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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 degrees Celsius (1.1 ± 0.4  degrees Fahrenheit) in the 20th century. The prevailing scientific opinion on climate change is that "most of the warming observed over the last 50 years is attributable to human activities" . The increased amounts of carbon dioxide (CO₂) and other greenhouse gases (GHGs) are the primary causes of the human-induced component of warming. They are released by the burning of fossil fuels, land clearing and agriculture, etc. and lead to an increase in the greenhouse effect.

The measure of the climate response to increased GHGs, climate sensitivity, is found by observational studies and climate models. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO₂ in the atmosphere. The current literature estimates sensitivity in the range 1.5-4.5  °C (2.7-8.1 °F). Models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures may increase by between 1.4 and 5.8  °C (2.5 to 10.5  °F) between 1990 and 2100. The uncertainty in this range results from both the difficulty of predicting the volume of future greenhouse gas emissions and uncertainty about climate sensitivity.

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, glacier retreat, reduced summer streamflows, 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 a long average atmospheric lifetime. .

Only a small minority of 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 what, if anything, should be done to reduce or reverse future warming, and how to cope 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.

Causes

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. This conclusion can be controversial, especially outside the scientific community.

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.

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, making it more difficult for the Earth to cool. 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 emissions of carbon dioxide to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is 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.

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.

Alternative theories

Various alternative hypotheses have been proposed to explain the observed increase in global temperatures, including but not limited to:

• The warming is within the range of natural variation.
• The warming is a consequence of coming out of a prior cool period — the Little Ice Age.
• The warming is a result of variances in solar irradiance.

At present, these have little support within the climate science community as primary explanations for the recent warming.

In an essay in the journal Science, researcher N. Oreskes reported a sampling of the abstracts of 928 articles on climate change, selected using the key phrase "global climate change", from the published peer-reviewed scientific literature. Oreskes found that of the 75% of abstracts that in any way considered the question of attribution, all of them either explicitly or implicitly supported the consensus view that a significant fraction of recent climate change is due to human activities .

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 3-4 than in the reconstructions used in the TAR (e.g. Hoyt and Schatten, 1993, Lean, 2000.). Other workers (e.g. Stott et al. 2003 ) believe that the impact of solar forcing is being underestimated and propose that solar forcing accounts for 16% or 36% of recent greenhouse warming. Other researchers (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 impact 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, though the impact of such a decline will probably be small compared to the impact of concurrent increases in greenhouse gas.

Predicted effects

The predicted effects of global warming are many and various, both for the environment and for human life. These effects include sea level rise, impacts on 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 being experienced, although it is difficult to attribute specific natural phenomena to long-term global warming. In particular, 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 likelihood of these consequences is a matter of considerable controversy. A summary of possible effects and recent understanding can be found in the report of the IPCC Working Group II . Some scientists believe 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

Secondary evidence of global warming — lessened snow cover, rising sea levels, weather changes — provides examples of consequences of global warming that may influence not only human activities but also ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may flourish.

Ocean PH is lowering as a result of increased carbon dioxide levels. Lowering of ocean PH along with changing water temperature and ocean deepth will have a direr impact 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 .

Impact 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)

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.

Environmental refugees

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 impacts on world-wide trade and economy. Presently, the IPCC predicts sea level rise of less than 1 meter (3 ft) 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 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

Global warming may extend the range of vectors conveying infectious diseases such as malaria. Bluetongue disease in domesticated ruminants associated with mite bites has recently spread to the north Mediterranean region. 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 effects

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, tax payers, 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 either was, or was not, affected by global warming . 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.

Biomass production

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 benefiting from global warming is a realistic scenario. 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 impacts: 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 higher temperature are increased; and the reductions in soil moisture in summer, which are likely to occur, would have a negative impact on productivity.

Satellite data shows that the productivity of the northern hemisphere has indeed increased from 1982 to 1991 . However, more recent studies , found that from 1991 to 2002, wide-spread 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 which are too big to fit through the Panama Canal and currently have to go around the tip of South America. 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.

Negative impacts of the melting of ice include a potential increase in the rate of global warming, as that ice reflects more sunlight than the open water which is replacing it. There are also ecological effects of melting polar ice: for example, polar bears use sea ice to reach their prey, and swim to another ice floe when one breaks up. Ice is now becoming further separated, and dead polar bears are being found in the water, believed to have drowned. More recently, observed cannibalistic behavior in polar bears has been suggested by some scientists to be the result of food shortages brought on by global warming (Amstrup et al. 2006).

Responses

The threat of possible global warming has led to attempts to mitigate global warming, which 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:

1. Reduction of energy use (conservation)
2. Shifting from carbon-based fossil fuels to alternative energy sources
3. Carbon capture and storage
4. Carbon sequestration
5. Planetary engineering to cool the earth

Strategies for mitigation of global warming include development of new technologies, wind power, nuclear power, solar power, renewable energy, biodiesel, electric or hybrid automobiles, fuel cells, energy conservation, carbon taxes, enhancing natural carbon dioxide sinks, population control and carbon capture and storage. Some environmentalist 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 combination of scientific consensus and economic incentives were enough to persuade the governments of more than 150 countries to ratify the Kyoto Protocol (notably excluding the United States and Australia), there is a continuing debate about just how much greenhouse gas emissions warm the planet. Some politicians, including President of the United States George W. Bush , Prime Minister of Australia John Howard and some public intellectuals such as Bjørn Lomborg and Ronald Bailey have argued the cost of mitigating global warming is too large to be justified. It should also be noted that the world's two largest populations, India and China, are currently exempt from the requirements of the protocol.

However, some segments of the business community have accepted both the reality of global warming and its attribution to anthropogenic causes, as well as the 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.

Climate models

Scientists have studied global warming with computer models of the climate (see below). For a climate model to be accepted by the scientific community as being valid it must first be shown that it does a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Niño. Once models which fail these tests are rejected, the remaining models all 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.

Other related issues

Relationship to ozone depletion

Although they are often interlinked in the mass media, the connection between global warming and ozone depletion is not strong. There are four areas of linkage:

• 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: reduced ozone allows more solar radiation to penetrate, thus warming the troposphere. But a colder stratosphere emits less long-wave radiation, tending to cool the troposphere. 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 difficult to use it as an attribution of recent climate change since similar cooling is caused by ozone depletion.

• 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 .

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 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 level that could have been as much as 350 times current levels. 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.

Paleoclimatologist William Ruddiman has argued (e.g. Scientific American, March 2005) that human influence on the global climate actually began around 8,000 years ago with the development of agriculture. In his opinion, this prevented carbon dioxide (and later methane) levels falling as rapidly as they would have done otherwise. Ruddiman argues that without this effect, the Earth would be entering, or already have entered, a new ice age. However other work in this area (Nature 2004) argues that the present interglacial is most analogous to the interglacial 400,000 years ago that lasted approximately 28,000 years, in which case there is no need to invoke the spread of agriculture for having delayed the next ice age.

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• RealClimate Scientists Baffled
• Ruddiman, William F. (2005). Plows, Plagues, and Petroleum: How Humans Took Control of Climate. New Jersey: Princeton University Press. ISBN 0691121648.
• Smith, T.M. and R.W. Reynolds, 2005: A global merged land and sea surface temperature reconstruction based on historical observations (1880–1997). J. Climate, 18, 2021–2036.
• UNEP summary (2002) Climate risk to global economy, Climate Change and the Financial Services Industry, United Nations Environment Programme Finance Initiatives Executive Briefing Paper (UNEP FI) (PDF) Accessed 7 January 2006
• Shaviv and Veizer (2004). "Forum: Comment". Eos. 85 (48): 510–511.
• S.K. Solanki, I.G. Usoskin, B. Kromer, M. Schussler, J. Beer (2004). "Unusual activity of the Sun during recent decades compared to the previous 11,000 years". Nature. 431: 1084–1087. doi:10.1038/nature02995.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• S. K. Solanki, I. G. Usoskin, B. Kromer, M. Schüssler and J. Beer (2005). "Climate: How unusual is today's solar activity? (Reply)". Nature. 436: E4–E5. doi:10.1038/nature04046.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Wang, Y.M., J.L. Lean, and N.R. Sheeley (2005). "Modeling the sun's magnetic field and irradiance since 1713". Astrophysical Journal. 625: 522–538.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Wired Careful Where You Put That Tree
• Kennett J. P., Cannariato K. G., Hendy I. L. & Behl R. J.American Geophysical Union, Special Publication, Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis. 54, (2003).
• Sowers T. (2006). "Late Quaternary Atmospheric CH₄ Isotope Record Suggests Marine Clathrates Are Stable". Science. 311 (5762): 838–840. doi:10.1126/science.1121235.
• Hinrichs K.U., Hmelo L. & Sylva S. (2003). "Molecular Fossil Record of Elevated Methane Levels in Late Pleistocene Coastal Waters". Science. 299 (5610): 1214–1217. doi:10.1126/science.1079601.
• Questions about Clathrate Gun Hypothesis (source of information)

See also

• Global cooling
• Glossary of climate change
• Economics of global warming
• Global warming controversy
• Climate Change Science Program
• Global Atmosphere Watch
• Iris Hypothesis
• National Assessment on Climate Change
• Phenology
• Global warming skeptics
• Timeline of environmental events
• United Kingdom Climate Change Programme
• United Nations Framework Convention on Climate Change
• An Inconvenient Truth
• Massachusetts v. Environmental Protection Agency
• Atlantic Multidecadal Oscillation

External links

Scientific

• Global Warming Information from the Ocean & Climate Change Institute, Woods Hole Oceanographic Institution
• Intergovernmental Panel on Climate Change (IPCC)
  • IPCC Third Assessment Report published in 2001
  • Climate Change 2001: Working Group II: Impacts, Adaptation and Vulnerability
  • A summary of the above IPCC report - by GreenFacts
• NASA's Global Hydrology and Climate Center
• Mauna Loa Observatory, Hawaii - Latest CO₂ Measurements and Data
• 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)
• RealClimate - A group blog of climate scientists
• 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

Polar ice-related links

• Polar bears drown as ice shelf melts The Sunday Times, December 18 2005
• Arctic melting will open new sea passages, Debora MacKenzie, New Scientist, 2 March 2002. (Full article by subscription)

Other

• 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.
• Climate Indymedia - An independent, open publishing, news media website about Climate Chaos. (A topical Indymedia )
• "Global Warming" at a glance - latest data of the global temperature from JunkScience.com
• Global Warming Newspaper Articles Archive - free archive of more than 50,000 historical newspaper articles on Global Warming
• BBC: Global warming risk 'much higher'
• Articles on the role of business organizations in sustainable development - includes actions on climate change.
• 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"
• Newest reports on US EPA website
• Boffey, Philip (July 4, 2006). "Talking Points: The Evidence for Global Warming". New York Times. {{cite news}}: Cite has empty unknown parameter: |coauthors= (help)

• Climate change feedbacks and causes
• History of climate