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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.

Global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.3 ± 0.32 °F) during the past century. The Intergovernmental Panel on Climate Change (IPCC) concludes, "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations," which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes have probably had a small warming effect from pre-industrial times to 1950, but a cooling effect since 1950. The basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. The American Association of Petroleum Geologists is the only scientific society that rejects these conclusions, and a few individual scientists also disagree with parts of them.

Climate models referenced by the IPCC 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 scenarios of future greenhouse gas emissions and results of models with differences in 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 millennium even if greenhouse gas levels are stabilized. This reflects the large heat capacity of the oceans.

An increase in global temperatures can in turn cause other changes, including sea level rise, and changes in the amount and pattern of precipitation. There may also be increases 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, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.

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 or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol aimed at combating greenhouse gas emissions.

Terminology
The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In principle, global warming is neutral as to the period or causes, but in both common and scientific usage the term generally refers to recent warming and implies a human influence. 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 variations in external forcing factors including solar activity, volcanic emissions, variations in the earth's orbit (orbital forcing) and greenhouse gases. The detailed causes of the recent warming remain an active field of research, but 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. Contrasting with the scientific consensus, other hypotheses have been proposed to explain some of the observed increase in global temperatures, including: the warming is within the range of natural variation; the warming is a consequence of coming out of a prior cool period, namely the Little Ice Age; or the warming is primarily a result of variances in solar radiation.

None of the effects of forcing are instantaneous. Due to the thermal inertia of the Earth's oceans and slow responses of other indirect effects, 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 present day levels, a further warming of about 0.5 °C (0.9 °F) would still occur.

Greenhouse gases in the atmosphere


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 warms a planet's atmosphere and surface.

Greenhouse gases create a natural greenhouse effect, without which, mean temperatures on Earth would be an estimated 30 °C (54 °F) lower, so that Earth would be uninhabitable. Thus scientists do not "believe in" or "oppose" the greenhouse effect as such; rather, the debate concerns the net effect of the addition of greenhouse gases, while allowing for associated positive and negative feedback mechanisms.

On Earth, 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%. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. 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. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago. "About three-quarters of the anthropogenic [man-made] emissions of CO2 to the atmosphere during the past 20 years are due to fossil fuel burning. The rest of the anthropogenic emissions are predominantly due to land-use change, especially deforestation."

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume. 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, 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. 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 complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. CO2 injected into the atmosphere causes a warming of the atmosphere and the earth's surface. The warming causes more water to be evaporated into the atmosphere. Since water vapor itself acts as 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. 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 and debate. Seen from below, clouds absorb infrared radiation and so exert a warming effect. Seen from above, the same clouds reflect sunlight and so exert a cooling effect. Increased global water vapor concentration may or may not cause an increase in global average cloud cover. The net effect of clouds thus has not been well modeled, however, 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.

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. Stratospheric warming has not been observed.

Other phenomena such as solar variation combined with volcanoes have probably had a warming effect from pre-industrial times to 1950, but a cooling effect since 1950. However, some research has suggested that the Sun's contribution may have been underestimated. Researchers at Duke University have estimated that the Sun may have minimally contributed about 10–30% of the global surface temperature warming over the period 1980–2002. Similarly, Stott et al. estimate in 2003 that climate models overestimate the relative effect of greenhouse gases compared to solar forcing but also that the cooling effect of volcanic dust and sulfate aerosols has been underestimated. 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.

From the present to the dawn of human settlement
Global temperatures on both land and sea have increased by 0.75 °C (1.4 °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. 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.

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.



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's interpretation of the historical record, with respect to the methane data, has been disputed.

Pre-human climate variations
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.

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9.0 °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.

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for 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).

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 still remains a considerable range of climate sensitivity.

Including uncertainties in the models and in future greenhouse gas concentrations, the IPCC anticipates 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.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding 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, 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 include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, adverse health effects from warmer temperatures.

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. 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, "There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones."

Additional anticipated effects include sea level rise of 110 to 770 mm (0.36 to 2.5 feet) 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. Mechanistic studies have documented extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change. Parmesan states, "Few studies have been conducted at a scale that encompasses an entire species" and McLaughlin et al. agreed "few mechanistic studies have linked extinctions to recent climate change."

Economics
Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach 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). 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 1%, and that in a worst-case scenario global consumption per head could fall 20%. The reports methodology, advocacy and conclusions has been criticized by many economists, while others have supported the general attempt to quantify economic risk, even if not the specific numbers.

The IPCC's last report says that to stabilize greenhouse-gas concentrations at 550 ppm would require a price of US$20-50 per tonne of carbon by 2020-30. The IPCC's economic models reckon, on average, that if the world adopted such a price the annual global economic growth would be 0.1% a year lower than it otherwise would have been, or put another way, at the end of 2050 the global economy would be 1.3% smaller than had the new pricing not been adopted.

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.

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 important 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 United Nations Framework Convention on Climate Change (UNFCCC), negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions. The United States, the world's largest greenhouse gas emitter; Australia; and Kazakhstan have refused to ratify the treaty. China and India, two other large emitters, have ratified the treaty but, as developing countries, are exempt from its provisions.

Controversy and politics
Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been negligible compared to the developed world, reports The New York Times. At the same time, developing world exemptions from provisions of the Kyoto treaty have been criticized by the United States and been used as part of its justification for continued non-ratification. In the Western world, the idea of human influence on climate and efforts to combat it has gained wider acceptance in Europe than the United States.

Fossil fuel companies such as ExxonMobil have spent large sums of money for public relations to downplay the risks of climate change, while environmental groups have launched campaigns emphasizing its impacts.

This issue has sparked debate in the U.S. about the benefits of reducing industrial emissions of greenhouse gases to help the environment, versus any resulting harm due to limitations in economic activity. There has also been discussion in several countries about the cost of adopting alternate, cleaner energy sources in order to reduce emissions.

Another point of debate is the degree to which newly developed economies, like India and China, have a right to increase their industrial emissions, especially since China is expected to exceed the United States in total greenhouse gas emissions by 2010, though the U.S. has less than one-fourth of China's population. At an IPCC conference in April 2007, delegates from 120 nations discussed the specific economic and societal costs of mitigating global warming; eventually, delegates approved a report indicating general consensus that the benefits of mitigation are worth any specific costs.

Related climatic issues
A variety of issues are often raised in relation to global warming. One is ocean acidification, the ongoing decrease in the pH of the Earth's oceans. 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 serious 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.

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.

Scientific

 * Intergovernmental Panel on Climate Change (IPCC)
 * NOAA's Global Warming FAQ
 * Discovery of Global Warming – An extensive introduction to the topic and the history of its discovery
 * Max Planck Society: Global Warming - The Blame Is not with the Plants
 * Caution urged on climate 'risks'
 * NASA Finds Sun-Climate Connection in Old Nile Records, March 19, 2007

Educational

 * 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 — NASA

Other

 * Help Prevent Global Warming A wikiHow article
 * UBS Launches First Global Warming Index "UBS-GWI"
 * UN: rearing cattle produces more greenhouse gases than driving cars
 * Trends in Atmospheric Carbon Dioxide - Mauna Loa
 * 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 US EPA website
 * IPS Inter Press Service – Independent news on global warming and its consequences.

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