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Global warming refers to the increases in the average temperature of the Earth's atmosphere and oceans that has been observed in recent decades. The scientific opinion on climate change is that the average global temperature has risen 0.6 ± 0.2 °C over the 20th century, and that it is likely that "most of the warming observed over the past 50 years is attributable to human activities". The increased volumes of carbon dioxide and other greenhouse gases (GHGs) released by the burning of fossil fuels, land clearing and agriculture, and other human activities, are the primary sources of the human-induced component of warming.

Observational sensitivity studies and climate models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures may increase by between 1.4 and 5.8 °C between 1990 and 2100. The range of uncertainty is primarily the results of the difficulty of predicting the volume of future carbon dioxide emissions, but there is also some uncertainty about the climate sensitivity.

The increase in global temperatures is expected to result in other changes, including rises in 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, and hurricanes. It may cause lower agricultural yields, glacier retreat, reduced summer streamflows, and contribute to biological extinctions. Although warming is expected to affect the number and magnitude of these events, 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. .

There are only a few scientists who contest the view that humanity's actions have played a significant role in increasing recent temperatures. However, more significant uncertainties exist 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 consequences.

The term 'global warming' is a specific case of the more general term 'climate change' (which can also refer to 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. 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. Over the one or two thousand years before 1850, world temperature is believed to have been relatively stable, with possibly local 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 timeframe, 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. 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 accept that the earth has warmed recently, but the cause or causes of this change is more controversial, especially outside the scientific community.

Adding carbon dioxide (CO2) or methane (CH4) to an atmosphere, with no other changes, will tend to make a planet's surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C 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 CH4 to the Earth's atmosphere will result in warmer surface temperatures on Earth, absent indirect mitigating effects. Rather, the debate is about what the net effect of the addition of carbon dioxide and CH4 will be.

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 GHG levels were stabilised at present day levels, there is a further warming of perhaps 0.5 °C to 1.0 °C already "committed", but not yet realised.

Greenhouse gases in the atmosphere


The atmospheric concentrations of carbon dioxide and CH4 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 from 315 ppmv as shown by the Keeling Curve. The concentration reached 376 ppmv in 2003. South Pole records show similar growth. The monthly measurements display small seasonal oscillations.

Methane is produced biologically and released from gas pipelines. Some biological sources are "natural" such as termites and others are attributable to human activity such as agriculture, e.g., rice paddies. Recent evidence suggests that forests may also be a source (RC; BBC).

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 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 possibly as much as 70,000 million tonnes of methane from permafrost peat bogs in Siberia, which have started melting due to the rising temperatures, may lead to significant additional sources of greenhouse gas emissions. . Note that anthropogenic emissions of other pollutants - notably sulphate aerosol - exert a cooling effect; this can account for the plateau/cooling seen in the temperature record in the middle of the 20th 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 &mdash; the Little Ice Age.
 * The warming is a result of variances in solar irradiance.

At present, none of these has much support within the climate science community as an explanation for recent warming.

Solar variation theory


In general the level of scientific understanding of the variance in direct solar irradiance is very low. Although the majority of scientists believe that direct variations in solar output appear too small to have substantially affected the climate, some researchers (e.g. ) have proposed that feedbacks from clouds or other processes enhance the effect. Proxy studies indicate that the level of solar activity during the last 70 years has probably been the highest in more than 8000 years.

In the IPCC Third Assessment Report (TAR), it was reported that volcanic and solar forcings might account for half of the temperature variations prior to 1950, but that the net effect of such natural forcings was roughly neutral since then. In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be 8 times larger than the change in forcing due to increasing solar activity over the same period.

Since the TAR, various 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.). Stott et al. estimated solar forcing to be 16% or 36% of greenhouse warming.

Potential negative 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 impossible to attribute specific natural phenomena to long-term global warming. In particular the relationship between global warming and hurricanes is still being debated. Four new papers correlating climate change with increased hurricane intensity seem to be making the case that the two phenomena are linked ; a draft WMO statement acknowledges the different viewpoints.

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. Global warming is already causing death and disease across the world through flooding, environmental destruction, heatwaves and other extreme weather events, according to some scientists. (Reuters, February 9, 2006; archived).

Effects on ecosystems
Secondary evidence of global warming &mdash; lessened snow cover, rising sea levels, weather changes &mdash; provides examples of consequences of global warming that may influence not only human activities but also the 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. Few of the terrestrial ecoregions on Earth could expect to be unaffected. Many of the species at risk are arctic fauna such as polar bears, emperor penguins, many salt wetland flora and fauna species, and any species that inhabit the low land areas near the sea. Species that rely on cold weather conditions such as gyrfalcons, and snowy owls that prey on lemmings that use the cold cold winter to their advantage will be hit hard.

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.

Destabilisation of ocean currents
There is also some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised 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.

See also: Shutdown of thermohaline circulation

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 metres almost every coastal city in the world would be severely affected, with the potential for major impacts on worldwide trade and economy. Presently, the IPCC predicts sea level rise of less than 1 meter 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 mm/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 150 billion US dollars 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 [a given hurricane] 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 200 billion U.S. dollars, with insured losses running at more than 70 billion U.S. dollars.

Potential positive effects
Global warming may also have positive effects.

Increased biomass production
Plants use solar energy to convert water, nutrients, and carbon dioxide into usable biomass using photosynthesis. Part of this biomass is used (directly or indirectly) as the energy source for nearly all other life forms. Examples of human use of biomass are feed-stock for cattle, fruits and grains for human consumption, and 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. Together, these effects can create a greener, richer planet, with more available biomass.

It is currently unclear if this is a realistic scenario. Plant growth can be limited by a number of factors, including soil fertility, water, temperature, and carbon dioxide concentration. Thus, the overall effect of increasing carbon dioxide and changing climate is very hard to estimate. IPCC models currently predict a modest increase in plant productivity that will taper of quickly with further temperature increases.

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, 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, which would cut 5,000 nautical miles 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.

Global Warming Controversy
The global warming controversy is a dispute regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature, especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, and whether the increase is wholly or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, and what the consequences of global warming will be. The controversy is significantly more pronounced in the popular media than in the scientific literature.

A minority of scientists oppose the global warming consensus
There are a minority of scientists who believe one or more of these categories; global warming is not occurring or has ceased, believe the accuracy of IPCC climate projections is questionable, global warming is primarily caused by natural processes, the cause of global warming is unknown, or believe the effects of global warming will not be significantly negative. The majority of climate scientists, across the globe, agree that the global average surface temperature has risen over the last century.

Politics of Global Warming
The politics of global warming have involved policy decisions, legislation, and political debate over the science of and response to global warming. The political struggle over global warming has involved various governmental bodies, special-interest groups, and scientific organizations.

Al Gore
Al Gore, former American Senator and vice-president has been involved in a number of environmental activities since 2004. Gore was the subject of a 2006 documentary film An Inconvenient Truth. This film won the Academy Award for Documentary Feature and became the subject of the book, An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. The book won a Grammy Award for Best Spoken Word Album in February, 2009. Later, in 2007, Gore was awarded the Nobel Peace Prize, which was shared by the Intergovernmental Panel on Climate Change, headed by Rajendra K. Pachauri (Delhi, India). The award was given "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change." Gore and Pachauri accepted the Nobel Peace Prize in Oslo, Norway on December 10, 2007. He also helped to organize the Live Earth benefit concerts.

Environmental criticism
Vice-president Gore's global warming position has been the subject of criticism. In 2008, he gave a speech at the DAR Constitution Hall in Washington, D.C. in which he called for a move towards replacing a dependence upon "carbon-based fuels" with green energy by the United States within 10 years." Some criticized this plan. According to the BBC, "Robby Diamond, of Securing America's Future Energy, said weaning the nation off fossil fuels could not be done in a decade. 'The country is not going to be able to go cold turkey [...] We have a hundred years of infrastructure with trillions of dollars of investment that is not simply going to be made obsolete.'"

Gore's personal use of electricity has made the news. In February 2007, the Tennessee Center for Policy Research (TCPR) stated that the Gore household uses "20 times as much electricity as the average household nationwide." Gore's spokeswoman Kalee Kreider countered the claim by stating that the Gores' "utility bills have gone down 40 percent since the green retrofit." of his home.

Contribution to global warming by deforestation
Deforestation is ongoing and is shaping climate and geography. and is often cited as one of the major causes of the enhanced greenhouse effect. Recent calculations suggest that carbon dioxide emissions from deforestation contribute about 12% of total anthropogenic carbon dioxide emissions with a range from 6 to 17%. Trees and other plants remove carbon (in the form of carbon dioxide) from the atmosphere during the process of photosynthesis and release oxygen back into the atmosphere during normal respiration. Deforestation may cause carbon stores held in soil to be released. Mature forests alternate between being net sinks and net sources of carbon dioxide (see carbon dioxide sink and carbon cycle). The incineration and burning of forest plants in order to clear land also releases tonnes of CO2, which contributes to global warming.

Reducing emissions from the tropical deforestation and forest degradation (REDD) in developing countries may reduce greenhouse gas. The idea consists in providing financial compensations for the reduction of greenhouse gas (GHG) emissions from deforestation and forest degradation".

Global cooling theory in the 1970's, 80's
Global cooling was a conjecture during the 1970s of imminent cooling of the Earth's surface and atmosphere along with a posited commencement of glaciation. This hypothesis never had significant scientific support, but gained temporary popular attention due to a combination of press reports that did not accurately reflect the scientific understanding of ice age cycles, and a slight downward trend of temperatures from the 1940s to the early 1970s. General scientific opinion is that the Earth has not durably cooled, but has indeed undergone global warming throughout the 20th century.

Climategate
The Climatic Research Unit e-mail hacking incident, also known as Climategate, began in November 2009 with the hacking of a server used by the Climatic Research Unit (CRU) of the University of East Anglia (UEA) in Norwich, England, U.K. An unknown party anonymously disseminated over a thousand e-mails and other documents to a web server. The university confirmed that a criminal breach of their security systems took place. Professor Phil Jones, Director of the CRU, stated that the leaked e-mails appeared to be genuine. The leaked material included more than 1,000 e-mails, pertaining to climate change research covering 1996 to 2009. Some of the e-mails contained discussions of how to combat the arguments of climate change sceptics, unflattering comments about sceptics, queries from journalists, and preclusion of contrary views from peer-review literature.

Critics have asserted that the e-mails show collusion by climate scientists to withhold scientific information. Prominent climate scientists, such as Richard Somerville, have called the incident a smear campaign. Kevin Trenberth of the National Center for Atmospheric Research declared it an attempt to sabotage the Copenhagen global climate summit in December, 2009.

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 four 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

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, and energy conservation, carbon taxes and carbon sequestration schemes. 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, 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.

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. Likewise, the opposition of the United States and Australia to action on climate change has been attributed to both countries' links to fossil fuels. The US is a major producer and consumer of fossil energy. Australia has the fourth-largest coal reserves in the world, and is a major exporter of coal.

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 this issue with computer models of the climate (see below). These models are accepted by the scientific community as being valid only after it has been shown that they do a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Niño. All climate models that pass these tests also predict that the net effect of adding greenhouse gases will be a warmer climate in the future. The amount of predicted warming varies by model; one of the most important sources of this uncertainty in climate sensitivity is believed to be different ways of handling clouds.

As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4 °C to 5.8 °C 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.

The most recent 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 ability of the environment to sink carbon dioxide suggested that rising fossil fuel emissions would decrease absorption from the atmosphere, amplifying climate warming beyond previous predictions, although "Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses" .

Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release the potent greenhouse gas, methane, that is trapped in large quantities in permafrost and ice clathrate compounds.

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 underpredict future global warming. 

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 &minus;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
It is thought by some geologists that the Earth experienced global warming in the early Jurassic period, with average temperatures rising by 5 °C. 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%. Rock weathering locks away carbon in calcite and dolomite, which are minerals with various degrees of carbon oxides. As a result of this, carbon dioxide levels dropped back to normal over roughly the next 150,000 years.

Sudden release of methane from clathrate compounds (the clathrate gun hypothesis), has been hypothesized as a cause of past global warming. Two events possibly linked in this way are the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last ice age is thought not to be due to methane release.

The greenhouse effect has also been invoked to explain how the Earth made it out of the Snowball Earth period. 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 about 350 times current levels. At this point temperatures were raised to an average of 50 °C, hot enough to melt the ice. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere. Thick layers of abiotic carbonate sediment which can be found on top of the glacial rocks from this period are believed to have been formed by this rapid carbon dioxide removal process.

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 coupled to carbon dioxide variations. Shaviv and Veizer (2003, ) extended this by arguing that the biggest long-term influence on temperature is actually the solar system's motion around the galaxy. Afterwards, they argued that over geologic time a change in carbon dioxide concentrations comparable to doubling preindustrial levels, only results in about 0.75 °C warming rather than the usual 1.5-4.5 °C reported by climate models. In turn Veizer's recent work has been discussed and criticised on RealClimate.org.

Palaeoclimatologist William Ruddiman has argued (e.g. Scientific American, March 2005) that human influence on the global climate began around 8000 years ago with the development of agriculture. 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.

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
 * 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 &mdash; 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 &mdash; Basic Science
 * NOAA ESRL Global Monitoring Division

Other

 * Climate Ark - climate change and global warming portal providing news, search, links and analysis
 * A large compendium of links to sites with information on global warming
 * Science and Technology Librarianship: Global Warming and Climate Change Science &mdash; Extensive commented list of Internet resources &mdash; 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
 * Global Warming Political and Environmental Policies