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 temperature of the Earth's near-surface air and oceans since the mid-20th 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 ] [ 2 ] The Intergovernmental Panel on Climate Change (IPCC) concludes that most of the temperature increase since the mid-twentieth century is "very likely" due to the increase in anthropogenicgreenhouse gas concentrations. [ 3 ] [ 2 ] 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 from 1950 onward. [ 4 ] [ 5 ] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, [ 6 ] including all of the national academies of science of the major industrialized countries. [ 7 ] [ 8 ] [ 9 ] While individual scientists have voiced disagreement with these findings, [ 10 ] the overwhelming majority of scientists working on climate change agree with the IPCC's main conclusions. [ 11 ] [ 12 ]
Climate model projections 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. [ 3 ] The uncertainty in this estimate arises from use of differing estimates of future greenhouse gas emissions and from use of models with differing climate sensitivity. Another uncertainty is how warming and related changes will vary from region to region around the globe. Although most studies focus on the period up to 2100, warming is expected to continue for more than a thousand years even if greenhouse gas levels are stabilized. This results from the large heat capacity of the oceans. [ 3 ]
Increasing global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, likely including an expanse of the subtropical desert regions. [ 13 ] Other likely effects include increases in the intensity of extreme weather events, changes in agricultural yields, modifications of trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.
Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions. Political and public debate continues regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences.
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Greenhouse effect
- Main articles: Greenhouse gas and Greenhouse effect
The causes of the recent warming are an active field of research. The scientific consensus [ 14 ] [ 15 ] 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. [ 16 ] This attribution is clearest for the most recent 50 years, being the period 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 [ 17 ] and 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. Existence of the greenhouse effect as such is not disputed. The question is instead how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of particular greenhouse gases.
Recent increases in atmospheric carbon dioxide (CO 2 ). The monthly CO 2 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 CO 2 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 ] [ 19 ] On Earth the major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect (not including clouds); carbon dioxide (CO 2 ), which causes 9–26 percent; methane (CH 4 ), which causes 4–9 percent; and ozone, which causes 3–7 percent. [ 20 ] [ 21 ]
Human activity since the industrial revolution has increased the atmospheric concentration of various greenhouse gases, leading to increased radiative forcing from CO 2 , methane, tropospheric ozone, CFCs and nitrous oxide. The atmospheric concentrations of CO 2 and methane have increased by 36% and 148% respectively since the beginning of the industrial revolution in the mid-1700s. [ 22 ] 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. [ 23 ] From less direct geological evidence it is believed that CO 2 values this high were last seen approximately 20 million years ago. [ 24 ] Fossil fuel burning has produced approximately three-quarters of the increase in CO 2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation. [ 25 ]
CO 2 concentrations are expected to continue 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. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO 2 scenarios, ranging from 541 to 970 ppm by the year 2100. [ 26 ] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited. [ 27 ]
