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Global warming
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This article is about the current period of increasing global temperature. For other periods of warming in Earth's history, see Paleoclimatology and Geologic temperature record.
Global mean surface temperature anomaly relative to 1961–1990
Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980
Global warming is the increase in the average measured temperature of the Earth's near-surface air and oceans since the mid-20th century, and its projected continuation. In media, it is synonymous with the term "climate change."
Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 years ending in 2005.[1][2] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[3][4] via an enhanced greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[5][6] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science,[7] including all of the national academies of science of the major industrialized countries.[8][9][10] While individual scientists have voiced disagreement with these findings,[11] the overwhelming majority of scientists working on climate change agree with the IPCC's main conclusions.[12][13]
Climate model projections summarized by the IPCC indicate that average 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.[3] This range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a thousand years even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[3]
Increasing global temperature is expected to cause sea levels to rise, an increase in the intensity of extreme weather events, and significant changes to the amount and pattern of precipitation, likely including an expanse of the subtropical desert regions.[14]. Other expected effects of global warming include changes in agricultural yields, modifications of trade routes, glacier retreat, mass species extinctions and increases in the ranges of disease vectors.
Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions, but there is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences.
Contents
[hide]
1 Greenhouse effect
1.1 Stored methane releases
1.1.1 Thawing permafrost
1.1.2 Clathrate gun hypothesis
2 Solar variation
3 Forcing and feedback
3.1 Climate variability
3.2 Feedback
4 Temperature changes
4.1 Recent
4.2 Pre-human climate variations
5 Climate models
6 Attributed and expected effects
6.1 Environmental
6.2 Economic
7 Adaptation and mitigation
8 Economic and political debate
9 Related climatic issues
10 See also
11 Notes and references
12 Further reading
13 External links
Greenhouse effect
Main articles: Greenhouse gas and Greenhouse effect
The detailed causes of the recent warming remain an active field of research, but the scientific consensus[15][16] is that the increase in atmospheric greenhouse gases due to human activity caused most of the warming observed since the start of the industrial era, and the observed warming cannot be satisfactorily explained by natural causes alone.[17] This attribution is clearest for the most recent 50 years, for which the most detailed data are available.
The greenhouse effect was theorized by Joseph Fourier in 1824[18][19][20] and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's lower atmosphere and surface.
Recent increases in atmospheric carbon dioxide (CO2). The 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.
Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[21][22] On Earth, 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.[23][24] The issue is how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of some greenhouse gases.
Human activity since the industrial revolution has increased the concentration of various greenhouse gases, leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs and nitrous oxide. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. Some other naturally occurring gases contribute small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively since the beginning of the industrial revolution in the mid-1700s. 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.[25] From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[26] 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.[27]
The present atmospheric concentration of CO2 is about 385 parts per million (ppm) by volume.[28] Human activities have caused the atmospheric concentrations of carbon dioxide and methane to be higher today than at any point during the last 650,000 years[29] . Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. 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.[30] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[31]
Inasmuch as the greenhouse effect is due to human activity, it is a forcing effect that is separate from forcing due to climate variability.
Stored methane releases
Thawing permafrost
Main article: Arctic methane release
Recent research carried out in 2008 in the Siberian Arctic has shown millions of tons of the greenhouse gas methane being released, apparently through perforations in the Arctic Ocean's seabed permafrost,[32] with concentrations in some regions reaching up to 100 times above normal.[33][34] Current methane release has previously been estimated at 0.5 megatonnes (Mt) per year.[35] Shakhova et al (2008) estimate that not less than 1,400 gigatonnes (Gt) of Carbon is presently locked up as methane and methane hydrates under the Arctic submarine permafrost, and 5-10% of that area is subject to puncturing by open taliks. They conclude that "release of up to 50 gigatonnes (Gt) of predicted amount of hydrate storage [is] highly possible for abrupt release at any time". That would increase the methane content of the planet's atmosphere by a factor of twelve,[36][37] equivalent in greenhouse effect to a doubling in the current level of CO2. Land-based permafrost in the Siberian Arctic was also recently observed to be releasing large amounts of methane, estimated at over 4 million tons.[38]
Clathrate gun hypothesis
Main article: Clathrate gun hypothesis
Methane clathrate, also known as methane hydrate, was once believed to only exist in space, as extremes of cold are required for its formation. Around 6.4 trillion tonnes (6.4 teratonnes/Tt) of methane[39] is trapped in deposits of methane clathrate on the deep ocean floor[40]. The Clathrate gun hypothesis states that warmer deep ocean temperatures can release the methane (CH4) from the deep ocean deposits of methane clathrate[41]. It is theorized that this was responsible for two extinction events in earth's history; the Permian-Triassic extinction event[41][42] and the Paleocene-Eocene Thermal Maximum. A concentration of 5-15% methane in the atmosphere, which would occur at and around the area of a CH4 release, is explosive[43]; the products of such an explosion would be 2 parts H2O to one of CO2. CH4 dissipates faster than carbon dioxide (CO2), but even after 20 years has a 62 times greater Global warming potential index.
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