food security in mountain regions in South America and Asia, and in various drylands, particularly in Africa. Regions dependent on glacier water, regions that are already dry, and small islands are also at increased risk of water stress due to climate change.
Health and security
Aerial view over southern Bangladesh after the passage of Cyclone Sidr. The combination of rising sea levels and increased rainfall from cyclones makes countries more vulnerable to floods, impacting people's livelihoods and health.
Generally, impacts on public health will be more negative than positive. Impacts include the direct effects of extreme weather, leading to injury and loss of life; and indirect effects, such as undernutrition brought on by crop failures. Temperature rise has been connected to increased numbers of suicides. Climate change may also lead to new human diseases. For example, while ordinary temperatures usually kill off the yeast Candida auris before it infects humans, three strains have recently appeared in widely separate regions, leading researchers to postulate that warmer temperatures are driving it to adapt to higher temperatures at which it can more readily infect humans. Climate change has been linked to an increase in violent conflict by amplifying poverty and economic shocks, which are well-documented drivers of these conflicts. Links have been made between a wide range of violent behaviour including fist fights, violent crimes, civil unrest, and wars.
The majority of severe impacts of climate change are expected in sub-Saharan Africa and South-East Asia, where existing poverty is exacerbated. Current inequalities between men and women, between rich and poor and between people of different ethnicity have been observed to worsen as a consequence of climate variability and climate change. Existing stresses include poverty, political conflicts, and ecosystem degradation. Regions may even become uninhabitable, with humidity and temperatures reaching levels too high for humans to survive.
Mitigation of and adaptation to climate change are two complementary responses to global warming. Successful adaptation is easier if there are substantial emission reductions. Many of the countries that have contributed least to global greenhouse gas emissions are among the most vulnerable to climate change, which raises questions about justice and fairness with regard to mitigation and adaptation.
Annual greenhouse gas emissions attributed to different sectors as of 2010. Emissions are given as a percentage share of total emissions, measured in carbon dioxide-equivalents, using global warming potentials from the IPCC Fifth Assessment Report.
Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, depend upon uncertain economic, sociological, technological, and natural developments. In some scenarios emissions continue to rise over the century, while others have reduced emissions. Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century. Emission scenarios can be combined with modelling of the carbon cycle to predict how atmospheric concentrations of greenhouse gases might change in the future. According to these combined models, by 2100 the atmospheric concentration of CO2 could be as low as 380 or as high as 1400 ppm, depending on the Shared Socioeconomic Pathway (SSP) the world takes and the mitigation scenario.
Near- and long-term trends in the global energy system are inconsistent with limiting global warming to below 1.5 or 2 °C relative to pre-industrial levels. Current pledges made as part of the Paris Agreement would lead to about 3.0 °C of warming at the end of the 21st century, relative to pre-industrial levels. To keep warming below 2 °C, more stringent emission reductions in the near-term would allow for less rapid reductions after 2030. To keep warming under 1.5 °C, a far-reaching system change on an unprecedented scale is necessary in energy, land, cities, transport, buildings, and industry. A report, published just before the 2019 UN Climate Action Summit, says that the full implementation of all pledges taken by international coalitions, countries, cities, regions and businesses (not only in the Paris Agreement) would be sufficient to limit temperature rise to 2 °C but not to 1.5 °C. All the information about the pledges is added to the UNFCCC Global Climate Action portal. The scientific community is checking their fulfilment.
Co-benefits of climate change mitigation may help society and individuals more quickly. For example, bicycling reduces greenhouse gas emissions while reducing the effects of a sedentary lifestyle at the same time. The development and scaling-up of clean technology, such as cement that produces less CO2, is critical to achieve sufficient emission reductions for the Paris agreement goals. Many integrated models are unable to meet the 2 °C target if pessimistic assumptions are made about the availability of mitigation technologies.
Climate change adaptation is "the adjustment in natural or human systems in response to actual or expected climatic stimuli or their effects, which moderates harm or exploits beneficial opportunities". Examples of adaptation are improved coastline protection, better disaster management, and the development of more resistant crops. The adaptation may be planned, either in reaction to or anticipation of global warming, or spontaneous, i.e. without government intervention.
The public sector, private sector, and communities are all gaining experience with adaptation, and adaptation is becoming embedded within certain planning processes. While some adaptation responses call for trade-offs, others bring synergies and co-benefits. Environmental organizations and public figures have emphasized changes in the climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions.
Adaptation is especially important in developing countries since they are predicted to bear the brunt of the effects of global warming. The capacity and potential for humans to adapt, called adaptive capacity, is unevenly distributed across different regions and populations, and developing countries generally have less capacity to adapt. In June 2019, U.N. special rapporteur Philip Alston warned of a "climate apartheid" situation developing, where global warming "could push more than 120 million more people into poverty by 2030 and will have the most severe impact in poor countries, regions, and the places poor people live and work".
Climate engineering (sometimes called geoengineering or climate intervention) is the deliberate modification of the climate. It has been investigated as a possible response to global warming by groups including NASA and the Royal Society. Techniques studied fall generally into the categories of solar radiation management and carbon dioxide removal, although various other schemes have been suggested. A study from 2014 investigated the most common climate engineering methods and concluded that they are either ineffective or have potentially severe side effects and cannot be stopped without causing rapid climate change.
Article 2 of the UN Framework Convention refers explicitly to "stabilization of greenhouse gas concentrations". To stabilize the atmospheric concentration of CO 2, emissions worldwide would need to be dramatically reduced from their present level.
During these negotiations, the G77 (a lobbying group in the United Nations representing developing countries) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions. This was justified on the basis that the developed countries' emissions had contributed most to the accumulation of greenhouse gases in the atmosphere, per-capita emissions were still relatively low in developing countries, and the emissions of developing countries would grow to meet their development needs.
This mandate was sustained in the 2005 Kyoto Protocol to the Framework Convention. In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to limit their emissions. These first-round commitments expired in 2012. United States President George W. Bush rejected the treaty on the basis that "it exempts 80% of the world, including major population centres such as China and India, from compliance, and would cause serious harm to the US economy". In 2009 several UNFCCC Parties produced the Copenhagen Accord, which has been widely portrayed as disappointing because of its low goals, leading poor nations to reject it. Parties associated with the Accord aim to limit the future increase in global mean temperature to below 2 °C.
Countries by Climate change performance Index
In 2015 all UN countries negotiated the Paris Agreement, which aims to keep climate change well below 2 °C. The agreement replaced the Kyoto Protocol. Unlike Kyoto, no binding emission targets are set in the Paris Agreement. Instead, the procedure of regularly setting ever more ambitious goals and reevaluating these goals every five years has been made binding. The Paris Agreement reiterated that developing countries must be financially supported. As of November 2019[update], 194 states and the European Union have signed the treaty and 186 states and the EU have ratified or acceded to the agreement. In November 2019 the Trump administration notified the UN that it would withdraw the United States from the Paris Agreement in 2020.
While the ozone layer and climate change are considered separate problems, the solution to the former has significantly mitigated global warming. The estimated mitigation of the Montreal Protocol, an international agreement to stop emitting ozone-depleting gases, is estimated to have been more impactful than the Kyoto Protocol, which was specifically designed to curb greenhouse gas emissions. It has been argued that the Montreal Protocol, may have done more than any other measure, as of 2017[update], to mitigate climate change as those substances were also powerful greenhouse gases.
National science academies have called on world leaders for policies to cut global emissions. In November 2017, a second warning to humanity signed by 15,364 scientists from 184 countries stated that "the current trajectory of potentially catastrophic climate change due to rising greenhouse gases from burning fossil fuels, deforestation, and agricultural production – particularly from farming ruminants for meat consumption" is "especially troubling". In 2018 the IPCC published a Special Report on Global Warming of 1.5 °C which warned that, if the current rate of greenhouse gas emissions is not mitigated, global warming is likely to reach 1.5 °C (2.7 °F) between 2030 and 2052, risking major crises. The report said that preventing such crises will require a swift transformation of the global economy that has "no documented historic precedent". In November 2019, a group of more than 11,000 scientists from 153 countries named climate change an "emergency" that would lead to "untold human suffering" if no big shifts in action takes place. The emergency declaration emphasized that economic growth and population growth "are among the most important drivers of increases in CO2 emissions from fossil fuel combustion" and that "we need bold and drastic transformations regarding economic and population policies".
The global warming problem came to international public attention in the late 1980s. Significant regional differences exist in how concerned people are about climate change and how much they understand the issue. In 2010, just a little over half the US population viewed it as a serious concern for either themselves or their families, while 73% of people in Latin America and 74% in developed Asia felt this way. Similarly, in 2015 a median of 54% of respondents considered it "a very serious problem", but Americans and Chinese (whose economies are responsible for the greatest annual CO2 emissions) were among the least concerned. Worldwide in 2011, people were more likely to attribute global warming to human activities than to natural causes, except in the US where nearly half of the population attributed global warming to natural causes. Public reactions to global warming and concern about its effects have been increasing, with many perceiving it as the worst global threat. In a 2019 CBS poll, 64% of the US population said that climate change is a "crisis" or a "serious problem", with 44% saying human activity was a significant contributor.
Due to confusing media coverage in the early 1990s, issues such as ozone depletion and climate change were often mixed up, affecting public understanding of these issues. Although there are a few areas of linkage, the relationship between the two is weak.
Global warming has been the subject of controversy, substantially more pronounced in the popular media than in the scientific literature, with disputes regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature, especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, whether humankind has contributed significantly to it, and whether the increase is completely or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, what the consequences of global warming will be, and what to do about it. One suggestion is that the best individual actions include having fewer children but some disagree with encouraging people to stop having children, saying that children "embody a profound hope for the future", and that more emphasis should be placed on lifestyle choices of the world's wealthy, fossil fuel companies, and government inaction.
In the 20th century and early 2000s some companies, such as ExxonMobil, challenged IPCC climate change scenarios, funded scientists who disagreed with the scientific consensus, and provided their own projections of the economic cost of stricter controls. In general, since the 2010s, global oil companies do not dispute that climate change exists and is caused by the burning of fossil fuels. As of 2019[update], however, some are lobbying against a carbon tax and plan to increase production of oil and gas, but others are in favour of a carbon tax in exchange for immunity from lawsuits which seek climate change compensation.
In Greco-Roman geography, climate was thought to be simply set by klima, the angle of the midday sun defining bands of latitude which suited different peoples. Theophrastus thought agriculture caused local climate changes. Colonists expected North American weather to match latitudes, but found winters unexpectedly harsh. 18th century naturalists, starting with Du Bos in 1719, said millennia of farming had given Europe the temperate climate needed for civilisation, and cultivation could bring rapid climate improvements to America. Thomas Jefferson and others sought evidence of this warming. Savants thought the earth had cooled from incandescent origins; in 1778 Buffon proposed climatic Epochs of diminishing warmth, shown by tropical fossils in the north.
Following from Mariotte's 1681 demonstration that glass, though transparent to sunlight, obstructs radiant heat,de Saussure showed that non-luminous warm objects emitted "obscure" (infrared) heat, and in 1774 measured heat from the sun with a thermometer in a glass-topped insulated box exposed to sunlight for an hour.
In an innovative 1824 memoir summarising research into heat transfer, Joseph Fourier assessed sources heating the globe, and the balancing emission of infrared radiation. He proposed, by analogy with de Saussure's device, a simple formulation of what was later called the greenhouse effect; transparent atmosphere lets through visible light, which warms the surface. The warmed surface emits infrared radiation, but the atmosphere is relatively opaque to infrared and slows the emission of energy, warming the planet.
Tyndall's sensitive ratio spectrophotometer measured the extent to which infrared radiation was impeded by various gases filling its central tube.
In detailed research starting in 1859, John Tyndall established that nitrogen and oxygen (99% of dry air) are transparent to infrared, but water vapour and traces of complex molecules (significantly methane and carbon dioxide) both absorb infrared and, when warmed, emit infrared radiation. His 1861 paper proposed changing concentrations of these gases could have caused "all the mutations of climate which the researches of geologists reveal" and would explain ice age changes.
By then, as Humboldt noted in 1850, decades of climate measurements showed stability in North America rather than the expected improvement, though popular belief that rain follows the plow continued.
Svante Arrhenius noted that water vapour in air continuously varied, but carbon dioxide (CO 2) was determined by long term geological processes. At the end of an ice age, warming from increased CO 2 would increase the amount of water vapour, amplifying its effect in a feedback process. In 1896, after laborious calculations, he published the first climate model of its kind, showing that halving of CO 2 could have produced the drop in temperature initiating the ice age. His source for geology, Arvid Högbom, had estimated industrial carbon output, and Arrhenius calculated the temperature increase expected from doubling CO 2. Other scientists highlighted flaws; Ångström said that saturation of the greenhouse effect meant adding more CO 2 made no difference. Experts thought climate would be self-regulating.
From 1938 Guy Stewart Callendar published evidence that climate was warming and CO 2 levels increasing, but his calculations met the same objections.
1950s military research found less saturation of the greenhouse effect at high altitudes. Earlier calculations treated the atmosphere as a single layer, Gilbert Plass used digital computers to model the different layers and found added CO 2 would cause warming. Hans Suess found evidence CO 2 levels had been rising, Roger Revelle showed the oceans would not absorb the increase, and together they helped Charles Keeling to begin a record of continued increase, the Keeling Curve. Revelle, Plass and other scientists alerted media to press for government attention, the dangers of global warming came to the fore at James Hansen's 1988 Congressional testimony. Scientific research on climate change expanded, and the Intergovernmental Panel on Climate Change, set up in 1988 to provide formal advice to the world's governments, has spurred unprecedented levels of exchange between different scientific disciplines.
Research in the 1950s suggested that temperatures were increasing, and a 1952 newspaper used the term "climate change". This phrase next appeared in a November 1957 report in The Hammond Times which described Roger Revelle's research into the effects of increasing human-caused CO 2 emissions on the greenhouse effect: "a large scale global warming, with radical climate changes may result". A 1971 MIT report referred to the human impact as "inadvertent climate modification", identifying many possible causes.
Both the terms global warming and climate change were used only occasionally until 1975, when Wallace Smith Broecker published a scientific paper on the topic, "Climatic Change: Are We on the Brink of a Pronounced Global Warming?". The phrase began to come into common use, and in 1976 Mikhail Budyko's statement that "a global warming up has started" was widely reported. An influential 1979 National Academy of Sciences study headed by Jule Charney followed Broecker in using global warming to refer to rising surface temperatures, while describing the wider effects of increased CO 2 as climate change.
There were increasing heatwaves and drought problems in the summer of 1988, and NASA climate scientist James Hansen's testimony in the U.S. Senate sparked worldwide interest. He said, "Global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and the observed warming." Public attention increased over the summer, and global warming became the dominant popular term, commonly used both by the press and in public discourse. In the 2000s, the term climate change increased in popularity.
In technical sources, the term climate change is also used to refer to past and future climate changes that persist for and extended period of time, and includes regional changes as well as global change. People who regard climate change as catastrophic, irreversible, or rapid might label climate change as a climate crisis or a climate emergency. One newspaper, The Guardian, has embraced this terminology (as well as global heating) in their editorial guidelines. In a statement explaining the paper's policy editor-in-chief, Katharine Viner said "We want to ensure that we are being scientifically precise, while also communicating clearly with readers on this very important issue".
Oxford Dictionary chose climate emergency as the word of the year 2019 and defines the term as "a situation in which urgent action is required to reduce or halt climate change and avoid potentially irreversible environmental damage resulting from it". Usage of the term soared more than 10,000% between September 2018 and September 2019.
^ abIPCC AR5 WG1 Summary for Policymakers 2013, p. 4: Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased
^Shaftel 2016: "'Climate change' and 'global warming' are often used interchangeably but have distinct meanings. .... Global warming refers to the upward temperature trend across the entire Earth since the early 20th century .... Climate change refers to a broad range of global phenomena ...[which] include the increased temperature trends described by global warming."
^IPCC AR5 SYR Glossary 2014, p. 124: Global warming refers to the gradual increase, observed or projected, in global surface temperature, as one of the consequences of radiative forcing caused by anthropogenic emissions.; IPCC SR15 Ch1 2018, p. 51: "Global warming is defined in this report as an increase in combined surface air and sea surface temperatures averaged over the globe and over a 30-year period. Unless otherwise specified, warming is expressed relative to the period 1850–1900, used as an approximation of pre-industrial temperatures in AR5.".
^Shaftel 2016; Associated Press, 22 September 2015: "The terms global warming and climate change can be used interchangeably. Climate change is more accurate scientifically to describe the various effects of greenhouse gases on the world because it includes extreme weather, storms and changes in rainfall patterns, ocean acidification and sea level.".
^IPCC AR5 WG1 Ch5 2013, pp. 389, 399–400: "5: Information from Paleoclimate Archives: The PETM [around 55.5–55.3 million years ago] was marked by ... global warming of 4 °C to 7 °C ..... Deglacial global warming occurred in two main steps from 17.5 to 14.5 ka [thousand years ago] and 13.0 to 10.0 ka.
^IPCC AR5 SYR Summary for Policymakers 2014, p. 2: SPM 1.1 .... Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. The period from 1983 to 2012 was likely the warmest 30-year period of the last 1400 years in the Northern Hemisphere, where such assessment is possible (medium confidence).
^Decision 1/CP.16, paragraph 4, in UNFCCC: Cancun 2010: "deep cuts in global greenhouse gas emissions are required according to science, and as documented in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, with a view to reducing global greenhouse gas emissions so as to hold the increase in global average temperature below 2 °C above preindustrial levels".
^ abIPCC SR15 Ch1 2018, p. 57: This report adopts the 51-year reference period, 1850–1900 inclusive, assessed as an approximation of pre-industrial levels in AR5 .... Temperatures rose by 0.0 °C–0.2 °C from 1720–1800 to 1850–1900 (Hawkins et al., 2017).
^Hawkins et al. 2017, p. 1844: "The period after 1800 is influenced by the Dalton Minimum in solar activity and the large eruptions of an unlocated volcano in 1808/09, Tambora (1815; Raible et al. 2016), and several others in the 1820s and 1830s. In addition, greenhouse gas concentrations had already increased slightly by this time .... The 1720–1800 period is most suitable to be defined as preindustrial in physical terms ... The 1850–1900 period is a reasonable pragmatic surrogate for preindustrial global mean temperature."
^IPCC AR5 WG1 Summary for Policymakers 2013, pp. 4–5: Global-scale observations from the instrumental era began in the mid-19th century for temperature and other variables ... the period 1880 to 2012 ... multiple independently produced datasets exist.
^IPCC AR5 WG1 Ch3 2013, p. 257: "Ocean warming dominates the global energy change inventory. Warming of the ocean accounts for about 93% of the increase in the Earth's energy inventory between 1971 and 2010 (high confidence), with warming of the upper (0 to 700 m) ocean accounting for about 64% of the total.
^United States Environmental Protection Agency 2016, p. 5: "Black carbon that is deposited on snow and ice darkens those surfaces and decreases their reflectivity (albedo). This is known as the snow/ice albedo effect. This effect results in the increased absorption of radiation that accelerates melting."
^NASA, 12 September 2018: "We are seeing a major shift in the circulation in the North Atlantic, likely related to a weakening Atlantic Meridional Overturning Circulation (AMOC)", said Pershing. "One of the side effects of a weaker AMOC is that the Gulf Stream shifts northward and the cold current flowing into the Gulf of Maine gets weaker. This means we get more warmer water pushing into the Gulf."
^IPCC AR4 WG1 Ch9 2007, p. 690: "Recent estimates indicate a relatively small combined effect of natural forcings on the global mean temperature evolution of the second half of the 20th century, with a small net cooling from the combined effects of solar and volcanic forcings."
^IPCC AR4 WG1 Ch1 2007, FAQ1.1: "To emit 240 W m−2, a surface would have to have a temperature of around −19 °C. This is much colder than the conditions that actually exist at the Earth's surface (the global mean surface temperature is about 14 °C). Instead, the necessary −19 °C is found at an altitude about 5 km above the surface."
^IPCC SRCCL DRAFT Ch2 2019, pp. 2–54:The global biophysical cooling alone has been estimated by a larger range of climate models and is −0.10 ± 0.14°C; it ranges from – 0.57°C to +0.06°C .........This cooling is essentially dominated by increases in surface albedo: historical land cover changes have generally led to a dominant brightening of land as discussed in AR5
^Wolff et al. 2015: "the nature and magnitude of these feedbacks are the principal cause of uncertainty in the response of Earth's climate (over multi-decadal and longer periods) to a particular emissions scenario or greenhouse gas concentration pathway."
^NASA, 16 June 2011: "So far, land plants and the ocean have taken up about 55 percent of the extra carbon people have put into the atmosphere while about 45 percent has stayed in the atmosphere. Eventually, the land and oceans will take up most of the extra carbon dioxide, but as much as 20 percent may remain in the atmosphere for many thousands of years."
^Melillo et al. 2017: Our first-order estimate of a warming-induced loss of 190 Pg of soil carbon over the 21st century is equivalent to the past two decades of carbon emissions from fossil fuel burning.
^"How the oceans absorb carbon dioxide is critical for predicting climate change". Archived from the original on 29 March 2019. Retrieved 24 February 2019. increasing CO 2 modifies the climate which in turn impacts ocean circulation and therefore ocean CO 2 uptake. Changes in marine ecosystems resulting from rising CO 2 and/or changing climate can also result in changes in air-sea CO 2 exchange. These feedbacks can change the role of the oceans in taking up atmospheric CO 2 making it very difficult to predict how the ocean carbon cycle will operate in the future.
^Lenton, Timothy M.; Rockström, Johan; Gaffney, Owen; Rahmstorf, Stefan; Richardson, Katherine; Steffen, Will; Schellnhuber, Hans Joachim (27 November 2019). "Climate tipping points — too risky to bet against". Nature. 575 (7784): 592–595. doi:10.1038/d41586-019-03595-0.
^Phys.org, 6 August 2018: "Hothouse Earth is likely to be uncontrollable and dangerous to many ... global average temperatures would exceed those of any interglacial period—meaning warmer eras that come in between Ice Ages—of the past 1.2 million years."; Steffen et al. 2018: "A Hothouse Earth trajectory would almost certainly flood deltaic environments, increase the risk of damage from coastal storms, and eliminate coral reefs (and all of the benefits that they provide for societies) by the end of this century or earlier". The Guardian, 7 August 2018.
^IPCC SROCC Ch4 2019, p. 324: GMSL (global mean sea level, red) will rise between 0.43 m (0.29–0.59 m, likely range) (RCP2.6) and 0.84 m (0.61–1.10 m, likely range) (RCP8.5) by 2100 (medium confidence) relative to 1986–2005.
^Keller, Feng & Oschlies 2014: "We find that even when applied continuously and at scales as large as currently deemed possible, all methods are, individually, either relatively ineffective with limited (<8%) warming reductions, or they have potentially severe side effects and cannot be stopped without causing rapid climate change."
^NOAA, 17 June 2015; IPCC AR5 SYR Glossary 2014, p. 120: "Climate change refers to a change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use."
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