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Global warming is the observed increase in the average temperature of the Earth's atmosphere and oceans in recent decades. The Earth's average near-surface atmospheric temperature rose 0.6 ± 0.2 °Celsius (1.1 ± 0.4 °Fahrenheit) in the 20th century .
The current scientific consensus is that "most of the observed warming over the last 50 years is likely to have been attributable to human activities". The extent of this consensus was the subject of a study—published in December 2004 in the journal Science—that considered the abstracts of 928 refereed scientific articles in the ISI citation database identified with the keywords "global climate change". This study concluded that 75% of the 928 articles either explicitly or implicitly accepted the consensus view — the remainder of the articles covered methods or paleoclimate and did not take any stance on recent climate change .
The primary causes of the human-induced component of warming are the increased amounts of carbon dioxide (CO2) and other greenhouse gases (GHGs). They are released by the burning of fossil fuels, land clearing and agriculture, etc. and lead to an increase in the greenhouse effect. This effect was first described by Joseph Fourier in 1824, and first investigated quantitatively in 1896 by the Swedish chemist Svante Arrhenius, although the greenhouse effect did not enter into popular awareness until the 1980's.
Climate sensitivity is a measure of the equilibrium response to increased GHGs, and other anthropogenic and natural climate forcings. It is found by observational  and model studies. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere. The 2001 IPCC report estimates a likelyhood between 66% and 90% for a climate sensitivity in the range 1.5–4.5 °C (2.7–8.1 °F). This should not be confused with the expected temperature change by a given date, which also includes a dependence on the future GHG emissions and a delayed response due to thermal lag, principally from the oceans. Models referenced by the Intergovernmental Panel on Climate Change (IPCC), using a range of SRES scenarios, project that global temperatures will increase between 1.4 and 5.8 °C (2.5 to 10.5 °F) between 1990 and 2100.
An increase in global temperatures can in turn cause other changes, including a rising sea level and changes in the amount and pattern of precipitation. These changes may increase the frequency and intensity of extreme weather events, such as floods, droughts, heat waves, hurricanes, and tornados. Other consequences include higher or lower agricultural yields, glacial retreat, reduced summer stream flows, species extinctions and increases in the ranges of disease vectors. Warming is expected to affect the number and magnitude of these events; however, it is difficult to connect particular events to global warming. Although most studies focus on the period up to 2100, warming (and sea level rise due to thermal expansion) is expected to continue past then, since CO2 has an estimated atmospheric lifetime of 50 to 200 years. . Only a small minority of climate scientists discount the role that humanity's actions have played in recent warming. However, the uncertainty is more significant regarding how much climate change should be expected in the future, and there is a hotly contested political and public debate over implementation of policies that deal with predicted consequences, what, if anything, should be done to reduce or reverse future warming, and how to deal with the predicted consequences.
The term "global warming" is a specific case of the more general term "climate change" (which can also refer to "global cooling," such as occurs during ice ages). In principle, "global warming" is neutral as to the causes, but in common usage, "global warming" generally implies a human influence. However, the UNFCCC uses "climate change" for human-caused change, and "climate variability" for other changes . Some organizations use the term "anthropogenic climate change" for human-induced changes.
Historical warming of the Earth
Relative to the period 1860–1900, global temperatures on both land and sea have increased by 0.75 °C (1.4 °F), according to the instrumental temperature record. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C/decade against 0.13 °C/decade (Smith, 2005). Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C per decade since 1979, according to satellite temperature measurements. Over the one or two thousand years before 1850, world temperature is believed to have been relatively stable, 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 Celsius. Similar estimates prepared by the World Meteorological Organization and the UK Climatic Research Unit concluded that 2005 was still only the second warmest year, behind 1998  .
Depending on the time frame, a number of temperature records are available. These are based on different data sets, with different degrees of precision and reliability. An approximately global instrumental temperature record begins in about 1860; contamination from the urban heat island effect is believed to be small and well controlled for. A longer-term perspective is available from various proxy records for recent millennia; see temperature record of the past 1000 years for a discussion of these records and their differences. The attribution of recent climate change is clearest for the most recent period of the last 50 years, for which the most detailed data are available. Satellite temperature measurements of the tropospheric temperature date from 1979.
The climate system varies both through natural, "internal" processes as well as in response to variations in external "forcing" from both human and non-human causes, including solar activity, volcanic emissions, and greenhouse gases. Climatologists agree that the earth has warmed recently. The detailed causes of this change remain an active field of research, but the scientific consensus identifies greenhouse gases as the primary cause of the recent warming. Outside of the scientific community, however, this conclusion can be controversial.
Adding carbon dioxide (CO2) or methane (CH4) to Earth's atmosphere, with no other changes, will make the planet's surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C (54 °F) lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the theory that adding carbon dioxide or methane to the Earth's atmosphere will, absent any mitigating actions or effects, result in warmer surface temperatures on Earth. Rather, the debate is about what the net effect of the addition of carbon dioxide and methane will be, when allowing for compounding or mitigating factors.
One example of an 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 less reflective than ice, and so absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues.
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 by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilized at present day levels, a further warming of perhaps 0.5 °C to 1.0 °C (0.9–1.8 °F) would still occur.
Greenhouse gases in the atmosphere
Greenhouse gases are transparent to shortwave radiation from the sun. However, they absorb some of the longer infrared radiation emitted as black body radiation from the Earth, thereby slowing radiational cooling and raising the 'equilibrium' temperature of the Earth. How much they warm the world by is shown in their global warming potential.
The atmospheric concentrations of carbon dioxide and methane have increased by 31% and 149% respectively above pre-industrial levels since 1750. 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 carbon dioxide values this high were last attained 40 million years ago. About three-quarters of the anthropogenic (man-made) emissions of carbon dioxide to the atmosphere during the past 20 years is due to fossil fuel burning. The rest of the anthropogenic emissions are predominantly due to land-use change, especially deforestation .
The longest continuous instrumental measurement of carbon dioxide mixing ratios began in 1958 at Mauna Loa. Since then, the annually averaged value has increased monotonically by approximately 21% from the initial reading of 315 ppmv, as shown by the Keeling curve, to over 380 ppmv in 2006  . The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend, with the maximum reached during the northern hemisphere's late spring (the growing season in the northern hemisphere temporarily removes some CO2 from the atmosphere).
Methane, the primary constituent of natural gas, enters the atmosphere both from biological production and leaks from natural gas pipelines and other infrastructure. Some biological sources are natural, such as termites, but others have been increased or created by agricultural activities, such as the cultivation of rice paddies . Recent evidence suggests that forests may also be a source (RC; BBC), and if so this would be an additional contribution to the natural greenhouse effect, and not to the anthropogenic greenhouse effect (Ealert).
Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage, though the actual trajectory will depend on uncertain economic, sociological, technological, and natural developments. The IPCC Special Report on Emissions Scenarios gives a wide range of future carbon dioxide scenarios , ranging from 541 to 970 parts per million by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal and tar sands are extensively used.
Globally, the majority of anthropogenic greenhouse gas emissions arise from fuel combustion. The remainder is accounted for largely by "fugitive fuel" (fuel consumed in the production and transport of fuel), emissions from industrial processes (excluding fuel combustion), and agriculture: these contributed 5.8%, 5.2% and 3.3% respectively in 1990. Current figures are broadly comparable.  Around 17% of emissions are accounted for by the combustion of fuel for the generation of electricity. A small percentage of emissions come from natural and anthropogenic biological sources, with approximately 6.3% derived from agriculturally produced methane and nitrous oxide.
Positive feedback effects, such as the expected release of methane 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. . Note that the anthropogenic emissions of other pollutants—notably sulfate aerosols—exert a cooling effect; this partially accounts for the plateau/cooling seen in the temperature record in the middle of the twentieth century , though this may also be due to intervening natural cycles.
The extent of the scientific consensus on global warming—that "most of the observed warming over the last 50 years is likely to have been attributable to human activities"—has been investigated: In the journal Science in December 2004, Dr Naomi Oreskes published a study of the abstracts of 928 refereed scientific articles in the ISI citation database identified with the keywords "global climate change". This study concluded that 75% of the 928 articles either explicitly or implicitly accepted the consensus view — the remainder of the articles covered methods or paleoclimate and did not take any stance on recent climate change. . The study did not report how many of the 928 abstracts explicitly endorsed the hypothesis of human-induced warming.
Contrasting with the consensus view, alternative hypotheses have been proposed to explain all or part of the observed increase in global temperatures. Some of these hypotheses (listed here without comment on their validity or lack thereof) include:
- 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.
- The warming is primarily a result of variances in solar irradiance, possibly via modulation of cloud cover . It is similar in concept to the operating principles of the Wilson cloud chamber, but on a global scale where earth's atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of cloud condensation nuclei.
- The observed warming actually reflects the Urban Heat Island, as most readings are done in heavily populated areas which are expanding with growing population .
Solar variation theory
Modeling studies reported in the IPCC Third Assessment Report (TAR) did not find that changes in solar forcing were needed in order to explain the climate record for the last four or five decades . These studies found that volcanic and solar forcings may account for half of the temperature variations prior to 1950, but the net effect of such natural forcings has been roughly neutral since then . In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be eight times larger than the change in forcing due to increasing solar activity over the same period .
Since the TAR, some studies (Lean et al., 2002, Wang et al., 2005) have suggested that changes in irradiance since pre-industrial times are less by a factor of 3 to 4 than in the reconstructions used in the TAR (e.g. Hoyt and Schatten, 1993, Lean, 2000.). Other researchers (e.g. Stott et al. 2003 ) believe that the effect of solar forcing is being underestimated and propose that solar forcing accounts for 16% or 36% of recent greenhouse warming. Others (e.g. Marsh and Svensmark 2000 ) have proposed that feedback from clouds or other processes enhance the direct effect of solar variation, which if true would also suggest that the effect of solar variability was being underestimated. In general the level of scientific understanding of the contribution of variations in solar irradiance to historical climate changes is "very low" .
The present level of solar activity is historically high. Solanki et al. (2004) suggest that solar activity for the last 60 to 70 years may be at its highest level in 8,000 years; Muscheler et al. disagree, suggesting that other comparably high levels of activity have occurred several times in the last few thousand years . Solanki concluded based on their analysis that there is a 92% probability that solar activity will decrease over the next 50 years. In addition, researchers at Duke University (2005) have found that 10–30% of the warming over the last two decades may be due to increased solar output . In a review of existing literature, Foukal et al. (2006) determined both that the variations in solar output were too small to have contributed appreciably to global warming since the mid-1970s and that there was no evidence of a net increase in brightness during this period. 
The expected effects of global warming are many and various, both for the environment and for human life. These effects include sea level rise, repercussions to agriculture, reductions in the ozone layer, increased intensity and frequency of extreme weather events, and the spread of disease. In some cases, the effects may already be manifest, although it is difficult to attribute specific incidents of natural phenomena to long-term global warming. Since the mid-1970s, the total power of hurricanes has increased markedly because they have been more intense and have lasted longer; in addition, there has been a high correlation of hurricane power with tropical sea-surface temperature. In spite of such strong evidence, the relationship between global warming and hurricanes is still being debated.  A draft statement by the World Meteorological Organization acknowledges the differing viewpoints on this issue .
The extent and probability of these consequences is a matter of considerable uncertainty. A summary of probable effects and recent understanding can be found in the report of the IPCC Working Group II . Some scientists have concluded global warming is already causing death and disease across the world through flooding, environmental destruction, heat waves and other extreme weather events. (Reuters, February 9, 2006; archived)
Effects on ecosystems
Both primary and secondary effects of global warming — such as higher temperatures, lessened snow cover, rising sea levels and weather changes — may influence not only human activities but also ecosystems. Some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may flourish. Similarly, changes in timing of life patterns, such as annual migration dates, may alter regional predator-prey balance. The effect of advanced spring arrival dates in Scandinavia of birds that over winter in sub-Saharan Africa has been ascribed to evolutionary adaptation of the species to climatic warming .
Ocean pH is lowering as a result of increased carbon dioxide levels. Lowering of ocean pH along with changing water temperature and ocean depth will have a damaging effect on coral reefs.
Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release significant amounts of the potent greenhouse gas methane that is trapped in permafrost and ice clathrate compounds .
There are also ecological effects of melting polar ice: for example, polar bears use sea ice to reach their prey, and they must swim to another ice floe when one breaks up. Ice is now becoming further separated, and dead polar bears have been found in the water, believed to have drowned. More recently, some scientists have suggested that the observed cannibalistic behavior in polar bears may be the result of food shortages brought on by global warming (Amstrup et al. 2006).
Effect on glaciers
Global warming has led to negative glacier mass balance, causing glacier retreat around the world. Oerlemans (2005) showed a net decline in 142 of the 144 mountain glaciers with records from 1900 to 1980. Since 1980 global glacier retreat has increased significantly. Similarly, Dyurgerov and Meier (2005) averaged glacier data across large-scale regions (e.g. Europe) and found that every region had a net decline from 1960 to 2002, though a few local regions (e.g. Scandinavia) have shown increases. Some glaciers that are in disequilibrium with present climate have already disappeared  and increasing temperatures are expected to cause continued retreat in the majority of alpine glaciers around the world. Upwards of 90% of glaciers reported to the World Glacier Monitoring Service have retreated since 1995 .
Of particular concern is the potential for failure of the Hindu Kush and Himalayan glacial melts. The melt of these glaciers is a large and reliable source of water for China, India, and much of Asia, and these waters form a principal dry-season water source. Increased melting would cause greater flow for several decades, after which "some areas of the most populated region on Earth are likely to 'run out of water'" (T. P. Barnett, J. C. Adam and D. P. Lettenmaier 2005) 
Miniature rock glaciers
Rock glaciers — caches of ice under boulders — are among other water signs such as drying meadows and warming lakes that scientists are studying in the Sierras in the western United States . Connie Millar searches for the rock glaciers in the Yosemite area of the Sierra crest. She hypothesizes that rock glaciers will be predictors of how ecosystems change with rising temperatures. Millar is leading an effort (the Consortium for Integrated Climate Research in Western Mountains ) to co-ordinate the work of many scientists to see how the pieces of the Global Warming puzzle may fit.
Destabilization of ocean currents
There is also some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localized cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift.
Sea level rise and environmental refugees
Rising global temperatures will melt glaciers and expand the water of the seas through the mechanism of thermal expansion, leading to sea level rise. Even a relatively small rise in sea level would make some densely settled coastal plains uninhabitable and create a significant refugee problem. If the sea level were to rise in excess of 4 meters (13 ft) almost every coastal city in the world would be severely affected, with the potential for major damage to world-wide trade and economy. Presently, the IPCC predicts sea level rise is most probable to be just short of half a metre, and at least between 9 and 88 cm through 2100  - but they also warn that global warming during that time may lead to irreversible changes in the Earth's glacial system and ultimately melt enough ice to raise sea level many meters over the next millennia. It is estimated that around 200 million people could be affected by sea level rise, especially in Vietnam, Bangladesh, China, India, Thailand, Philippines, Indonesia, Nigeria and Egypt.
An example of the ambiguous nature of environmental refugees is the emigration from the island nation of Tuvalu, which has an average elevation of approximately one meter above sea level. Tuvalu already has an ad hoc agreement with New Zealand to allow phased relocation  and many residents have been leaving the islands. However, it is far from clear that rising sea levels from global warming are a substantial factor - best estimates are that sea level has been rising there at approximately 1–2 millimeters per year (~1/16th in/yr), but that shorter timescale factors—ENSO, or tides—have far larger temporary effects    .
Spread of disease
One of the largest known outbreaks of Vibrio parahaemolyticus gastroenteritis has been attributed to generally rising ocean temperature where infected oysters were harvested in Prince William Sound, Alaska in 2005. Before this, the northernmost reported risk of such infection was in British Columbia, 1000 km to the south (McLaughlin JB, et al.).
Global warming may extend the range of vectors conveying infectious diseases such as malaria. A warmer environment boosts the reproduction rate of mosquitoes and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse . Global warming has been implicated in the recent spread to the north Mediterranean region of bluetongue disease in domesticated ruminants associated with mite bites (Purse, 2005). Hantavirus infection, Crimean-Congo hemorrhagic fever, tularemia and rabies increased in wide areas of Russia during 2004–2005. This was associated with a population explosion of rodents and their predators but may be partially blamed on breakdowns in governmental vaccination and rodent control programs. Similarly, despite the disappearance of malaria in most temperate regions, the indigenous mosquitoes that transmitted it were never eliminated and remain common in some areas. Thus, although temperature is important in the transmission dynamics of malaria, many other factors are influential .
Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warned in a 2002 study (UNEP summary) that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost US$150 billion each year in the next decade. These costs would, through increased costs related to insurance and disaster relief, burden customers, taxpayers, and industry alike.
According to the Association of British Insurers, limiting carbon emissions could avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s. According to Choi and Fisher (2003) each 1% increase in annual precipitation could enlarge catastrophe loss by as much as 2.8%.
The United Nations' Environmental Program recently announced that severe weather around the world has made 2005 the most costly year on record . Although there is "no way to prove that [a given hurricane] either was, or was not, affected by global warming" , global warming is thought to increase the probability of hurricanes emerging. Preliminary estimates presented by the German insurance foundation Munich Re put the economic losses at more than US$200 billion, with insured losses running at more than US$70 billion.
Nicholas Stern in the Stern Review has warned that one percent of global GDP is required to be invested in order to mitigate the effects of climate change, and that failure to do so could risk a recession worth up to twenty percent of global GDP . Stern’s report suggests that climate change threatens to be the greatest and widest-ranging market failure ever seen. The report has had significant political effects: Australia reported two days after the report was released that they would allott AU$60 million to projects to help cut greenhouse gas emissions. Tony Blair said the Stern Review showed that scientific evidence of global warming was "overwhelming" and its consequences "disastrous".
The creation of biomass by plants is influenced by the availability of water, nutrients, and carbon dioxide. Part of this biomass is used (directly or indirectly) as the energy source for nearly all other life forms, including feed-stock for domestic animals, and fruits and grains for human consumption. It also includes timber for construction purposes.
A rise in atmospheric carbon dioxide can increase the efficiency of the metabolism of most plants, potentially allowing them to create more biomass. A rising temperature can also increase the growing season in colder regions. It is sometimes argued that these effects can create a greener, richer planet, with more available biomass. However, there are many other factors involved, and it is currently unclear if plants really benefit from global warming. Plant growth can be limited by a number of factors, including soil fertility, water, temperature, and carbon dioxide concentration.
IPCC models currently predict a possible modest increase in plant productivity. However, there are several negative ramifications: decreases in productivity may occur at above-optimal temperatures; greater variation in temperature is likely to decrease wheat yields; in experiments, grain and forage quality declines if CO2 and temperature are increased; and the reductions in soil moisture in summer, which are likely to occur, would have a negative effect on productivity. 
Satellite data show that the productivity of the northern hemisphere did indeed increase from 1982 to 1991 . However, more recent studies  found that from 1991 to 2002, widespread droughts had actually caused a decrease in summer photosynthesis in the mid and high latitudes of the northern hemisphere.
Opening up of the Northwest Passage in summer
Melting Arctic ice may open the Northwest Passage in summer in approximately ten years, which would cut 5,000 nautical miles (9,300 km) from shipping routes between Europe and Asia. This would be of particular relevance for supertankers that are too big to fit through the Suez Canal and currently have to go around the southern tip of Africa. According to the Canadian Ice Service, the amount of ice in Canada's eastern Arctic Archipelago decreased by 15% between 1969 and 2004 . A similar opening is possible in the Arctic north of Siberia, allowing much faster East Asian to Europe transport.
Adverse effects of the melting of ice include a potential increase in the rate of global warming, since ice reflects 90% of solar heat, while open water absorbs 90% .
Further global warming (positive feedback)
Some effects of global warming themselves contribute directly to further global warming:
- Melting of permafrost may lead to methane release; methane clathrate deposits on the ocean floor might release more methane (the clathrate gun hypothesis).
- There have been predictions, and some evidence, that global warming might cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO2 levels  
- Sea ice and seasonal snow cover are more reflective than the underlying sea; hence any meltback leads to further warming
The likelihood that global temperatures will continue to significantly increase has led to proposals to mitigate global warming. Mitigation covers all actions aimed at reducing the negative effects or the likelihood of global warming.
There are five categories of actions that can be taken to mitigate global warming:
- Reduction of energy use (conservation)
- Shifting from carbon-based fossil fuels to alternative energy sources
- Carbon capture and storage
- Carbon sequestration
- Planetary engineering to cool the earth, including screening out sunlight or increasing albedo
Strategies for mitigation of global warming include development of new technologies; carbon offsets; renewable energy such as biodiesel, wind power, and solar power; nuclear power; electric or hybrid automobiles; fuel cells; energy conservation; carbon taxes; enhancing natural carbon dioxide sinks; population control; and carbon capture and storage. Many environmental groups encourage individual action against global warming, often aimed at the consumer, and there has been business action on climate change.
The world's primary international agreement on combating climate change is the Kyoto Protocol. The Kyoto Protocol is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC). Countries that ratify this protocol commit to reduce their emissions of carbon dioxide and five other greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases.
Although the governments of 163 countries ratified the Kyoto Protocol, (notably excluding the United States and Australia), there is a growing debate about how effective the Kyoto protocol has been. Some politicians, including President of the United States George W. Bush , Prime Minister of Australia John Howard  had argued that the cost of mitigating global warming via the Kyoto protocol is too large to be practical. This view may be proving correct, as the signatories of the Kyoto protocol are currently struggling to meet their targets , including Europe and Japan. After only five years, Canada has given up entirely. Also, of the 163 countries that have signed and ratified Kyoto, only 31 are actually required to lower greenhouse emissions.
Some segments of the business community have accepted global warming and its attribution to anthropogenic causes as valid, as well as a need for actions such as carbon emissions trading and carbon taxes.
Adaptation strategies accept some warming as a foregone conclusion and focus on preventing or reducing undesirable consequences. Examples of such strategies include defense against rising sea levels or ensuring food security.
Scientists have studied global warming with computer models of the climate (see below). Before the scientific community accepts a climate model, it has to be validated against observed climate variations. As of 2006, sufficiently high-resolution models successfully simulate summer/winter differences, the North Atlantic Oscillation, and El Niño . All validated current models predict that the net effect of adding greenhouse gases will be a warmer climate in the future. However, the amount of predicted warming varies by model, and there still remains a considerable range of climate sensitivity predicted by the models which survive these tests; one of the most important sources of this uncertainty is believed to be different ways of handling clouds. Part of the technical summary of the IPCC TAR includes a recognition of the need to quantify this uncertainty: "In climate research and modeling, we should recognize that we are dealing with a coupled non-linear system, and therefore that the prediction of a specific future climate is not possible. Rather the focus must be on the probability distribution of the system's possible future states by the generation of ensembles of model solutions." (See , page 78.) An example of a study which aims to do this is the Climateprediction.net project; their methodology is to investigate the range of climate sensitivities predicted for the 21st century by those models which are first shown to give a reasonable simulation of late 20th century climate change.
As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4 °C to 5.8 °C (2.5 °F–10.4 °F) between 1990 and 2100 . They have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors. In addition to having their own characteristic climate sensitivity, models have also been used to derive independent assessments of climate sensitivity.
Climate models can produce a good match to observations of global temperature changes over the last century . 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. Adding simulation of the carbon cycle to the models 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 .
Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds . There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small-scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considered climate models "to be suitable tools to provide useful projections of future climates" .
In December, 2005 Bellouin et al. suggested in Nature that the reflectivity effect of airborne pollutants was about double that previously expected, and that therefore some global warming was being masked. If supported by further studies, this would imply that existing models under-predict future global warming. 
Dangerous global warming
Although global warming has been seen as potentially dangerous for some time, the first international attempt to define what constitutes a 'dangerous' level occurred at the Avoiding Dangerous Climate Change scientific conference in February 2005. This took place in Exeter, United Kingdom under the UK presidency of the G8 .
At the conference it was said that increasing damage was forecast if the globe warms to about 1 to 3 °Celsius (1.8 to 5.4 °Fahrenheit) above pre-industrial levels. It was concluded that the stabilization of greenhouse gases at the equivalent of 450 ppmv CO2 would provide a 50% likelihood of limiting global warming to the average figure of 2 °C (3.6 °F). Stabilization below 400 ppm would give a relatively high certainty of not exceeding 2 °C, while stabilization at 550 ppm would mean it was likely that 2 °C would be exceeded.
It was stated that unless 'urgent and strenuous mitigation actions' were taken in the next 20 years, it was almost certain that by 2050 global temperatures will have risen to between 0.5 and 2 °C (0.9 and 3.6°F) above current levels. With carbon dioxide levels currently around 381 ppm and rising by 2ppm per year, without such action greenhouse gasses are likely to reach 400ppm by 2016, 450ppm by 2041, and 550ppm by around 2091.
Other related issues
Increased atmospheric carbon dioxide increases the amount of CO2 dissolved in the oceans. This ameliorates the greenhouse effect by removing the greenhouse gas from the atmosphere. Unfortunately, carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid. The net effect, even accounting for warming of the oceans, is an observed and accelerating ocean acidification. Since biosystems are adapted to a narrow range of pH this is a very serious concern directly driven by increased atmospheric CO2 and not global warming. Ocean acidification would not be created by any other postulated climate change mechanisms. In fact, the result of these other forcings would be to decrease the acidity of the oceans as the sea temperature warmed. This is exactly contrary to what has been observed. Roger Revelle and Hans Suess first described the chemistry of the process.
Relationship to ozone depletion
- Global warming from carbon dioxide radiative forcing is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes.
- Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposed effects: 1) reduced ozone allows more solar radiation to penetrate, thus warming the troposphere instead of the stratosphere. 2) The resulting colder stratosphere emits less long-wave radiation down to the troposphere, thus having a cooling effect. Overall, the cooling dominates: the IPCC concludes that observed stratospheric O3 losses over the past two decades have caused a negative forcing of the surface-troposphere system  of about −0.15 ± 0.10 W/m² .
- One of the strongest predictions of the greenhouse effect theory is that the stratosphere will cool. However, although this is observed, it is not trivial to separate the effects of changes in the concentration of greenhouse gases and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the NOAA Geophysical Fluid Dynamics Laboratory show that above 20 km, the greenhouse gases dominate the cooling. 
- Ozone depleting chemicals are also greenhouse gases, representing 0.34 ±0.03 W/m², or about 14% of the total radiative forcing from well-mixed greenhouse gases .
- Decreased ozone leads to an increase in ultraviolet levels. Ultraviolet radiation may be responsible for the death of ocean algae, which operate as a carbon dioxide sink in the ocean. Increased UV, therefore, may lead to a decrease in carbon dioxide uptake, thereby raising global carbon dioxide levels. 
Possible compounding effects
Another concern is the possibility of a positive feedback loop: i.e., that global warming can cause further global warming in a vicious circle, the nature of which may be difficult to predict in advance. For example, the melting of ice caps appears to be causing the release of large amounts of additional carbon dioxide or methane from decaying vegetation trapped beneath   ; it could also lead to increased heat absorption because ice reflects more solar heat (has higher albedo) than land or water.
Relationship to global dimming
Some scientists now consider that the effects of global dimming (the reduction in sunlight reaching the surface of the planet, possibly due to aerosols) may have masked some of the effect of global warming. If this is so, the indirect aerosol effect is stronger than previously believed, which would imply that the climate sensitivity to greenhouse gases is also stronger. Concerns about the effect of aerosol on the global climate were first researched as part of concerns over global cooling in the 1970s.
Pre-human global warming
The Earth has experienced natural global warming and cooling many times in the past, and can offer useful insights into present processes. It is thought by some geologists that a rapid buildup of greenhouse gases caused the Earth to experience global warming in the early Jurassic period, with average temperatures rising by 5 °C (9.0 °F). Research by the Open University published in Geology (32: 157–160, 2004 ) indicates that this caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, carbon dioxide 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 past global warming events, including the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last glacial period is thought not to be due to methane release . Instead, natural variations in the Earth's orbit (Milankovitch cycles) are believed to have triggered the retreat of ice sheets by changing the amount of solar radiation received at high latitude and led to deglaciation.
The greenhouse effect is also invoked to explain how the Earth made it out of the Snowball Earth period 600 million years ago. During this period all silicate rocks were covered by ice, thereby preventing them from combining with atmospheric carbon dioxide. The atmospheric carbon dioxide level gradually increased until it reached a level that could have been as much as 350 times the current level. At this point temperatures were raised enough to melt the ice, even though the reflective ice surfaces had been reflecting most sunlight back into space. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere, and thick layers of abiotic carbonate sediment have been found on top of the glacial rocks from this period.
Using paleoclimate data for the last 500 million years Veizer et al. (2000, Nature 408, pp. 698–701) concluded that long-term temperature variations are only weakly related to carbon dioxide variations. Most paleoclimatologists believe this is because other factors, such as continental drift and mountain building have larger effects in determining very long term climate. However, Shaviv and Veizer (2003, ) proposed that the biggest long-term influence on temperature is actually the solar system's motion around the galaxy, and the ways in which this influences the atmosphere by altering the flux of cosmic rays received by the Earth. Afterwards, they argued that over geologic times a change in carbon dioxide concentrations comparable to doubling pre-industrial levels, only results in about 0.75 °C (1.3 °F) warming rather than the usual 1.5–4.5 °C (2.7–8.1 °F) reported by climate models . They acknowledge (Shaviv and Veizer 2004) however that this conclusion may only be valid on multi-million year time scales when glacial and geological feedback have had a chance to establish themselves. Rahmstorf et al. 2004  argue that S+V have highly and arbitrarily tuned their data, and that their conclusions are unreliable.
Pre-industrial global warming
Paleoclimatologist William Ruddiman has argued (e.g., Scientific American, March 2005) 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. He contends that forest clearing explains the rise in carbon dioxide levels in the current interglacial that started 8,000 years ago, contrasting with the decline in carbon dioxide levels seen in the previous three interglacials. He further contends that the spread of rice irrigation explains the breakdown in the last 5,000 years of the correlation between the Northern Hemisphere solar radiation and global methane levels, which had been maintained over at least the last 11 22,000-year cycles. Ruddiman argues that without these effects, the Earth would be nearly 2 °C cooler and "well on the way" to a new ice age. Ruddimann's interpretation of the historical record, with respect to the methane data, has been disputed .
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Global Warming in popular culture
- Catastrophic climate change (due to abrupt shutdown of thermohaline circulation) was dramatized in the 2004 blockbuster, The Day After Tomorrow.
- Global warming was spoofed in five South Park episodes: Two Days Before the Day After Tomorrow, Spontaneous Combustion, Goobacks, Smug Alert! and Manbearpig.
|Scientific opinion | Attribution of causes | Effects | Mitigation | Adaptation | Controversy | Politics | Economics|
|Greenhouse effect | Greenhouse gases | Temperature data | Kyoto Protocol | Long-term climate change |
Intergovernmental Panel on Climate Change
- Global Warming and Ozone Depletion Freeview video interview with F.Sherwood Rowland (Nobel Prize for work on Ozone), by the Vega Science Trust.
- Global Warming Information from the Ocean & Climate Change Institute, Woods Hole Oceanographic Institution
- Intergovernmental Panel on Climate Change (IPCC)
- NASA's Global Hydrology and Climate Center
- Mauna Loa Observatory, Hawaii - Latest CO2 Measurements and Data
- Maps & Graphics on Global Warming from the UN Environment Programme GRID-Arendal
- NOAA's Global Warming FAQ
- RealClimate - A group blog of climate scientists
- National Center for Atmospheric Research - Overview of NCAR research on climate change
- Potsdam Institute for Climate Impact Research
- Discovery of Global Warming — An extensive introduction to the topic and the history of its discovery
- Introduction to climate change: Lecture notes for meteorologists (World Meteorological Organization) (PDF)
- Pew Center on Global Climate Change — basic science
- NOAA ESRL Global Monitoring Division
- Global Warming Site, U.S. Environmental Protection Agency
- Final Report of U.S. Climate Change Science Program
- Melting lakes in Siberia emit greenhouse gas
- Danish National Space Centre: SKY Experiment
Polar ice-related links
- How rapidly is permafrost changing? What are the impacts of these changes? from *NOAA
- Melting Russian Permafrost Could Accelerate Global Warming - ENS (7 September 2006)
- Ad hoc committee report on the ‘hockey stick’ global climate reconstruction - The Wegman Report, discussing statistical errors in Global Warming studies.
- Climate Ark - climate change and global warming portal providing news, search, links and analysis
- 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
- BBC: Global warming risk 'much higher'
- Watch and read 'Tipping Point', Australian science documentary about effects of global warming on rare, common, and endangered wildlife
- Summary by "Physicians and Scientists for Responsible Application of Science and Technology"
- A report by the Competitive Enterprise Institute, a pro-business group of global warming skeptics
- Newest reports on US EPA website
- An optimistic outlook on Global Warming from PBNV.com Apr 25, 2006
- The Discovery of Global Warming from historian Spencer Weart, Director of the Center for History of Physics of the American Institute of Physics (AIP).
- IPS Inter Press Service - Independent news on global warming and its consequences.
- Boffey, Philip. "Talking Points: The Evidence for Global Warming", New York Times, July 4, 2006.
- Borenstein, Saul. "Hot Summer Nights Getting Hotter", LiveScience.com, August 2, 2006. Retrieved on 2006-08-06.