Global warming

Annual average global warming by the year 2060 simulated and plotted using EdGCM

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.

Global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the past century. The prevailing scientific view, as represented by the science academies of the major industrialized nations and the Intergovernmental Panel on Climate Change (IPCC), is that most of the temperature increase since the mid-20th century has been caused by increases in atmospheric greenhouse gas concentrations produced by human activity.

Climate models project that global surface temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100. The range of values reflects the use of differing assumptions of future greenhouse gas emissions and results of models that differ in their sensitivity to increases in greenhouse gases. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. This is because adjustment of the climate is slowed by the large heat capacity of the oceans.

An increase in global temperatures will in turn cause sea level rise and changes in the amount and pattern of precipitation. There may also be changes in the frequency and intensity of extreme weather events, though it is difficult to connect specific events to global warming. Other effects may include changes in agricultural yields, glacier retreat, species extinctions, and effects on human health.

Remaining scientific uncertainties include the exact degree of climate change expected in the future, and how changes will vary from region to region around the globe. There is ongoing political and public debate regarding what, if any, action should be taken to reduce future warming or to adapt to its consequences. The Kyoto Protocol, an international agreement aimed at reducing greenhouse gas emissions, was adopted by 169 nations.

Terminology

The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes. The term "anthropogenic climate change" is sometimes used when focusing on human-induced changes.

Causes

The climate system varies through natural, internal processes and in response to external forcing factors. Climate forcing includes solar activity, volcanic emissions, variations in Earth's orbit (orbital forcing), and atmospheric composition. The scientific consensus identifies increased levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses have been offered to explain most of the observed increase in global temperatures but are less broadly supported. Among these are that the warming is caused by natural fluctuations in the climate, that warming is mainly a result of variations in solar radiation, or that warming is caused by changes in cloud cover due to variations in galactic cosmic rays.

None of the effects of forcing are instantaneous. Due to the thermal inertia of the Earth's oceans and slow responses of some other indirect effects, the Earth's climate is never in perfect equilibrium with the imposed forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at present day levels, there would be a further warming of about 0.5 °C (0.9 °F) as the climate continued to adjust toward equilibrium.

Greenhouse gases in the atmosphere

Existence of the greenhouse effect itself is not disputed. It is the process by which absorption and emission of infrared radiation by atmospheric gases warms a planet's atmosphere and surface. Naturally occurring greenhouse gases warm the earth by about 33 °C (59 °F); without this natural greenhouse effect, the average temperature of Earth would be about -18 °C (0 °F) making the planet uninhabitable. The major natural 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%. Some other naturally occurring gases such as nitrous oxide (N₂O) contribute very small fractions of the greenhouse effect.

The present atmospheric concentration of CO₂ is about 383 parts per million (ppm) by volume. This is considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO₂ values this high were last attained 20 million years ago.

About three-fourths of mad-made CO₂ emissions over the past 20 years come from the burning of fossil fuels. The rest is largely the result of land-use change, mainly deforestation. Analysis of carbon isotope ratios confirms that the oceans are a net sink of atmospheric CO₂ and not a source.

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, 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. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands, or methane clathrates are extensively used.

Positive feedback effects such as the expected release of CH₄ from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes) may lead to significant additional sources of greenhouse gas emissions not included in climate models cited by the IPCC.

Feedbacks

The effects of forcing agents on the climate are modified by various feedback processes. One of the most important feedback effects relates to the evaporation of water. Increased CO₂ causes a warming of the atmosphere and the Earth's surface, which increases the evaporation of water into the atmosphere. Since water vapor itself is a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached at a slight increase in humidity and with a much larger greenhouse effect than that due to CO₂ alone.

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, the same clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Increased global water vapor content may or may not cause an increase in global or regional cloud cover, since cloud cover is affected by relative humidity rather than the absolute concentration of water vapor. Although the net effect of clouds is one of the main uncertainties in present day climate models, cloud feedback is second only to water vapor feedback and is positive in all the models that contributed to the IPCC Fourth Assessment Report.

Another important feedback process is ice-albedo feedback. The increased CO₂ in the atmosphere warms the Earth's surface and leads to melting of ice near the poles. 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 is an additional mechanism contributing to warming. Possible positive feedback due to CH₄ release from melting seabed ices is a further mechanism to be considered.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.

Solar variation

Variations in solar output, possibly amplified by cloud feedbacks, may have contributed to recent warming. A difference between this mechanism and greenhouse warming is that an increase in solar activity should produce a warming of the stratosphere while greenhouse warming should produce a cooling of the stratosphere. Reduction of stratospheric ozone also has a cooling influence but substantial ozone depletion did not occur until the late 1970s. Observations show that the lower stratosphere has been cooling since at least 1960, which is inconsistent with the solar variation hypothesis.

Natural phenomena such as solar variation combined with volcanoes have probably had a warming effect from pre-industrial times to 1950, but a small cooling effect since 1950. Some research indicate that the Sun's contribution may have been underestimated, suggesting that the Sun may have contributed about 40–50% of the global surface temperature warming between 1900 and 2000, and about 25–35% between 1980 and 2000. Stott and coauthors suggest 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. Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.

Climate change since the Industrial Revolution

According to the instrumental temperature record, mean global temperatures (both land and sea) have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900. This measured temperature increase is not significantly affected by the urban heat island effect. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade). 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.

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. Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998. Global temperatures in 1998 were exceptionally warm because the strongest El Niño in the instrumental record occurred in that year.

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, though the cooling may also be due in part to natural variability.

Climate models

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 some simplifications being necessary because of limitations in computer power. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, even when the same assumptions of fossil fuel consumption and CO₂ emission are used, the amount of projected warming varies between models and there is a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC report projects a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) between 1990 and 2100. 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.

Climate models can produce a good match to observations of global temperature changes over the last century, but cannot yet simulate all aspects of climate. 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 scenarios, generally one 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.

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

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 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. While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects as a result of warmer temperatures include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects.

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 minor benefits, such as fewer deaths due to cold exposure. 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, repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity and frequency 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. McLaughlin et al. have documented two populations of Bay checkerspot butterfly being threatened by precipitation change, though they state few mechanistic studies have documented extinctions due to recent climate change.

Mitigation and adaptation

The broad agreement among climate scientists that global temperatures will continue to increase has led nations, states, corporations, and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. There has been business action on climate change, including efforts at increased energy efficiency and (still limited) moves to alternative fuels. One innovation has been the development 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% of global greenhouse gas emissions. The United States, Australia, and Kazakhstan have not ratified the treaty. China and India, two other large emitters, have ratified the treaty but, as developing countries, are exempt from its provisions. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.

The world's primary body for crafting a response is the Intergovernmental Panel on Climate Change (IPCC), a UN-sponsored activity which holds periodic meetings between national delegations on the problems of global warming, and issues working papers and assessments on the current status of the science of climate change, impacts, and mitigation. It convenes four different working groups examining various specific issues.

Related climatic issues

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. CO₂ dissolved in the ocean reacts with water to form carbonic acid resulting in acidification. Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era, and it is estimated that it will drop by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO₂. 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.

Another related issue that may have partially mitigated global warming in the late twentieth century is global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface. 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 global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.

Further reading

• Amstrup, Steven C.; Ian Stirling, Tom S. Smith, Craig Perham, Gregory W. Thiemann (2006-04-27). "Recent observations of intraspecific predation and cannibalism among polar bears in the southern Beaufort Sea". Polar Biology 29 (11): 997–1002. DOI:10.1007/s00300-006-0142-5. Research Blogging.

• Association of British Insurers (2005-06). Financial Risks of Climate Change (PDF). 
• Barnett, Tim P.; J. C. Adam, D. P. Lettenmaier (2005-11-17). "Potential impacts of a warming climate on water availability in snow-dominated regions". Nature 438 (7066): 303–309. DOI:10.1038/nature04141. Research Blogging.

• Behrenfeld, Michael J.; Robert T. O'Malley, David A. Siegel, Charles R. McClain, Jorge L. Sarmiento, Gene C. Feldman, Allen G. Milligan, Paul G. Falkowski, Ricardo M. Letelier, Emanuel S. Boss (2006-12-07). "Climate-driven trends in contemporary ocean productivity" (PDF). Nature 444 (7120): 752–755. DOI:10.1038/nature05317. Research Blogging.

• Choi, Onelack; Ann Fisher (May 2005). "The Impacts of Socioeconomic Development and Climate Change on Severe Weather Catastrophe Losses: Mid-Atlantic Region (MAR) and the U.S.". Climate Change 58 (1–2): 149–170. DOI:10.1023/A:1023459216609. Research Blogging.

• Dyurgerov, Mark B.; Mark F. Meier (2005). Glaciers and the Changing Earth System: a 2004 Snapshot (PDF). Institute of Arctic and Alpine Research Occasional Paper #58. 
• Emanuel, Kerry A. (2005-08-04). "Increasing destructiveness of tropical cyclones over the past 30 years." (PDF). Nature 436 (7051): 686–688. DOI:10.1038/nature03906. Research Blogging.

• Hansen, James; Larissa Nazarenko, Reto Ruedy, Makiko Sato, Josh Willis, Anthony Del Genio, Dorothy Koch, Andrew Lacis, Ken Lo, Surabi Menon, Tica Novakov, Judith Perlwitz, Gary Russell, Gavin A. Schmidt, Nicholas Tausnev (2005-06-03). "Earth's Energy Imbalance: Confirmation and Implications" (PDF). Science 308 (5727): 1431–1435. DOI:10.1126/science.1110252. Research Blogging.

• Hinrichs, Kai-Uwe; Laura R. Hmelo, Sean P. Sylva (2003-02-21). "Molecular Fossil Record of Elevated Methane Levels in Late Pleistocene Coastal Waters". Science 299 (5610): 1214–1217. DOI:10.1126/science.1079601. Research Blogging.

• Hirsch, Tim. Plants revealed as methane source, BBC, 2006-01-11.

• Hoyt, Douglas V.; Kenneth H. Schatten (1993–11). "A discussion of plausible solar irradiance variations, 1700–1992". Journal of Geophysical Research 98 (A11): 18,895–18,906.

• Kenneth, James P.; Kevin G. Cannariato, Ingrid L. Hendy, Richard J. Behl (2003-02-14). Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis. American Geophysical Union. 
• Keppler, Frank, Marc Brass, Jack Hamilton, Thomas Röckmann. Global Warming - The Blame Is not with the Plants, Max Planck Society, 2006-01-18.

• Kurzweil, Raymond (2006–07). "Nanotech Could Give Global Warming a Big Chill" (PDF). Forbes / Wolfe Nanotech Report 5 (7).

• Lean, Judith L.; Y.M. Wang, N.R. Sheeley (2002–12). "The effect of increasing solar activity on the Sun's total and open magnetic flux during multiple cycles: Implications for solar forcing of climate". Geophysical Research Letters 29 (24). DOI:10.1029/2002GL015880. Research Blogging.

• Lerner, K. Lee; Brenda Wilmoth Lerner (2006-07-26). Environmental issues : essential primary sources.. Thomson Gale. ISBN 1414406258. 
• McLaughlin, Joseph B.; Angelo DePaola, Cheryl A. Bopp, Karen A. Martinek, Nancy P. Napolilli, Christine G. Allison, Shelley L. Murray, Eric C. Thompson, Michele M. Bird, John P. Middaugh (2005-10-06). "Outbreak of Vibrio parahaemolyticus gastroenteritis associated with Alaskan oysters". New England Journal of Medicine 353 (14): 1463–1470.

(online version requires registration)

• Muscheler, Raimund; Fortunat Joos, Simon A. Müller, Ian Snowball (2005-07-28). "Climate: How unusual is today's solar activity?" (PDF). Nature 436 (7012): 1084–1087. DOI:10.1038/nature04045. Research Blogging.

• Oerlemans, J. (2005-04-29). "Extracting a Climate Signal from 169 Glacier Records" (PDF). Science 308 (5722): 675–677. DOI:10.1126/science.1107046. Research Blogging.

• Oreskes, Naomi (2004-12-03). "Beyond the Ivory Tower: The Scientific Consensus on Climate Change" (PDF). Science 306 (5702): 1686. DOI:10.1126/science.1103618. Research Blogging.

• Purse, Bethan V.; Philip S. Mellor, David J. Rogers, Alan R. Samuel, Peter P. C. Mertens, Matthew Baylis (February 2005). "Climate change and the recent emergence of bluetongue in Europe". Nature Reviews Microbiology 3 (2): 171–181. DOI:10.1038/nrmicro1090. Research Blogging.

• Revkin, Andrew C. Rise in Gases Unmatched by a History in Ancient Ice, The New York Times, 2005-11-05.

• Ruddiman, William F. (2005-12-15). Earth's Climate Past and Future. New York: Princeton University Press. ISBN 0-7167-3741-8. 
• Ruddiman, William F. (2005-08-01). Plows, Plagues, and Petroleum: How Humans Took Control of Climate. New Jersey: Princeton University Press. ISBN 0-691-12164-8. 
• Solanki, Sami K.; I.G. Usoskin, B. Kromer, M. Schussler, J. Beer (2004-10-23). "Unusual activity of the Sun during recent decades compared to the previous 11,000 years." (PDF). Nature 431: 1084–1087. DOI:10.1038/nature02995. Research Blogging.

• Solanki, Sami K.; I. G. Usoskin, B. Kromer, M. Schüssler, J. Beer (2005-07-28). "Climate: How unusual is today's solar activity? (Reply)" (PDF). Nature 436: E4-E5. DOI:10.1038/nature04046. Research Blogging.

• Sowers, Todd (2006-02-10). "Late Quaternary Atmospheric CH₄ Isotope Record Suggests Marine Clathrates Are Stable". Science 311 (5762): 838–840. DOI:10.1126/science.1121235. Research Blogging.

• Svensmark, Henrik; Jens Olaf P. Pedersen, Nigel D. Marsh, Martin B. Enghoff, Ulrik I. Uuggerhøj (2007-02-08). "Experimental evidence for the role of ions in particle nucleation under atmospheric conditions". Proceedings of the Royal Society A 463 (2078): 385–396. DOI:10.1098/rspa.2006.1773. Research Blogging.

(online version requires registration)

• Walter, K. M.; S. A. Zimov, Jeff P. Chanton, D. Verbyla, F. S. Chapin (2006-09-07). "Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming". Nature 443 (7107): 71–75. DOI:10.1038/nature05040. Research Blogging.

• Wang, Y.-M.; J.L. Lean, N.R. Sheeley (2005-05-20). "Modeling the sun's magnetic field and irradiance since 1713" (PDF). Astrophysical Journal 625: 522–538. DOI:10.1086/429689. Research Blogging.

External links

Scientific

• Intergovernmental Panel on Climate Change (IPCC)
• Nature Reports Climate Change
• NOAA's Global Warming FAQ
• Outgoing Longwave Radiation pentad mean - NOAA Climate Prediction Center
• Discovery of Global Warming — An extensive introduction to the topic and the history of its discovery
• Caution urged on climate 'risks'
• NASA Finds Sun-Climate Connection in Old Nile Records
• News in Science - Night flights are worse for global warming - 15/06/2006

Educational

• What Is Global Warming? Simulation from National Geographic
• The EdGCM (Educational Global Climate Modelling) Project free research-quality simulation for students, educators, and scientists alike, with a user-friendly interface that runs on desktop computers
• Daily global temperatures and trends from satellites Interactive graphics from NASA
• The Pew Center on global climate change

Other

• The Global Warming Survival Guide from Time.com
• UBS Launches First Global Warming Index "UBS-GWI"
• Global Warming News & Articles Portal
• UN: rearing cattle produces more greenhouse gases than driving cars
• Science and Technology Librarianship: Global Warming and Climate Change Science – Extensive commented list of Internet resources – Science and Technology Sources on the Internet.
• Union of Concerned Scientists Global Warming page
• Watch and read 'Tipping Point', Australian science documentary about effects of global warming on rare, common, and endangered wildlife
• Newest reports on U.S. EPA website
• IPS Inter Press Service — Independent news on global warming and its consequences.
• Indonesia Counts Its Islands Before It Is Too Late
• World Environment Day 2007 "Melting Ice" image gallery at The Guardian
• Climate Counts - corporate watchdog