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Global warming is the increase in the average temperature of the Earth\'s near-surface air and oceans in recent decades and its projected continuation.
The global average air temperature near the Earth\'s surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the hundred years ending in 2005.Summary for Policymakers (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2007-02-05). Retrieved on 2007-02-02. “The updated hundred-year linear trend (1906 to 2005) of 0.74 °C [0.56 °C to 0.92 °C] is therefore larger than the corresponding trend for 1901 to 2000 given in the TAR of 0.6 °C [0.4 °C to 0.8 °C].” 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" via the 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. Hegerl, Gabriele C.; et al. (2007-05-07). Understanding and Attributing Climate Change (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 690. Intergovernmental Panel on Climate Change. Retrieved on 2007-05-20. “Recent estimates (Figure 9.9) indicate a relatively small combined effect of natural forcings on the global mean temperature evolution of the seconds half of the twentieth century, with a small net cooling from the combined effects of solar and volcanic forcings” Ammann, Caspar; et al. (2007-04-06). "Solar influence on climate during the past millennium: Results from ransient simulations with the NCAR Climate Simulation Model" (PDF). Proceedings of the National Academy of Sciences of the United States of America 104 (10): 3713–3718. “However, because of a lack of interactive ozone, the model cannot fully simulate features discussed in (44)." "While the NH temperatures of the high-scaled experiment are often colder than the lower bound from proxy data, the modeled decadal-scale NH surface temperature for the medium-scaled case falls within the uncertainty range of the available temperature reconstructions. The medium-scaled simulation also broadly reproduces the main features seen in the proxy records." "Without anthropogenic forcing, the 20th century warming is small. The simulations with only natural forcing components included yield an early 20th century peak warming of ≈0.2 °C (≈1950 AD), which is reduced to about half by the end of the century because of increased volcanism.”
These basic conclusions have been endorsed by at least thirty scientific societies and academies of science,The 2001 joint statement was signed by the scientific academies of Australia, Belgium, Brazil, Canada, the Caribbean, China, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, New Zealand, Sweden, and the UK. The 2005 statement added Japan, Russia, and the U.S. The 2007 statement added Mexico, and South Africa. Professional societies include American Meteorological Society, American Geophysical Union, American Institute of Physics, American Astronomical Society, American Association for the Advancement of Science, Stratigraphy Commission of the Geological Society of London, Geological Society of America, American Chemical Society, and Engineers Australia. including all of the national academies of science of the major industrialized countries.The Science Of Climate Change. Royal Society (May 2001). Retrieved on 2008-01-04.Joint science academies\' statement: Global response to climate change. Royal Society (June 2005). Retrieved on 2008-01-04.Joint science academies\' statement on growth and responsibility: sustainability, energy efficiency and climate protection. Potsdam Institute for Climate Impact Research (May 2007). Retrieved on 2008-01-04. While individual scientists have voiced disagreement with some findings of the IPCC, Don\'t fight, adapt. National Post (December 2007). Retrieved on 2007-11-18. the overwhelming majority of scientists working on climate change agree with the IPCC\'s main conclusions.A guide to facts and fictions about climate change. Royal Society (March 2005). Retrieved on 2007-11-18. “"However, the overwhelming majority of scientists who work on climate change agree on the main points"”Beyond the Ivory Tower: The Scientific Consensus on Climate Change. Science Magazine (December 2004). Retrieved on 2008-01-04.
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. The 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.
Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, 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.
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The term "global warming" is a specific example of global climate change. The term "climate change" can also refer to other periods of overall temperature change such as global cooling. In common usage, the term "global warming" refers to the warming in recent decades and implies a human influence. Climate Change: Basic Information. United States Environmental Protection Agency (2006-12-14). Retrieved on 2007-02-09. “In common usage, \'global warming\' often refers to the warming that can occur as a result of increased emissions of greenhouse gases from human activities.” The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes. United Nations Framework Convention on Climate Change, Article I. United Nations Framework Convention on Climate Change. Retrieved on 2007-01-15. The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.
Components of the current radiative forcing as estimated by the IPCC Fourth Assessment Report.
The Earth\'s climate changes in response to external forcing, including variations in its orbit around the Sun (orbital forcing),Berger, A.; et al. (2005-12-10). "On the origin of the 100-kyr cycles in the astronomical forcing". Paleoceanography 20 (4). PA4019. Retrieved on 2007-11-05. Genthon, C.; et al. (1987-10-01). "Vostok Ice Core - Climatic response to CO2 and orbital forcing changes over the last climatic cycle". Nature 329 (6138): 414–418. Retrieved on 2007-11-05. Alley, Richard B.; et al. (January 2002). "A northern lead in the orbital band: north-south phasing of Ice-Age events". Quaternary Science Reviews 21 (1-3): 431–441. Retrieved on 2007-11-05. volcanic eruptions,Robock, Alan, and Clive Oppenheimer, Eds., 2003: Volcanism and the Earth’s Atmosphere, Geophysical Monograph 139, American Geophysical Union, Washington, DC, 360 pp. and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus Joint science academies\' statement: The science of climate change (ASP). Royal Society (2001-05-17). Retrieved on 2007-04-01. “The work of the Intergovernmental Panel on Climate Change (IPCC) represents the consensus of the international scientific community on climate change science” (2007-10-18) "Rising to the climate challenge". Nature 449 (7164). Retrieved on 2007-11-06. 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. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the temperature increase. One such hypothesis proposes that warming may be the result of variations in solar activity.Svensmark, Henrik (February 2007). "Cosmoclimatology: a new theory emerges". Astronomy & Geophysics 48 (1): 18-24. doi:10.1111/j.1468-4004.2007.48118.x. Forster, Piers; et al. (2007-02-05). Changes in Atmospheric Constituents and in Radiative Forcing (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 188-193. Intergovernmental Panel on Climate Change. Retrieved on 2007-09-17. Bard, Edouard; Frank, Martin (2006-06-09). "Climate change and solar variability: What\'s new under the sun?". Earth and Planetary Science Letters 248 (1-2): 1-14. Retrieved on 2007-09-17.
None of the effects of forcing are instantaneous. The thermal inertia of the Earth\'s oceans and slow responses of other indirect effects mean that the Earth\'s current climate is not in equilibrium with the forcing imposed. 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. Meehl, Gerald A.; et al. (2005-03-18). "How Much More Global Warming and Sea Level Rise" (PDF). Science 307 (5716): 1769–1772. doi:10.1126/science.1106663. Retrieved on 2007-02-11.
The greenhouse effect was discovered by Joseph Fourier in 1824 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.
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.IPCC WG1 AR4 Report — Chapter 1: Historical Overview of Climate Change Science (PDF). IPCC WG1 AR4 Report p97 (pdf page 5 of 36). IPCC (2007). Retrieved on 2007-10-07. “To emit 240 W m–2, a surface would have to have a temperature of around –19 °C. This is much colder than the conditions that actually exist at the Earth’s surface (the global mean surface temperature is about 14 °C). Instead, the necessary –19 °C is found at an altitude about 5 km above the surface.”Note that the Greenhouse Effect produces a temperature increase of about 33 °C (59 °F) with respect to black body predictions and not a surface temperature of 33 °C (91 °F) which is 32 °F (0 °C) higher. The average surface temperature is about 14 °C (57 °F). Also note that both the Celsius and Fahrenheit temperatures are expressed to 2 significant figures even though the conversion formula produces 3. Rather, the issue is how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of some greenhouse gases.
On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.Kiehl, J. T.; Kevin E. Trenberth (February 1997). "Earth’s Annual Global Mean Energy Budget" (PDF). Bulletin of the American Meteorological Society 78 (2): 197-208. Retrieved on 2006-05-01. Water vapour: feedback or forcing?. RealClimate (6 Apr 2005). Retrieved on 2006-05-01. 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 very 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.Neftel, A., E. Moor, H. Oeschger, and B. Stauffer. (1985). "Evidence from polar ice cores for the increase in atmospheric CO2 in the past two centuries". Nature 315:45-47. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago. Pearson, Paul N.; Palmer, Martin R. (2000-08-17). "Atmospheric carbon dioxide concentrations over the past 60 million years". Nature 406 (6797): 695–699. doi:10.1038/35021000. Fossil fuel burning has produced about 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. Summary for Policymakers. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2001-01-20). Retrieved on 2007-01-18.
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.
The present atmospheric concentration of CO2 is about 385 parts per million (ppm) by volume. Tans, Pieter. Trends in Atmospheric Carbon Dioxide – Mauna Loa. National Oceanic and Atmospheric Administration. Retrieved on 2008-02-15. 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. Prentice, I. Colin; et al. (2001-01-20). 3.7.3.3 SRES scenarios and their implications for future CO2 concentration. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change. Retrieved on 2007-04-28. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used. 4.4.6. Resource Availability. IPCC Special Report on Emissions Scenarios. Intergovernmental Panel on Climate Change. Retrieved on 2007-04-28.
The effects of forcing agents on the climate are complicated by various feedback processes.
One of the most pronounced feedback effects relates to the evaporation of water. Warming by the addition of long-lived greenhouse gases such as CO2 will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, the atmosphere warms further; this warming causes more water vapor to be evaporated, a positive feedback loop, and so on until a new dynamic equilibrium concentration of water vapor is reached with 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. Soden, Brian J.; Held, Isacc M. (2005-11-01). "An Assessment of Climate Feedbacks in Coupled Ocean–Atmosphere Models" (PDF). Journal of Climate 19 (14). Retrieved on 2007-04-21. “Interestingly, the true feedback is consistently weaker than the constant relative humidity value, implying a small but robust reduction in relative humidity in all models on average" "clouds appear to provide a positive feedback in all models” This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.
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. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.
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 warming will reduce the decrease of temperature with height, 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.Panel on Climate Change Feedbacks, Climate Research Committee, National Research Council, 2004: Understanding Climate Change Feedbacks.
Another important feedback process is ice-albedo feedback. Stocker, Thomas F.; et al. (2001-01-20). 7.5.2 Sea Ice. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change. Retrieved on 2007-02-11. 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.
Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism that could contribute to warming.Sample, Ian. "Warming Hits \'Tipping Point\'", The Guardian, 2005-08-11. Retrieved on 2007-01-18. Similarly a massive release of CH4 from methane clathrates in the ocean could cause rapid warming, according to the clathrate gun hypothesis.
The ocean\'s ability to sequester carbon is expected to decline as it warms. This is because the resulting low nutrient levels of the mesopelagic zone (about 200 to 1000 m depth) limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.Buesseler, Ken O.; et al. (2007-04-27). "Revisiting Carbon Flux Through the Ocean\'s Twilight Zone". Science 316 (5824): 567–570. Retrieved on 2007-11-16.
Solar variation over the last thirty years.
A few papers suggest that the Sun\'s contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000. Scafetta, Nicola; West, Bruce J. (2006-03-09). "Phenomenological solar contribution to the 1900–2000 global surface warming" (PDF). Geophysical Research Letters 33 (5). doi:10.1029/2005GL025539. L05708. Retrieved on 2007-05-08. A paper by Peter Stott and other researchers suggests 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.Stott, Peter A.; et al. (2003-12-03). "Do Models Underestimate the Solar Contribution to Recent Climate Change?". Journal of Climate 16 (24): 4079–4093. doi:10.1175/1520-0442(2003)016%3C4079:DMUTSC%3E2.0.CO;2. Retrieved on 2007-04-16. 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.
A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.Marsh, Nigel; Henrik, Svensmark (November 2000). "Cosmic Rays, Clouds, and Climate" (PDF). Space Science Reviews 94: 215–230. doi:10.1023/A:1026723423896. Retrieved on 2007-04-17. It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.Svensmark, Henrik (July 2000). "Cosmic Rays and Earth\'s Climate" (PDF). Space Science Reviews 93 (1-2): 175-185. Retrieved on 2007-09-17.
One predicted effect of an increase in solar activity would be a warming of most of the stratosphere, whereas greenhouse gas theory predicts cooling there Hegerl, Gabriele C.; et al.. Understanding and Attributing Climate Change (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 675. Intergovernmental Panel on Climate Change. Retrieved on 2008-02-01. . The observed trend since at least 1960 has been a cooling of the lower stratosphereClimate Change 2001:Working Group I: The Scientific Basis (Fig. 2.12) (2001). Retrieved on 2007-05-08. . Reduction of stratospheric ozone also has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.Ozone History Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since. In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last thirty years. This effect is far too small to contribute significantly to global warming. Foukal, Peter; et al. (2006-09-14). "Variations in solar luminosity and their effect on the Earth\'s climate.". Nature. Retrieved on 2007-04-16. National Center for Atmospheric Research (2006-09-14). "Changes in Solar Brightness Too Weak to Explain Global Warming". Press release. Retrieved on 2007-07-13. A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.Lockwood, Mike; Claus Fröhlich. "Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature". Proceedings of the Royal Society A. doi:10.1098/rspa.2007.1880. Retrieved on 2007-07-21. “Our results show that the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified.” Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed this criticism of their hypothesis.Reply to Lockwood and Fröhlich - The persistent role of the Sun in climate forcing — Spacecenter
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 on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.Working group I, section 3.2.2.2 of the 2007 IPPC page 243 Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade). Smith, Thomas M.; Reynolds, Richard W. (2005-05-15). "A Global Merged Land–Air–Sea Surface Temperature Reconstruction Based on Historical Observations (1880–1997)" (PDF). Journal of Climate 18 (12): 2021–2036. ISSN 0894-8755. Retrieved on 2007-03-14. 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.
Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.Rowan T. Sutton, Buwen Dong, Jonathan M. Gregory (2007). "Land/sea warming ratio in response to climate change: IPCC AR4 model results and comparison with observations". Geophysical Research Letters 34. doi:10.1029/2006GL028164. Retrieved on 2007-09-19. 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. More greenhouse gases are emitted in the Northern than Southern Hemisphere, but this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.
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. Hansen, James E.; et al. (2006-01-12). Goddard Institute for Space Studies, GISS Surface Temperature Analysis. NASA Goddard Institute for Space Studies. Retrieved on 2007-01-17. Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998. Global Temperature for 2005: second warmest year on record (PDF). Climatic Research Unit, School of Environmental Sciences, University of East Anglia (2005-12-15). Retrieved on 2007-04-13. WMO STATEMENT ON THE STATUS OF THE GLOBAL CLIMATE IN 2005 (PDF). World Meteorological Organization (2005-12-15). Retrieved on 2007-04-13. Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.Data @ NASA GISS: GISS Surface Temperature Analysis: 2005 Summation
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, Mitchell, J. F. B.; et al. (2001-01-20). 12.4.3.3 Space-time studies. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change. Retrieved on 2007-01-04. 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 largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.Global warming in the twenty-first century: An alternative scenario - Hansen et al. 97 (18): 9875 - Proceedings of the National Academy of Sciences
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. Ruddiman, William F. (March 2005). "How Did Humans First Alter Global Climate?" (PDF). Scientific American 292 (3): 46–53. Retrieved on 2007-03-05. Ruddiman\'s interpretation of the historical record, with respect to the methane data, has been disputed. Schmidt, Gavin; et al. (2004-12-10). "A note on the relationship between ice core methane concentrations and insolation". Geophysical Research Letters 31 (23). doi:10.1029/2004GL021083. L23206. Retrieved on 2007-03-05.
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. Hansen, James; et al. (2006-09-26). "Global temperature change" (PDF). PNAS 103: 14288–14293. Retrieved on 2007-04-20.
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. Open University (2004-01-30). "The Open University Provides Answers on Global Warming" (PDF). Press release. Retrieved on 2007-03-04. Cohen, Anthony S.; et al. (February 2004). "Osmium isotope evidence for the regulation of atmospheric CO2 by continental weathering" (PDF). Geology 32 (2): 157–160. doi:10.1130/G20158.1. Retrieved on 2007-03-04.
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).
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 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).
Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity 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.Chapter 7, "Couplings Between Changes in the Climate System and Biogeochemistry" (PDF). Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2007-02-05). Retrieved on 2008-02-21.These models predict that the effect of adding greenhouse gases is to produce a warmer climate.Hansen, James (2000). Climatic Change: Understanding Global Warming. One World: The Health & Survival of the Human Species in the 21st Century. Health Press. Retrieved on 2007-08-18. However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.
Including uncertainties in future greenhouse gas concentrations and climate modeling, 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. Models have also been 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.
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. Summary for Policymakers. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2001-01-20). Retrieved on 2007-04-28. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.
Global climate model projections of future climate are forced by imposed greenhouse gas emission scenarios, most often from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback. Torn, Margaret; Harte, John (2006-05-26). "Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming". Geophysical Research Letters 33 (10). L10703. Retrieved on 2007-03-04. Harte, John; et al. (2006-10-30). "Shifts in plant dominance control carbon-cycle responses to experimental warming and widespread drought". Environmental Research Letters 1 (1). 014001. Retrieved on 2007-05-02. Scheffer, Marten; et al. (2006-05-26). "Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change.". Geophysical Research Letters 33. doi:10.1029/2005gl025044. Retrieved on 2007-05-04.
The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem. Stocker, Thomas F.; et al. (2001-01-20). 7.2.2 Cloud Processes and Feedbacks. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change. Retrieved on 2007-03-04.
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 WGMS and the NSIDC.
Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,Macey, Jennifer. "Global warming opens up Northwest Passage", ABC News, September 19, 2007. Retrieved on 2007-12-11. reduced summer streamflows, species extinctions, and increases in the range of disease vectors.
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 being attributed in part to global warming. Climate Change 2001: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2001-02-16). Retrieved on 2007-03-14. While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. 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.McMichael AJ, Woodruff RE, Hales S (2006). "Climate change and human health: present and future risks". Lancet 367 (9513): 859-69. doi:10.1016/S0140-6736(06)68079-3. PMID 16530580.
Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some benefits, such as fewer deaths due to cold exposure. Summary for Policymakers (PDF). Climate Change 2007: Impacts, Adaptation and Vulnerability. Working Group II Contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report. Intergovernmental Panel on Climate Change (2007-04-13). Retrieved on 2007-04-28. 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. 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, 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.
Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,Church, John A.; et al. (2001-01-20). Executive Summary of Chapter 11. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change. Retrieved on 2005-12-19. repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. 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.Thomas, Chris D.; et al. (2004-01-08). "Extinction risk from climate change" (PDF). Nature 427 (6970): 145-138. doi:10.1038/nature02121. Retrieved on 2007-03-18. However, few mechanistic
