Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events are attributable in part to global warming.^[82] Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.^[83]

Social and economic effects of global warming may be exacerbated by growing population densities in affected areas. Temperate regions are projected to experience some benefits, such as fewer deaths due to cold exposure.^[84] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.^[82] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature (see Atlantic Multidecadal Oscillation), but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.^[3]

Additional anticipated effects include sea level rise of 180 to 590 millimeters (0.59 to 1.9 ft) in 2090-2100 relative to 1980-1999, ^[3] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increasingly intense (but less frequent)^[85] hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever.^[86][87] One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.^[88] However, few mechanistic studies have documented extinctions due to recent climate change^[89] and one study suggests that projected rates of extinction are uncertain.^[90]

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).^[84]

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.^[91] 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.^[92] Others have supported the general attempt to quantify economic risk, even if not the specific numbers.^[93][94]

Preliminary studies suggest that costs and benefits of mitigating global warming are broadly comparable in magnitude.^[95]

According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others.^[96] Developing countries dependent upon agriculture will be particularly harmed by global warming.^[97]

Adaptation and mitigation

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

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. 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.^[98][99]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One recently developed concept is that of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the UNFCCC negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55 percent of global greenhouse gas emissions.^[100] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.^[101] China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies.^[102] Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.^[103]

U.S. President George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns, claiming it that it "exempts 80 percent of the world, including major population centers such as China and India, from compliance, and would cause serious harm to the U.S. economy."^[104] Bush has instead promoted improved energy technology as a means to combat climate change,^[105] while various state and city governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis, such as the Regional Greenhouse Gas Initiative.^[106] The U.S. Climate Change Science Program is a joint program of over 20 U.S. federal agencies working together to investigate climate change.

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.^[107] According to Working Group III, to limit temperature rise to 2 degrees Celsius, "developed countries as a group would need to reduce their emissions to below 1990 levels in 2020 (on the order of –10 percent to 40 percent below 1990 levels for most of the considered regimes) and to still lower levels by 2050 (40 percent (Sic. 80 percent in Box 13.7, p776) to 95 percent below 1990 levels), even if developing countries make substantial reductions."^[108]

Economic and political debate

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

See also: Climate change denial, List of countries by greenhouse gas emissions per capita, List of countries by carbon dioxide emissions per capita, List of countries by carbon dioxide emissions, and List of countries by ratio of GDP to carbon dioxide emissions

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.^[109] 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.^[110] 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.^[111]In the Western world, the idea of human influence on climate has gained wider public acceptance in Europe than in the United States.^[112][113]

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.^[114] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.^{[115][116][117][118]} Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,^[119] or called for policies to reduce global warming.^[120]

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.^{[121][122][123][124]} 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.^[125] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.^[126] The U.S. contends that if it must bear the cost of reducing emissions, then China should do the same.^[127]

Related climatic issues

Main articles: Ocean acidification, global cooling, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO₂ increases the amount of CO₂ dissolved in the oceans.^[128]CO₂ dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,^[129]and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO₂.^[3][130] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO₂, that could disrupt food webs and impact human societies that depend on marine ecosystem services.^[131]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late 20th century. From 1960 to 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 global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.^[3]

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

See also

• Glossary of climate change
• List of climate change topics
• Paleoclimatology

Notes and references