global warming

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Global mean land-ocean temperature change from 1880–2011, relative to the 1951–1980 mean. The black line is the annual mean and the red line is the 5-year running mean. The green bars show uncertainty estimates. Source: NASA GISS

The map shows the 10-year average (2000–2009) global mean temperature anomaly relative to the 1951–1980 mean. The largest temperature increases are in the Arctic and the Antarctic Peninsula. Source: NASA Earth Observatory^[1]

Fossil fuel related CO2 emissions compared to five of IPCC's emissions scenarios. The dips are related to global recessions. Data from IPCC SRES scenarios; Data spreadsheet included with International Energy Agency's "CO2 Emissions from Fuel Combustion 2010 – Highlights"; and Supplemental IEA data. Image source: Skeptical Science

Global warming refers to the rising average temperature of Earth's atmosphere and oceans, which started to increase in the late 19th century and is projected to keep going up without significant mitigation policies. Since the early 20th century, Earth's average surface temperature has increased by about 0.8 °C (1.4 °F), with about two thirds of the increase occurring since 1980.^[2] Warming of the climate system is unequivocal, and scientists are more than 90% certain that most of it is caused by increasing concentrations of greenhouse gases produced by human activities such as deforestation and burning fossil fuels.^[3][4][5][6] These findings are recognized by the national science academies of all the major industrialized nations.^[7][A]

Climate model projections are summarized in the 2007 Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC). They indicate that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F) for their lowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest.^[8] The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations.^[9][10]

An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, and a probable expansion of subtropical deserts.^[11] Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent occurrence of extreme-weather events including heat waves, droughts and heavy rainfall, species extinctions due to shifting temperature regimes, and changes in crop yields. Warming and related changes will vary from region to region around the globe, with projections being more robust in some areas than others.^[12] If global mean temperature increases to 4 °C above preindustrial levels, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved.^[13]

Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC),^[14] whose ultimate objective is to prevent "dangerous" anthropogenic (i.e., human-induced) climate change.^[15] Parties to the UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions^{[16]:10[17][18][19]:9} and to assist in adaptation to global warming.^{[16]:13[19]:10[20][21]} Parties to the UNFCCC have agreed that deep cuts in emissions are required,^[22] and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre-industrial level.^[22][B] A 2011 report of analyses by the United Nations Environment Programme^[23] and International Energy Agency^[24] suggest that efforts as of the early 21st century to reduce emissions may be inadequately stringent to meet the UNFCCC's 2 °C target.

Observed temperature changes

Two millennia of mean surface temperatures according to different reconstructions from climate proxies, each smoothed on a decadal scale, with the instrumental temperature record overlaid in black.

Evidence for warming of the climate system includes observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.^[25][26][27] The Earth's average surface temperature, expressed as a linear trend, rose by 0.74±0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13±0.03 °C per decade, versus 0.07±0.02 °C per decade). The urban heat island effect is very small, estimated to account for less than 0.002 °C of warming per decade since 1900.^[28] Temperatures in the lower troposphere have increased between 0.13 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Climate proxies show the temperature to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.^[29]

Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree.^[30][31][32] Estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, “the error estimate for individual years ... is at least ten times larger than the differences between these three years.”^[33] The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, “The 2010 nominal value of +0.53 °C ranks just ahead of those of 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not statistically significant...”^[34]

NOAA graph of Global Annual Temperature Anomalies 1950–2011, showing the El Niño-Southern Oscillation

Temperatures in 1998 were unusually warm because global temperatures are affected by the El Niño-Southern Oscillation (ENSO), and the strongest El Niño in the past century occurred during that year.^[35] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.^[36][37] 2010 was also an El Niño year. On the low swing of the oscillation, 2011 as an La Niña year was cooler but it was still the 11th warmest year since records began in 1880. Of the 13 warmest years since 1880, 11 were the years from 2001 to 2011. Over the more recent record, 2011 was the warmest "La Niña year" in the period from 1950 to 2011, and was close to 1997 which was not at the lowest point of the cycle.^[38]

Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).^[39] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.^[40] The Northern Hemisphere warms faster than the Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.^[41]

The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.^[42]

Initial causes of temperature changes (external forcings)

Greenhouse effect schematic showing energy flows between space, the atmosphere, and earth's surface. Energy exchanges are expressed in watts per square meter (W/m²).

This graph, known as the "Keeling Curve", shows the long-term increase of atmospheric carbon dioxide (CO₂) concentrations from 1958–2008. Monthly CO₂ measurements display seasonal oscillations in an upward trend; each year's maximum occurs during the Northern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO₂.

External forcing refers to processes external to the climate system (though not necessarily external to Earth) that influence climate. Climate responds to several types of external forcing, such as radiative forcing due to changes in atmospheric composition (mainly greenhouse gas concentrations), changes in solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.*^[43]:0 Attribution of recent climate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens of thousands of years and at present are in an overall cooling trend which would be expected to lead towards an ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures.^[44]

Greenhouse gases

The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere and surface. It was proposed by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.^[45]

Naturally occurring amounts of greenhouse gases have a mean warming effect of about 33 °C (59 °F).^[46] </ref>^[C] The major greenhouse gases are water vapor, 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%.^[47][48][49] Clouds also affect the radiation balance through cloud forcings similar to greenhouse gases.

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. The concentrations of CO₂ and methane have increased by 36% and 148% respectively since 1750.^[50] 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.^{[51][52][53][54]} Less direct geological evidence indicates that CO₂ values higher than this were last seen about 20 million years ago.^[55] Fossil fuel burning has produced about three-quarters of the increase in CO₂ from human activity over the past 20 years. The rest of this increase is caused mostly by changes in land-use, particularly deforestation.^[56]

Per capita greenhouse gas emissions in 2005, including land-use change.

Total greenhouse gas emissions in 2005, including land-use change.

Over the last three decades of the 20th century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions.^[57] CO₂ emissions are continuing to rise due to the burning of fossil fuels and land-use change.^[58][59]:71 Emissions can be attributed to different regions. The two figures opposite show annual greenhouse gas emissions for the year 2005, including land-use change. Attribution of emissions due to land-use change is a controversial issue.^[60][61]:289

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.^[62] In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced.^[63][64] Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century.^[65] 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. Using the six IPCC SRES "marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO₂ could range between 541 and 970 ppm.^[66] This is an increase of 90–250% above the concentration in the year 1750.

The popular media and the public often confuse global warming with ozone depletion, i.e., the destruction of stratospheric ozone by chlorofluorocarbons.^[67][68] Although there are a few areas of linkage, the relationship between the two is not strong. Reduced stratospheric ozone has had a slight cooling influence on surface temperatures, while increased tropospheric ozone has had a somewhat larger warming effect.^[69]

Particulates and soot

Ship tracks over the Atlantic Ocean on the east coast of the United States. The climatic impacts from particulate forcing could have a large effect on climate through the indirect effect.

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.^{[70][dated info]} The main cause of this dimming is particulates produced by volcanoes and human made pollutants, which exerts a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion – CO₂ and aerosols – have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO₂ greenhouse gases such as methane.^[71] Radiative forcing due to particulates is temporally limited due to wet deposition which causes them to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in particulate concentrations will only delay climate changes due to carbon dioxide.^[72]

In addition to their direct effect by scattering and absorbing solar radiation, particulates have indirect effects on the radiation budget.^[73] Sulfates 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, known as the Twomey effect.^[74] 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.^[75] Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of particulates represent the largest uncertainty in radiative forcing.^[76]

Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated 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.^[77] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.^[78] The influences of particulates, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.^[79]

Satellite observations of Total Solar Irradiance from 1979–2006.

Solar activity

Variations in solar activity have been the cause of past climate changes.^[80] The effect of changes in solar forcing in recent decades is uncertain, but small, with some studies showing a slight cooling effect,^[81] while others studies suggest a slight warming effect.*^{[43][82][83][84]}

Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere.*^[43] Radiosonde (weather balloon) data show the stratosphere has cooled over the period since observations began (1958), though there is greater uncertainty in the early radiosonde record. Satellite observations, which have been available since 1979, also show cooling.^[85]

A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.^[86] Other research has found no relation between warming in recent decades and cosmic rays.^[87][88] The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.^[89]

Studies in 2011 have indicated that solar activity may be slowing, and that the next solar cycle could be delayed. To what extent is not yet clear; Solar Cycle 25 is due to start in 2020, but may be delayed to 2022 or even longer. It is even possible that Sol could be heading towards another Maunder Minimum. While there is not yet a definitive link between solar sunspot activity and global temperatures, the scientists conducting the solar activity study believe that global greenhouse gas emissions would prevent any possible cold snap.^[90]

Feedback

Feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback increases the change in the first quantity while negative feedback reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process.

The main positive feedback in the climate system is the water vapor feedback. The main negative feedback is radiative cooling through the Stefan–Boltzmann law, which increases as the fourth power of temperature. Positive and negative feedbacks are not imposed as assumptions in the models, but are instead emergent properties that result from the interactions of basic dynamical and thermodynamic processes.

A wide range of potential feedback processes exist, such as Arctic methane release and ice-albedo feedback. Consequentially, potential tipping points may exist, which may have the potential to cause abrupt climate change.^[91]

For example, the "emission scenarios" used by IPCC in its 2007 report primarily examined greenhouse gas emissions from human sources. In 2011, a joint study by the US National Snow and Ice Data Center and National Oceanic and Atmospheric Administration calculated the additional greenhouse gas emissions that would emanate from melted and decomposing permafrost, even if policymakers attempt to reduce human emissions from the A1FI scenario to the A1B scenario.^[92] The team found that even at the much lower level of human emissions, permafrost thawing and decomposition would still result in 190 Gt C of permafrost carbon being added to the atmosphere on top of the human sources. Importantly, the team made three extremely conservative assumptions: (1) that policymakers will embrace the A1B scenario instead of the A1FI scenario, (2) that all of the carbon would be released as carbon dioxide instead of methane, which is more likely and over a 20 year lifetime has 72x the greenhouse warming power of CO2, and (3) their model did not project additional temperature rise caused by the release of these additional gases.^[92][93] These very conservative permafrost carbon dioxide emissions are equivalent to about 1/2 of all carbon released from fossil fuel burning since the dawn of the Industrial Age,^[94] and is enough to raise atmospheric concentrations by an additional 87±29 ppm, beyond human emissions. Once initiated, permafrost carbon forcing (PCF) is irreversible, is strong compared to other global sources and sinks of atmospheric CO2, and due to thermal inertia will continue for many years even if atmospheric warming stops.^[92] A great deal of this permafrost carbon is actually being released as highly flammable methane instead of carbon dioxide.^[95] IPCC 2007's temperature projections did not take any of the permafrost carbon emissions into account and therefore underestimate the degree of expected climate change.^[92][93]

Other research published in 2011 found that increased emissions of methane could instigate significant feedbacks that amplify the warming attributable to the methane alone. The researchers found that a 2.5-fold increase in methane emissions would cause indirect effects that increase the warming 250% above that of the methane alone. For a 5.2-fold increase, the indirect effects would be 400% of the warming from the methane alone.^[96]

Climate models

Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions and regionally divided economic development.

The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

A climate model is a computerized representation of the five components of the climate system: Atmosphere, hydrosphere, cryosphere, land surface, and biosphere.^[97] Such models are based on physical principles including fluid dynamics, thermodynamics and radiative transfer. There can be components which represent air movement, temperature, clouds, and other atmospheric properties; ocean temperature, salt content, and circulation; ice cover on land and sea; the transfer of heat and moisture from soil and vegetation to the atmosphere; chemical and biological processes; and others.

Although researchers attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. Results from models can also vary due to different greenhouse gas inputs and the model's climate sensitivity. For example, the uncertainty in IPCC's 2007 projections is caused by (1) the use of multiple models with differing sensitivity to greenhouse gas concentrations, (2) the use of differing estimates of humanities' future greenhouse gas emissions, (3) any additional emissions from climate feedbacks that were not included in the models IPCC used to prepare its report, i.e., greenhouse gas releases from permafrost.^[92]

The models do not assume the climate will warm due to increasing levels of greenhouse gases. Instead the models predict how greenhouse gases will interact with radiative transfer and other physical processes. One of the mathematical results of these complex equations is a prediction whether warming or cooling will occur.^[98]

Recent research has called special attention to the need to refine models with respect to the effect of clouds^[99] and the carbon cycle.^{[100][101][102]}

Models are also used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1970 is dominated by man-made greenhouse gas emissions.*^[43]

The physical realism of models is tested by examining their ability to simulate contemporary or past climates.^[103]

As of 2012, climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.^[104] Not all effects of global warming are accurately predicted by the climate models used by the IPCC. Observed Arctic shrinkage has been faster than that predicted.^[105] Precipitation increased proportional to atmospheric humidity, and hence significantly faster than global climate models predict.^[106][107]

Expected effects

"Detection" is the process of demonstrating that climate has changed in some defined statistical sense, without providing a reason for that change. Detection does not imply attribution of the detected change to a particular cause. "Attribution" of causes of climate change is the process of establishing the most likely causes for the detected change with some defined level of confidence.^[108] Detection and attribution may also be applied to observed changes in physical, ecological and social systems.^[109]

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the World Glacier Monitoring Service (WGMS) and the National Snow and Ice Data Center (NSIDC)

Natural systems

Global warming has been detected in a number of systems. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on temperature changes. Rising sea levels and observed decreases in snow and ice extent are consistent with warming.^[110] Most of the increase in global average temperature since the mid-20th century is, with high probability,^[D] attributable to human-induced changes in greenhouse gas concentrations.^[111]

Even with policies to reduce emissions, global emissions are still expected to continue to grow over time.^[112]

In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21st century (the year 2090–2099, relative to 1980–1999) range from 0.18 to 0.59 m. These estimates, however, were not given a likelihood due to a lack of scientific understanding, nor was an upper bound given for sea level rise. On the timescale of centuries to millennia, the melting of ice sheets could result in even higher sea level rise. Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic Ice Sheet, could contribute 4–6 metres (13 to 20 ft) or more to sea level rise.^[113]

Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean.^[112] Snow cover area and sea ice extent are expected to decrease, with the Arctic expected to be largely ice-free in September by 2037.^[114] The frequency of hot extremes, heat waves, and heavy precipitation will very likely increase.

Ecological systems

In terrestrial ecosystems, the earlier timing of spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming.^[110] Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs.^[112] It is expected that most ecosystems will be affected by higher atmospheric CO₂ levels, combined with higher global temperatures.^[115] Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.^[116]

Social systems

Vulnerability of human societies to climate change mainly lies in the effects of extreme-weather events rather than gradual climate change.^[117] Impacts of climate change so far include adverse effects on small islands,^[118] adverse effects on indigenous populations in high-latitude areas,^[119] and small but discernable effects on human health.^[120] Over the 21st century, climate change is likely to adversely affect hundreds of millions of people through increased coastal flooding, reductions in water supplies, increased malnutrition and increased health impacts.^[121]

Future warming of around 3 °C (by 2100, relative to 1990–2000) could result in increased crop yields in mid- and high-latitude areas, but in low-latitude areas, yields could decline, increasing the risk of malnutrition.^[118] A similar regional pattern of net benefits and costs could occur for economic (market-sector) effects.^[120] Warming above 3 °C could result in crop yields falling in temperate regions, leading to a reduction in global food production.^[122] Most economic studies suggest losses of world gross domestic product (GDP) for this magnitude of warming.^[123][124]

Responses to global warming

Mitigation

Reducing the amount of future climate change is called mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere.^[125] Many countries, both developing and developed, are aiming to use cleaner, less polluting, technologies.^[59]:192 Use of these technologies aids mitigation and could result in substantial reductions in CO₂ emissions. Policies include targets for emissions reductions, increased use of renewable energy, and increased energy efficiency. Studies indicate substantial potential for future reductions in emissions.^[126]

In order to limit warming to within the lower range described in the IPCC's "Summary Report for Policymakers"^[127] it will be necessary to adopt policies that will limit greenhouse gas emissions to one of several significantly different scenarios described in the full report.^[128] This will become more and more difficult with each year of increasing volumes of emissions and even more drastic measures will be required in later years to stabilize a desired atmospheric concentration of greenhouse gases. Energy-related carbon-dioxide (CO2) emissions in 2010 were the highest in history, breaking the prior record set in 2008.^[129]

Since even in the most optimistic scenario, fossil fuels are going to be used for years to come, mitigation may also involve carbon capture and storage, a process that traps CO₂ produced by factories and gas or coal power stations and then stores it, usually underground.^[130]

Adaptation

Other policy responses include adaptation to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention.^[131] The ability to adapt is closely linked to social and economic development.^[126] Even societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood.

Geoengineering

A body of the scientific literature has developed which considers alternative geoengineering techniques for climate change mitigation.^[132] In the IPCC's Fourth Assessment Report (published in 2007) Working Group III (WG3) assessed some "apparently promising" geoengineering techniques, including ocean fertilization, capturing and sequestering CO₂, and techniques for reducing the amount of sunlight absorbed by the Earth's atmospheric system.^[132] The IPCC's overall conclusion was that geoengineering options remained "largely speculative and unproven, (...) with the risk of unknown side-effects."^[133] In the IPCC's^[133] judgement, reliable cost estimates for geoengineering options had not yet been published.

As most geoengineering techniques would affect the entire globe, deployment would likely require global public acceptance and an adequate global legal and regulatory framework, as well as significant further scientific research.^[134]

Views on global warming

There are different views over what the appropriate policy response to climate change should be.^[135] These competing views weigh the benefits of limiting emissions of greenhouse gases against the costs. In general, it seems likely that climate change will impose greater damages and risks in poorer regions.^[136]

Global warming controversy

The global warming controversy refers to a variety of disputes, significantly more pronounced in the popular media than in the scientific literature,^[137][138] 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 wholly or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, and what the consequences of global warming will be.

In the scientific literature, there is a strong consensus that global surface temperatures have increased in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases. No scientific body of national or international standing disagrees with this view,^[139][140] though a few organisations hold non-committal positions.

From 1990-1997 in the United States, conservative think tanks mobilized to undermine the legitimacy of global warming as a social problem. They challenged the scientific evidence; argued that global warming will have benefits; and asserted that proposed solutions would do more harm than good.^[141]

Politics

Article 2 of the UN Framework Convention refers explicitly to "stabilization of greenhouse gas concentrations."^[142] In order to stabilize the atmospheric concentration of CO₂, emissions worldwide would need to be dramatically reduced from their present level.^[143]

Most countries are Parties to the United Nations Framework Convention on Climate Change (UNFCCC).^[144] The ultimate objective of the Convention is to prevent "dangerous" human interference of the climate system.^[145] As is stated in the Convention, this requires that GHG 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 proceed in a sustainable fashion.^[146] The Framework Convention was agreed in 1992, but since then, global emissions have risen.^[147] During negotiations, the G77 (a lobbying group in the United Nations representing 133 developing nations)^[148]:4 pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions.^[149] This was justified on the basis that: the developed world's emissions had contributed most to the stock of GHGs 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.^[61]:290 This mandate was sustained in the Kyoto Protocol to the Framework Convention,^[61]:290 which entered into legal effect in 2005.^[150]

In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to limit their emissions. These first-round commitments expire in 2012.^[150] US President George W. Bush rejected the treaty on the basis that "it exempts 80% of the world, including major population centers such as China and India, from compliance, and would cause serious harm to the US economy."^[148]:5

At the 15th UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC Parties produced the Copenhagen Accord.^[151] Parties associated with the Accord (140 countries, as of November 2010)^[152]:9 aim to limit the future increase in global mean temperature to below 2 °C.^[153] A preliminary assessment published in November 2010 by the United Nations Environment Programme (UNEP) suggests a possible "emissions gap" between the voluntary pledges made in the Accord and the emissions cuts necessary to have a "likely" (greater than 66% probability) chance of meeting the 2 °C objective.^{[152]:10–14} The UNEP assessment takes the 2 °C objective as being measured against the pre-industrial global mean temperature level. To having a likely chance of meeting the 2 °C objective, assessed studies generally indicated the need for global emissions to peak before 2020, with substantial declines in emissions thereafter.

The 16th Conference of the Parties (COP16) was held at Cancún in 2010. It produced an agreement, not a binding treaty, that the Parties should take urgent action to reduce greenhouse gas emissions to meet a goal of limiting global warming to 2 °C above pre-industrial temperatures. It also recognized the need to consider strengthening the goal to a global average rise of 1.5 °C.^[154]

Public opinion

The examples and perspective in this section deal primarily with English-speaking territories and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (October 2011)

Based on Rasmussen polling of 1,000 adults in the USA conducted 29–30 July 2011.^[155]

In 2007–2008 Gallup Polls surveyed 127 countries. Over a third of the world's population was unaware of global warming, with people in developing countries less aware than those in developed, and those in Africa the least aware. Of those aware, Latin America leads in belief that temperature changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few countries from the Former Soviet Union lead in the opposite belief.^[156] In the Western world, opinions over the concept and the appropriate responses are divided. Nick Pidgeon of Cardiff University said that "results show the different stages of engagement about global warming on each side of the Atlantic", adding, "The debate in Europe is about what action needs to be taken, while many in the US still debate whether climate change is happening."^[157][158] A 2010 poll by the Office of National Statistics found that 75% of UK respondents were at least "fairly convinced" that the world's climate is changing, compared to 87% in a similar survey in 2006.^[159] A January 2011 ICM poll in the UK found 83% of respondents viewed climate change as a current or imminent threat, while 14% said it was no threat. Opinion was unchanged from an August 2009 poll asking the same question, though there had been a slight polarisation of opposing views.^[160]

A survey in October, 2009 by the Pew Research Center for the People & the Press showed decreasing public perception in the US that global warming was a serious problem. All political persuasions showed reduced concern with lowest concern among Republicans, only 35% of whom considered there to be solid evidence of global warming.^[161] The cause of this marked difference in public opinion between the US and the global public is uncertain but the hypothesis has been advanced that clearer communication by scientists both directly and through the media would be helpful in adequately informing the American public of the scientific consensus and the basis for it.^[162] The US public appears to be unaware of the extent of scientific consensus regarding the issue, with 59% believing that scientists disagree "significantly" on global warming.^[163]

By 2010, with 111 countries surveyed, Gallup determined that there was a substantial decrease in the number of Americans and Europeans who viewed Global Warming as a serious threat. In the US, a little over half the population (53%) now viewed it as a serious concern for either themselves or their families; this was 10% below the 2008 poll (63%). Latin America had the biggest rise in concern, with 73% saying global warming was a serious threat to their families.^[164] That global poll also found that people are more likely to attribute global warming to human activities than to natural causes, except in the USA where nearly half (47%) of the population attributed global warming to natural causes.^[165]

On the other hand, in May 2011 a joint poll by Yale and George Mason Universities found that nearly half the people in the USA (47%) attribute global warming to human activities, compared to 36% blaming it on natural causes. Only 5% of the 35% who were "disengaged", "doubtful", or "dismissive" of global warming were aware that 97% of publishing US climate scientists agree global warming is happening and is primarily caused by humans.^[166]

Researchers at the University of Michigan have found that the public's belief as to the causes of global warming depends on the wording choice used in the polls.^[167]

In the United States, according to the Public Policy Institute of California's (PPIC) eleventh annual survey on environmental policy issues, 75% said they believe global warming is a very serious or somewhat serious threat to the economy and quality of life in California.^[168]

A July 2011 Rasmussen Reports poll found that 69% of adults in the USA believe it is at least somewhat likely that some scientists have falsified global warming research.^[155]

A September 2011 Angus Reid Public Opinion poll found that Britons (43%) are less likely than Americans (49%) or Canadians (52%) to say that "global warming is a fact and is mostly caused by emissions from vehicles and industrial facilities." The same poll found that 20% of Americans, 20% of Britons and 14% of Canadians think "global warming is a theory that has not yet been proven."^[169]

Other views

Most scientists agree that humans are contributing to observed climate change.^[58][170] National science academies have called on world leaders for policies to cut global emissions.^[171] However, some scientists and non-scientists question aspects of climate-change science.^{[170][172][173]}

Organizations such as the libertarian Competitive Enterprise Institute, conservative commentators, and some companies such as ExxonMobil have challenged IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.^{[174][175][176][177]} In the finance industry, Deutsche Bank has set up an institutional climate change investment division (DBCCA),^[178] which has commissioned and published research^[179] on the issues and debate surrounding global warming.^[180] 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.^[181] Some fossil fuel companies have scaled back their efforts in recent years,^[182] or called for policies to reduce global warming.^[183]

Etymology

The term global warming was probably first used in its modern sense on 8 August 1975 in a science paper by Wally Broecker in the journal Science called "Are we on the brink of a pronounced global warming?".^{[184][185][186]} Broecker's choice of words was new and represented a significant recognition that the climate was warming; previously the phrasing used by scientists was "inadvertent climate modification," because while it was recognized humans could change the climate, no one was sure which direction it was going.^[187] The National Academy of Sciences first used global warming in a 1979 paper called the Charney Report, it said: "if carbon dioxide continues to increase, [we find] no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible."^[188] The report made a distinction between referring to surface temperature changes as global warming, while referring to other changes caused by increased CO₂ as climate change.^[187]

Global warming became more widely popular after 1988 when NASA climate scientist James Hansen used the term in a testimony to Congress.^[187] 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."^[189] His testimony was widely reported and afterward global warming was commonly used by the press and in public discourse.^[187]

See also

Global warming portal

	Book: Global warming
Wikipedia books are collections of articles that can be downloaded or ordered in print.

• Environmental impact of the coal industry
• Glossary of climate change
• History of climate change science
• Solar variation
• Index of climate change articles

Notes

Citations

References

Further reading

External links

Find more about Global warming on Wikipedia's sister projects:
	Definitions and translations from Wiktionary
	Images and media from Commons
	Learning resources from Wikiversity
	News stories from Wikinews
	Quotations from Wikiquote
	Source texts from Wikisource
	Textbooks from Wikibooks

Research

• NASA Goddard Institute for Space Studies  – Global change research
• NOAA State of the Climate Report  – U.S. and global monthly state of the climate reports
• Climate Change at the National Academies – repository for reports
• Nature Reports Climate Change – free-access web resource
• Met Office: Climate change – UK National Weather Service
• Global Science and Technology Sources on the Internet – commented list
• Educational Global Climate Modelling (EdGCM) – research-quality climate change simulator
• DISCOVER – satellite-based ocean and climate data since 1979 from NASA
• Global Warming Art – collection of figures and images

Educational

• What Is Global Warming? – by National Geographic
• Global Climate Change Indicators – from NOAA
• NOAA Climate Services – from NOAA
• Global Warming Frequently Asked Questions – from NOAA
• Understanding Climate Change – Frequently Asked Questions – from UCAR
• Global Warming: Center for Global Studies at the University of Illinois
• Global Climate Change: NASA's Eyes on the Earth – from NASA's JPL and Caltech
• OurWorld 2.0 – from the United Nations University
• Center for Climate and Energy Solutions – business and politics
• Best Effort Global Warming Trajectories – Wolfram Demonstrations Project – by Harvey Lam
• Koshland Science Museum – Global Warming Facts and Our Future – graphical introduction from National Academy of Sciences
• Climate Change: Coral Reefs on the Edge – A video presentation by Prof. Ove Hoegh-Guldberg, University of Auckland
• Climate Change Indicators in the United States Report by United States Environmental Protection Agency, 80 pp.
• Global Warming
• Video on the effects of global warming on St. Lawrence Island in the Bering Sea

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