Global warming

Global mean surface temperature anomaly relative to 1961–1990

Mean surface temperature anomalies during the period 1999 to 2008 with respect to the average temperatures from 1940 to 1980

Global warming is the increase in the average temperature of the Earth's near-surface air and the oceans since the mid-twentieth century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 years ending in 2005.^[1]^[A] The Intergovernmental Panel on Climate Change (IPCC) concludes that anthropogenic greenhouse gases are responsible for most of the observed temperature increase since the middle of the twentieth century,^[1] and that natural phenomena such as solar variation and volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect afterward.^[2]^[3] These basic conclusions have been endorsed by more than 40 scientific societies and academies of science,^[B] including all of the national academies of science of the major industrialized countries.^[4]^[5]

Climate model projections summarized in the latest IPCC report indicate that global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century.^[1] The uncertainty in this estimate arises from the use of models with differing climate sensitivity, and the use of differing estimates of future greenhouse gas emissions. Some other uncertainties include how warming and related changes will vary from region to region around the globe. Most studies focus on the period up to 2100. However, warming is expected to continue beyond 2100, even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere.^[6]^[7]

Increasing global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, likely including expansion of subtropical deserts.^[8] The continuing retreat of glaciers, permafrost and sea ice is expected, with the Arctic region being particularly affected. Other likely effects include shrinkage of the Amazon rainforest and Boreal forests, increases in the intensity of extreme weather events, species extinctions and changes in agricultural yields.

Political and public debate continues regarding the appropriate response to global warming. The available options are mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions. A successor to the first commitment period of the Kyoto protocol is expected to be agreed at the COP15 talks in December 2009.

Forcing

Components of the current radiative forcing as estimated by the IPCC Fourth Assessment Report.

The Earth's climate changes in response to external forcings, including changes in greenhouse gas concentrations, variations in Earth's orbit around the Sun,^[9]^[10]^[11] changes in solar luminosity, and volcanic eruptions.^[12] 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.^[13] Planetary albedo also significantly affects the energy balance of the Earth, resulting in net forcing.

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is sometimes cited in relation to global warming. Although there are a few areas of linkage the relationship between the two is not strong.

Greenhouse effect

Main articles: Greenhouse gas and Greenhouse effect

The scientific consensus^[14]^[15] is that the increase in atmospheric greenhouse gases due to human activity has caused most of the warming observed since the start of the industrial era and that the observed warming cannot be satisfactorily explained by natural causes alone.^[16] This attribution is clearest for the most recent 50 years, which is the period when most of the increase in greenhouse gas concentrations took place and for which the most complete measurements exist.

The greenhouse effect was discovered by Joseph Fourier in 1824 and first investigated quantitatively by Svante Arrhenius in 1896.^[17] It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's lower atmosphere and surface. Existence of the greenhouse effect as such is not disputed even by those who do not agree that the recent temperature increase is attributable to human activity. The question is instead how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of greenhouse gases.

Recent increases in atmospheric carbon dioxide (CO₂). Monthly CO₂ measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO₂ from the atmosphere.

Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.^[18]^[C] The major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect (not including clouds); carbon dioxide (CO₂), which causes 9–26 percent; methane (CH₄), which causes 4–9 percent; and ozone, which causes 3–7 percent.^[19]^[20]

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 the mid-1700s.^[21] These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores.^[22] Less direct geological evidence indicates that CO₂ values this high were last seen approximately 20 million years ago.^[23] Fossil fuel burning has produced approximately three-quarters of the increase in CO₂ from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.^[24]

CO₂ concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic, sociological, technological, and natural developments. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO₂ scenarios, ranging from 541 to 970 ppm by the year 2100.^[25] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.^[26]

Solar variation

Main article: Solar variation

An alternative hypothesis is that recent warming may be the result of variations in solar activity.^[27]^[28] Stott and colleagues have suggested that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.^[29] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases. Another paper suggests that the Sun may have contributed about 45–50 percent of the increase in the average global surface temperature over the period 1900–2000, and about 25–35 percent between 1980 and 2000.^[30] In 2006, Peter Foukal and colleagues found no net increase of solar brightness over the last 1,000 years. Solar cycles led to a small increase of 0.07 percent in brightness over the last 30 years. This effect is too small to contribute significantly to global warming.^[31]^[32] The general view is that the combined effect of the two main sources of natural climate forcing, solar variation and changes in volcanic activity, probably had a warming effect from pre-industrial times to 1950 but a cooling effect since.^[1]

Solar variation over the last thirty years.

An increase in solar activity should warm the stratosphere, whereas an increase in greenhouse gases should produce cooling there.^[33] The observed trend since at least 1960 has been a cooling of the lower stratosphere.^[34] Reduction of stratospheric ozone also has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.^[35]

Svensmark and colleagues have proposed another hypothesis related to solar activity, which is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.^[36]^[37]^[38] Another paper found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.^[39] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed this criticism of their hypothesis.^[40] A 2007 paper found that in the last 20 years there has been no significant link between changes in cosmic rays coming to Earth and cloudiness and temperature.^[41]^[42]

Albedo effects

The energy balance of the Earth is affected by the planet's albedo (essentially it's 'whiteness'). Various human activities have changed this albedo significantly in certain regions. Arctic shrinkage, attributed to global warming, has affected the albedo of the Arctic as white ice is replaced by dark ocean or forest, steppe and tundra. This results in ice-albedo feedback, exaggerating the effect.^[43] In other regions, notably Asia, albedo effects include the creation of black carbon, which may not have been properly considered in climate models.^[44] Additionally, aerosol pollution causes cooling.^[45] Efforts to reduce this pollution may have unintended effects on global warming: "Strong aerosol cooling in the past and present would then imply that future global warming may proceed at or even above the upper extreme of the range projected by the Intergovernmental Panel on Climate Change." ^[46].

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially counteracted global warming during the period 1960-1990. Human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the warming effect of increasing greenhouse gases.^[1]

Feedback

A drunken forest in Siberia caused by melting permafrost. Melting of permafrost releases methane into the atmosphere, accelerating global warming.

Main article: Effects of global warming

When a warming trend results in effects that induce further warming, the process is referred to as a positive feedback; when the warming results in effects that act to reduce the original warming, the process is referred to as a negative feedback. The main positive feedback involves the tendency of warming to increase the amount of water vapor in the atmosphere. The main negative feedback is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature.

Water vapor feedback: If the atmosphere is warmed the saturation vapour pressure increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO₂ alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.^[47]

Clouds: Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud, details that have been difficult to represent in climate models.^[47]

Lapse rate: A subtler feedback process relates to changes in the lapse rate as the atmosphere warms. The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation emitted from the upper atmosphere is less than that emitted from the lower atmosphere. Most of the radiation emitted from the upper atmosphere escapes to space, while most of the radiation emitted from the lower atmosphere is re-absorbed by the surface or the atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height: if the rate of temperature decrease is greater the greenhouse effect will be stronger, and if the rate of temperature decrease is smaller then the greenhouse effect will be weaker. Both theory and climate models indicate that with increased greenhouse gas content the rate of temperature decrease with height will be reduced, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.^[48]

Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to the ice-albedo feedback.

Ice-albedo feedback: When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.^[49] Rapid Arctic shrinkage is already occurring, with 2007 being the lowest ever recorded sea ice area. Some models suggest that tipping points exist, leading to a potentially rapid collapse of sea ice cover in the Arctic.^[50]

Arctic methane release: Warming is also the triggering variable for the release of methane from sources both on land and on the deep ocean floor, making both of these possible feedback effects. Thawing permafrost, such as the frozen peat bogs in Siberia, creates a positive feedback due to the potentially rapid release of CO₂ and CH₄.^[51]^{[unreliable source?]} Methane discharge from permafrost is presently under intensive study.^{[citation needed]}

Clathrate gun hypothesis: Over thousands of years, warmer deep ocean temperatures could release methane from deep ocean deposits of 'frozen' methane clathrate. A further release of methane from shallow cold water clathrates is also expected, and is predicted to be faster.^[52]

Reduced absorption of CO₂ by the oceans: Ocean ecosystems' ability to sequester carbon are expected to decline as the oceans warm. This is because warming reduces the nutrient levels of the mesopelagic zone (about 200 to 1000 m depth), which limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.^[53]

Temperature changes

Main article: Temperature record

Recent

Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Global temperatures have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. The urban heat island effect is unlikely to have significantly influenced this value and is estimated to account for about 0.02 °C of warming since 1900.^[54] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).^[55] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.^{[citation needed]}

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.^[56] The Northern Hemisphere has more land than the Southern Hemisphere so it warms faster. The Northern Hemisphere also has extensive areas of seasonal snow and sea-ice cover subject to the 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.^[57]

Based on estimates by NASA's Goddard Institute for Space Studies 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.^[58] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.^[59]^[60] Temperatures in 1998 were unusually warm because the strongest El Niño-Southern Oscillation in the past century occurred during that year.^[61]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,^[62] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO₂ and aerosols—have, for the short term, largely offset one another, so that net warming in recent decades has been driven mainly by non-CO₂ greenhouse gases.^[63]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.^[64]^[65] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.^[66]

Pre-human climate variations

Further information: Paleoclimatology

Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.^[67]

A rapid buildup of greenhouse gases amplified warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO₂ levels dropped back to normal over roughly the next 150,000 years.^[68]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as both a cause for and an effect of other warming events in the distant past, including the Permian–Triassic extinction event (about 251 million years ago) and the Paleocene–Eocene Thermal Maximum (about 55 million years ago).

Climate models

Main article: Global climate model

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.

The geographic distribution of surface warming during the 21^st 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).

The main tool for projecting future climate changes are computer models of the climate. These models are based on physical principles including fluid dynamics and radiative transfer. Although they 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. All modern climate models include an atmospheric model that is coupled to an ocean model and models for ice cover on land and sea. Some models also include treatments of chemical and biological processes.^[69] These models project a warmer climate due to increasing levels of greenhouse gases.^[70] Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.^[71]

Global climate model projections of future climate depend on estimates of greenhouse gas emissions, most often those from the IPCC Special Report on Emissions Scenarios (SRES). In addition to human-caused emissions, some models also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.^[72]^[73]^[74]

Including uncertainties in future greenhouse gas concentrations and climate sensitivity, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.^[1] A 2008 paper predicts that the global temperature will not increase during the next decade because of short-term natural climate cycles.^[75]

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 1975 is dominated by man-made greenhouse gas emissions.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.^[76] The physical realism of models is tested by examining their ability to simulate current or past climates.^[77] While a 2007 study by David Douglass and colleagues found that the models did not accurately predict observed changes in the tropical troposphere,^[78] a 2008 paper published by a 17-member team led by Ben Santer noted errors in the Douglass study, and found instead that the models and observations were not statistically different.^[79] Not all effects of global warming are accurately predicted by the climate models used by the IPCC. For example, observed Arctic shrinkage has been faster than that predicted.^[80]

Attributed and expected effects

Environmental

Main article: Effects of global warming

Economic

Main articles: Economics of global warming and Low-carbon economy

The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far yielded no conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).^[85]

One widely publicized report on potential economic impact is the Stern Review. It suggests that extreme weather might reduce global gross domestic product by up to one percent, and that in a worst-case scenario global per capita consumption could fall 20 percent.^[98] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios.^[99] Others have supported the general attempt to quantify economic risk, even if not the specific numbers.^[100]^[101]

Preliminary studies suggest that costs and benefits of mitigating global warming are broadly comparable in magnitude.^[102]

According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others.^[103] Developing countries dependent upon agriculture will be particularly harmed by global warming.^[104]

Responses to global warming

Main articles: Mitigation of global warming, Kyoto Protocol, Geoengineering, and Adaptation to global warming

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement responses. These responses to global warming can be divided into mitigation of the causes and effects of global warming, and adaptation to the changing global environment.

Mitigation

Emissions reduction

The world's primary international agreement on reducing greenhouse gas emissions is the Kyoto Protocol, an amendment to the UNFCCC negotiated in 1997. The Protocol now covers more than 160 countries and over 55 percent of global greenhouse gas emissions.^[105] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gases. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.^[106]

Many environmental groups encourage individual action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO₂ emissions.^[107]^[108]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. In January 2005 the European Union introduced its European Union Emission Trading Scheme, a greenhouse gas emissions trading scheme through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances. Australia announced its Carbon Pollution Reduction Scheme in 2008. United States President Barack Obama has announced he will introduce an economy wide cap and trade scheme.^[109]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6 percent increase and three percent decrease in global gross domestic product.^[110]

Geoengineering

Geoengineering would involve the deliberate modification of Earth's environment on a large scale "to suit human needs and promote habitability".^[111] It can be divided two major approaches. The first is remediation, in which greenhouse gases would be removed from the atmosphere, principally by carbon sequestration methods such as air capture.^[112] The second is solar radiation management, in which incoming solar radiation would be reduced, such as by the insertion of stratospheric sulfur aerosols.^[113] The slow pace of action to reduce greenhouse gas emissions have led some scientists to suggest that these techniques may be necessary.^[114] Whilst some cool roof and tree planting projects are underway, no planetary-scale geoengineering has yet been attempted.

Adaptation

The effects of global warming are wide in their scope, and a similarly wide variety of measures have been suggested for adaptation to global warming. These range from the trivial, such as the installation of air-conditioning equipment, up to major infrastructure projects, such as abandonment of settlements threatened by rising sea levels. Measures including water conservation,^[115] changes to agricultural practices,^[116] construction of flood defences,^[117] changes to medical care,^[118] and interventions to protect threatened species^[119] have all been suggested. A wide ranging study of the possible opportunities for adaptation of infrastructure has been published by the Institute of Mechanical Engineers^[120]

Economic and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming

Per capita greenhouse gas emissions in 2000, including land-use change.

Per country greenhouse gas emissions in 2000, including land-use change.

Increased publicity of the scientific findings surrounding global warming has resulted in political and economic debate.^[121] Poor regions, particularly Africa, appear at greatest risk from the projected effects of global warming, while their emissions have been small compared to the developed world.^[122] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and used as part of a rationale for continued non-ratification by the U.S.^[123] In the Western world, the idea of human influence on climate has gained wider public acceptance in Europe than in the United States.^[124]^[125]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.^[126] Business-centered organizations, conservative commentators, and companies such as the Competitive Enterprise Institute and ExxonMobil have downplayed IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.^[127]^[128]^[129]^[130] Likewise, environmental organizations and a number of public figures have emphasized the potential risks of climate change and promote the implementation of GHG emissions reduction measures. Some fossil fuel companies have scaled back their efforts in recent years,^[131] or called for policies to reduce global warming.^[132]

Another point of contention is the degree to which emerging economies such as India and China should be expected to constrain their emissions. According to recent reports, China's gross national CO₂ emissions may now exceed those of the U.S.^[133]^[134]^[135]^[136] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.^[137] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.^[138] The U.S. contends that if it must bear the cost of reducing emissions, then China should do the same.

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