globalwarming.txt

Global warming is a long-term rise in the average temperature of the Earth's climate system, an aspect of climate change shown by temperature measurements and by multiple effects of the warming. The term commonly refers to the mainly human-caused observed increase in global surface temperatures and its projected continuation, though there were also much earlier periods of global warming. In the modern context the terms global warming and climate change are commonly used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region.
Many observed changes since mid 20th century have been unprecedented compared to records over decades to thousands of years.

In 2013, the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century." The largest human influence has been the emission of greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Climate model projections summarized in the report indicated that during the 21st century, the global surface temperature is likely to rise a further 0.3 to 1.7 °C (0.5 to 3.1 °F) in a moderate scenario, or as much as 2.6 to 4.8 °C (4.7 to 8.6 °F) in an extreme scenario, depending on the rate of future greenhouse gas emissions and on climate feedback effects.
These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing.

Effects of global warming include rising sea levels, regional changes in precipitation, more frequent extreme weather events such as heat waves, and expansion of deserts. Surface temperature increases are greatest in the Arctic, with the continuing retreat of glaciers, permafrost, and sea ice. Overall, higher temperatures bring more rain and snowfall, but for some regions droughts and wildfires increase instead. Climate change threatens to diminish crop yields, harming food security, and rising sea levels may flood costal infrastructure and eventually force the abandonment of many coastal cities.
Environmental impacts include the extinction or relocation of many species as their ecosystems change, most immediately the enviroments of coral reefs,[18] mountains, and the Arctic.[19] Because the climate system has a large "inertia" and greenhouse gases persist in the atmosphere, climatic changes and their effects will intensify for many centuries even if further greenhouse emissions stop.

Globally, a majority of people consider global warming a serious or very serious issue. Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future climate engineering. Every country in the world is a party to the United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate objective is to prevent dangerous anthropogenic climate change. 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) compared to pre-industrial levels, 
has already experienced almost 1/2 of the 2.0 °C (3.6 °F) described in the Cancún Agreement. In the last 100 years, Earth's average surface temperature increased by about 0.8 °C (1.4 °F) with about two-thirds of the increase occurring over just the last three decades. With efforts made to limit warming to 1.5 °C (2.7 °F).[26] Some scientists call into question climate adaptation feasibility, with higher emissions scenarios, or the two degree temperature target.

Multiple independently produced datasets confirm that between 1880 and 2012, the global average (land and ocean) surface temperature increased by 0.85 [0.65 to 1.06] °C.[29] Currently, surface temperature rise with about 0.2 °C degrees per decade.[30] Since 1950, the number of cold days and nights have decreased, and the number of warm days and night have increased.[31] These trends can be compared to historical temperature trends: patterns of warming and cooling like the Medieval Climate Anomaly and the Little Ice Age were not as synchronous as current warming, but did reach temperatures as high as late-20th century regionally.

Although the increase of the average near-surface atmospheric temperature is commonly used to track global warming, over 90% of the additional energy stored in the climate system over the last 50 years is in warmer ocean water. The rest has melted ice and warmed the continents and the atmosphere. Melting ice (including Arctic sea ice, ice sheets and glaciers) and warming of the continents and atmosphere account for the remainder of the change in energy." 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.

Global warming refers to global averages, with the amount of warming varying by region. Since 1979, global average land temperatures have increased about twice as fast as global average ocean temperatures. This is due to the larger heat capacity of the oceans and because oceans lose more heat by evaporation. Where greenhouse gas emissions occur does not impact the location of warming because the major greenhouse gases persist long enough to diffuse across the planet, although localized black carbon deposits on snow and ice do contribute to Arctic warming.
The Northern Hemisphere and North Pole have heated much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, its arrangement 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. 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. 

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 overlay 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 by the media and some scientists. 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 heating and cooling in the Pacific Ocean from natural variability such as El Niño and La Nina events, reduced solar activity, and a number of volcanic eruptions that inserted sunlight-reflecting particles into the atmosphere.

By itself, the climate system experiences various cycles which can last for years (such as the El Niño–Southern Oscillation) to decadesor centuries at a time. Other changes are caused by so-called external forcings. These forcings are "external" to the climate system, but not always external to Earth.[54] Examples of external forcings include changes in the composition of the atmosphere (e.g., increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.
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. So 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 the observed pattern and evolution of the 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 in the current climate change are greenhouse gases, land use changes, and aerosols and soot. 

Greenhouse gases trap heat radiating from Earth to space.[57] This heat, in the form of infrared radiation, gets absorbed and emitted by these gases in the planet's atmosphere thus warming the lower atmosphere and the surface. On Earth, an atmosphere containing naturally occurring amounts of greenhouse gases causes air temperature near the surface to be warmer by about 33 °C (59 °F) than it would be in their absence.[58][c] Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing temperature of water.[59] The major greenhouse gases are water vapour, which causes about 36–70% of the greenhouse effect; carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone (O3), which causes 3–7%.[60][61][62]

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. In 2011, the concentrations of CO2 and methane had increased by about 40% and 150% respectively since pre-industrial times,[63] with CO2 readings taken at the world's primary benchmark site in Mauna Loa surpassing 400 ppm in 2013 for the first time.[64] These levels are much higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores.[65] Less direct geological evidence indicates that CO2 values haven't been this high for millions of years.[64]

Global anthropogenic greenhouse gas emissions in 2010 were 49 billion tonnes of carbon dioxide-equivalents per year (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 methane, 6.2% was nitrous oxide, and 2.0% were fluorinated gases.[66] Using life-cycle assessment to estimate emissions relating to final consumption, the dominant sources of 2010 emissions were: food (26–30% of emissions);[67] washing, heating and lighting (26%); personal transport and freight (20%); and building construction (15%).[68]
Land use change

Changing the type of vegetation in a region impacts the local temperature by changing how much sunlight gets reflected back into space and how much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, and causes it to reflect more sunlight: an increase in albedo. Humans change the land surface mainly to create more agricultural land.[69] Since the pre-industrial era, albedo increased due to land use change, which has a cooling effect on the planet. Other processes linked to land use change however have had the opposite effect, so that the net effect remains unclear.

Aerosols and soot
Solid and liquid particles known as aerosols – from volcanoes, plankton and human-made pollutants – reflect incoming sunlight,[71] cooling the climate.[72] From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed, a phenomenon popularly known as global dimming,[73] typically attributed to aerosols from biofuel and fossil fuel burning.[74] Aerosol removal by precipitation gives tropospheric aerosols an atmospheric lifetime of only about a week, while stratospheric aerosols can remain in the atmosphere for a few years.[75] Global aerosols have been declining since 1990, removing some of the masking of global warming that aerosols had been providing.[76]

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, a phenomenon known as the Twomey effect.[77] This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect.[78] Indirect effects of aerosols are the largest uncertainty in radiative forcing.[79][needs update]

While aerosols typically limit global warming by reflecting sunlight, if black carbon in soot falls on snow or ice, it can also increase global warming. Not only does it increase the absorption of sunlight, it also increases melting and sea level rise.[80] Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 degrees Celsius by 2050.[81] 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.[82] The influences of atmospheric particles, including black carbon, are most pronounced in the tropics and northern mid-latitudes, with the effects of greenhouse gases dominant in the other parts of the world.[83][84]
Minor forcings: the sun and ozone
Further information: Solar activity and climate

As the Sun is Earth's primary energy source, changes in incoming sunlight directly affect the climate system.[85] Solar irradiance has been measured directly by satellites[86] and indirect measurements are available beginning in the early 1600s.[85] 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.[87] 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.[88]

Another line of evidence for the warming not being due to the Sun is the temperature changes at different levels in the Earth's atmosphere.[89] According to basic physical principles, the greenhouse effect produces warming of the lower atmosphere (the troposphere), but cooling of the upper atmosphere (the stratosphere).[90] 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.[91]

Climate change feedback
The response of the climate system to an initial forcing is increased by positive feedbacks and reduced by negative feedbacks.[93] The main negative feedback to global temperature change is radiative cooling to space as infrared radiation, which increases strongly with increasing temperature.[94] The main positive feedbacks are the water vapour feedback, the ice–albedo feedback, and probably the net effect of clouds.[95] Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.[96]

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.[95] The reduction of snow cover and sea ice in the Arctic reduces the albedo (reflectivity) of the Earth's surface.[97] More of the sun's energy is now absorbed in these regions, contributing to Arctic amplification, which has caused Arctic temperatures to increase at almost twice the rate of the rest of the world.[45] 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.[98]

Cloud cover may change in the future. To date, cloud changes have had a cooling effect, with NASA estimating that aerosols produced by the burning of hydrocarbons have limited warming by half from 1850 to 2010.[71] An analysis of satellite data between 1983 and 2009 reveals that cloud tops are reaching higher into the atmosphere and that cloudy storm tracks are shifting toward Earth's poles, suggesting clouds will be a positive feedback in the future.[99]

Carbon dioxide stimulates plant growth so the carbon cycle has been a negative feedback so far: roughly half of each year's CO2 emissions have been absorbed by plants on land and in oceans,[100] with an estimated 30% increase in plant growth from 2000 to 2017.[101] The limits and reversal point for this feedback are an area of uncertainty.[102] 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.[103] On land, greater plant growth will be constrained by nitrogen levels and can be reversed by plant heat stress, desertification, and the release of carbon from soil as the ground warms.[104]

A concern is that positive 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.

Mitigation
Mitigation of climate change is the reduction of greenhouse gas emissions, or the enhancement of the capacity of carbon sinks to absorb greenhouse gases from the atmosphere.[175] There is a large potential for future reductions in emissions by a combination of activities, including energy conservation and increased energy efficiency; the use of low-carbon energy technologies, such as renewable energy, nuclear energy, and carbon capture and storage; decarbonizing buildings and transport; and enhancing carbon sinks through, for example, reforestation and preventing deforestation.[176][177] A 2015 report by Citibank concluded that transitioning to a low carbon economy would yield positive return on investments.
Drivers of greenhouse gas emissions

Over the last three decades of the twentieth century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions.[179] CO2 emissions are continuing to rise due to the burning of fossil fuels and land-use change.[180] Emissions can be attributed to different regions. Attribution of emissions due to land-use change are subject to considerable uncertainty.[181]

Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological, technological, and natural developments.[182] In some scenarios emissions continue to rise over the century, while others have reduced emissions.[183] Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century.[184] Emission scenarios, combined with modelling of the carbon cycle, have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future.[185] Depending both on the Shared Socioeconomic Pathway (SSP) the world takes and the mitigation scenario, model suggest that by the year 2100, the atmospheric concentration of CO2 could range between 380 and 1400 ppm.[186]
Reducing greenhouse gases

Near- and long-term trends in the global energy system are inconsistent with limiting global warming at below 1.5 or 2 °C, relative to pre-industrial levels.[187] 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.[188] In limiting warming at below 2 °C, more stringent emission reductions in the near-term would allow for less rapid reductions after 2030,[189] and be cheaper overall.[190] Many integrated models are unable to meet the 2 °C target if pessimistic assumptions are made about the availability of mitigation technologies.[191]

Co-benefits of climate change mitigation may help society and individuals more quickly. For example, cycling reduces greenhouse gas emissions[192] while reducing the effects of a sedentary lifestyle at the same time.[193] The development and scaling-up of clean technology, such as cement that produces less CO2.[194] is critical to achieve sufficient emission reductions for the Paris agreement goals.[195]

It has been suggested that the most effective and comprehensive policy to reduce carbon emissions is a carbon tax[196] or the closely related emissions trading.[197] There are diverse opinions on how people could mitigate their carbon footprint. One suggestion is that the best approach is having fewer children, and to a lesser extent living car-free, forgoing air travel, and adopting a plant-based diet.[198] 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 overconsumption, lifestyle choices of the world's wealthy, fossil fuel companies and government inaction.[199] Still others, such as Mayer Hillman, contend that both individual action and political action by national governments will not be enough, and only a global transition to zero GHG emissions throughout the entire economy and a reduction in human population growth will be sufficient to mitigate global warming.[200]
Adaptation
Main article: Climate change adaptation

Climate change adaptation is the process of adjusting to actual or expected climate and its effects.[201] Humans can strive to moderate or avoid harm due to climate change and exploit opportunities.[201] Examples of adaptation are improved coastline protection, better disaster management and the development of crops that are more resistant.[202] The adaptation may be planned, either in reaction to or anticipation of global warming, or spontaneous, i.e., without government intervention.[203]

The public section, private sector and communities are all gaining experience with adaptation and adaptation is becoming embedded within certain planning processes.[204] While some adaptation responses call for trade-offs, others bring synergies and co-benefits.[204] 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.[205]

Adaptation is especially important in developing countries since those countries are predicted to bear the brunt of the effects of global warming.[206] That is, 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.[207] 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."[208]
Climate engineering
Main article: Climate engineering

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, e.g. by NASA[209] and the Royal Society.[210] Techniques under research fall generally into the categories 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 they are either ineffective or have potentially severe side effects and cannot be stopped without causing rapid climate change.[211]

Society and culture
Political response
As of 2019 all countries in the world are parties to the United Nations Framework Convention on Climate Change (UNFCCC), but 12 countries have not ratified it,[214] which means they are not legally bound by the agreement.[215] The ultimate objective of the Convention is to prevent dangerous human interference to the climate system.[216] 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.[217] The Framework Convention was agreed on in 1992, but global emissions have risen since then.[218] Its yearly conferences are the stage of global negotiations.[219]

During these negotiations, the G77 (a lobbying group in the United Nations representing developing countries)[220] pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions.[221] 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 (i.e., emissions per head of population) were still relatively low in developing countries, and the emissions of developing countries would grow to meet their development needs.[222]

This mandate was sustained in the 2005 Kyoto Protocol to the Framework Convention.[223] In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to limit their emissions. These first-round commitments expired in 2012.[224] 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".[225]

In 2009 several UNFCCC Parties produced the Copenhagen Accord,[226] which has been widely portrayed as disappointing because of its low goals, leading poor nations to reject it.[227] Parties associated with the Accord aim to limit the future increase in global mean temperature to below 2 °C.[228] 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.[229] The Paris agreement reiterated that developing countries must be financially supported.

Scientific discussion
In the scientific literature, there is an overwhelming consensus that global surface temperatures have increased in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases.[230] No scientific body of national or international standing disagrees with this view.[231] Scientific discussion takes place in journal articles that are peer-reviewed, which scientists subject to assessment every couple of years in the Intergovernmental Panel on Climate Change reports.[232] The scientific consensus as of 2013 stated in the IPCC Fifth Assessment Report is that it "is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century".[233]

National science academies have called on world leaders for policies to cut global emissions.[234] 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".[235] 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".[236]
Fossil fuel companies
See also: Fossil fuels lobby

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.[237] In general, since the 2010s, global oil companies do not dispute that climate change exists and is caused by the burning of fossil fuels.[238] As of 2019, however, some are lobbying against a carbon tax and plan to increase production of oil and gas[239] but others are in favour of a carbon tax in exchange for immunity from lawsuits which seek climate change compensation.