Global warming is an increase in the average temperature of the Earth's atmosphere and oceans. The term is also used for the more specific scientific theory of anthropogenic global warming, which states that much of the recent observed and projected global warming is human-induced. The theory states that this is a result of a strengthened greenhouse effect caused by man-made increases in carbon dioxide and other greenhouse gases, through the burning of fossil fuels and deforestation. The natural greenhouse effect keeps the Earth 30 °C warmer than it otherwise would be; adding carbon dioxide to an atmosphere, with no other changes, will make a planet's surface warmer. Current research is attempting to find out more details about the processes and factors that would affect a temperature increase especially about positive and negative feedback mechanisms, to allow a more precise quantification of the effects of global warming.
Temperature change is just one aspect of the broader subject of (human-induced) climate change. The scientific opinion on climate change, as expressed by the UN Intergovernmental Panel on Climate Change (IPCC) and explicitly endorsed by the national science academies of the G8 nations, is that the average global temperature has risen 0.6 ± 0.2 °C since the late 19th century, and that it is likely that "most of the warming observed over the last 50 years is attributable to human activities" . A small minority of qualified scientists contest the view that humanity's actions have played a significant role in increasing recent temperatures. Uncertainties do exist regarding how much climate change should be expected in the future, and a hotly contested political and public debate exists over what actions, if any, should be taken in light of global warming.
Based on basic science, observational sensitivity studies, and the climate models referenced by the IPCC, temperatures may increase by 1.4 to 5.8 °C between 1990 and 2100 . This is expected to result in other climate changes including rises in sea level and changes in the amount and pattern of precipitation. Such changes may increase extreme weather events such as floods, droughts, heat waves, and hurricanes, change agricultural yields, or contribute to biological extinctions. Although warming is expected to affect the frequency and magnitude of these events, it is very difficult to connect any particular event to global warming.
The scientific consensus on global warming is that the Earth is warming, and that humanity's greenhouse gas emissions are making a significant contribution. This consensus is summarized by the findings of the Intergovernmental Panel on Climate Change (IPCC). In the Third Assessment Report, the IPCC concluded that "most of the warming observed over the last 50 years is attributable to human activities". This position was recently supported by an international group of science academies from the G8 countries and Brazil, China and India .
Over the past century or so the global (land and sea) temperature has increased by 0.6 ± 0.2 °C . The effects of global warming are increasingly visible. At the same time, atmospheric carbon dioxide has increased from around 280 parts per million (by volume) in 1800 to around 315 in 1958 and 367 in 2000, a 31% increase over 200 years. Other greenhouse gas emissions have also increased. 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 SRES gives a wide range of future carbon dioxide scenarios , ranging from 540-970 parts per million by 2100.
Climate models, driven by estimates of increasing carbon dioxide and to a lesser extent by generally decreasing sulphate aerosols, predict that temperatures will increase (with a range of 1.4 to 5.8 °C for change between 1990 and 2100 ). Much of this uncertainty results from not knowing future carbon dioxide emissions, but there is also uncertainty about the accuracy of climate models. Climate commitment studies predict that even if levels of greenhouse gases and solar activity were to remain constant, the global climate is committed to 0.5 °C of warming (some model results are as high as 1.0 °C) over the next one hundred years due to the lag in warming caused by the oceans.
Although the combination of scientific consensus and economic incentives (especially for Russia) were enough to persuade the governments of more than 150 countries to ratify the Kyoto Protocol - there are issues about just how much greenhouse gas emissions warm the planet. Uncertainties remain and have been emphasized by some politicians, and others questioning the costs needed to reduce future global warming; however, the business position on climate change is increasingly changing to accept global warming as both real and anthropogenic, and that action such as carbon emissions trading and carbon taxes is needed. The scientific consensus is questioned by a small minority of scientists.
Warming of the Earth
Relative to 1860-1900 the global (land and sea) temperature has increased by 0.75 °C. Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C per decade since 1979. Over the past 1-2 thousand years before 1850 the temperature is believed to have been relatively stable, with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age.
The length of time over which one is interested in change may vary according to the focus of the user of the term and the datasets available for investigation. Temperature record holds a discussion of the various records. An approximately global instrumental temperature record begins in about 1860; contamination from the urban heat island is believed to be small. A longer-term perspective is available from various proxy records for recent millenia; see Temperature record of the past 1000 years for a discussion of these records and their differences. Attribution of recent climate change is clearest for the most recent period (the last 50 years) for which the most detailed data is available. Satellite temperature measurements of the tropospheric temperature date from 1979.
Causes of global warming
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 changes in the Earth's orbit around the Sun (Milankovitch cycles), solar activity, and volcanic emissions as well as greenhouse gases. See Climate change for further discussion of these forcing processes. Climatologists accept that the earth has warmed recently. Somewhat more controversial is what may have caused this change. See attribution of recent climate change for further discussion.
Atmospheric scientists know that adding carbon dioxide (carbon dioxide) or methane (CH4) to an atmosphere, with no other changes, will tend to make a planet's surface warmer. Indeed, greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be 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, on average. Rather, the debate is about what the net effect of the addition of carbon dioxide and CH4 will be, and whether changes in water vapor, clouds, the biosphere and various other climate factors will cancel out its warming effect. The observed warming of the Earth over the past 50 years appears to be at odds with the idea that climate feedbacks will cancel out the warming.
Greenhouse gas emissions
Coal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the human environment contribute about 22 billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. Animal agriculture, manure, natural gas, rice paddies, landfills, coal, and other sources contribute about 250 million tons of methane each year. About half of human emissions have remained 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 (see the Keeling Curve). The concentration reached 376 ppmv in 2003. South Pole records show similar growth . The monthly measurements display small seasonal oscillations.
Note that anthropogenic emissions of other pollutants - notably sulphate aerosol - can exert a cooling effect; this accounts for the plateau/cooling seen in the temperature record in the middle of the century .
Solar variation theory
- Main article: Solar variation theory
Direct variations in solar output appear too small to have substantially affected the climate; nonetheless some researchers (e.g. ) have proposed that feedbacks from clouds or other processes enhance the effect.
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 irradiance changes over pre-industrial are less by a factor of 3-4 than in the reconstructions of, e.g. Hoyt and Schatten (1993), Lean (2000) used in the TAR. Stott et al.  estimated solar forcing to be 16% or 36% of greenhouse warming.
Various other hypotheses have been proposed, including but not limited to:
- The warming is within the range of natural variation and needs no particular explanation.
- The warming is a consequence of coming out of a prior cool period — the Little Ice Age — and needs no other explanation.
- The warming trend itself has not been clearly established, and therefore does not need any explanation.
At present, none of these has more than a small number of supporters within the climate science community.
- Main article: General circulation model
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, however, which probably reflects the way different models depict clouds differently.
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 determine 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 clathrates .
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 considers 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. 
The relation between global warming and ozone depletion
- Main article: Ozone depletion
Although they are often interlinked in the popular press, 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 m–2 .
- One of the strongest predictions of the GW theory is that the stratosphere should cool. However, although this is observed, it is difficult to use it for attribution (for example, warming induced by increased solar radiation would not have this upper cooling effect) because 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 GHG's .
The relation between global warming and global dimming
- Main article: Global dimming
Some scientists now consider that the effects of the recently recognized phenomenon 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%. As a result of this, carbon dioxide levels dropped back to normal over roughly the next 150,000 years.
Sudden release of methane (a greenhouse gas) from its ice complex (clathrate) 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 .
Leading 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.
- Main article: Global warming controversy
The global warming controversy is an ongoing dispute about the effects of humans on the global climate and the policies that should be followed to avoid future effects. Although not fully settled, the current consensus from the official scientific communities on climate change is that recent warming is largely human-caused.
- Main article: Effects of global warming
The predicted effects of global warming are many and various, both for the environment and for human life. The primary effect of global warming is increasing carbon dioxide and increasing global average surface temperature. From this flow a variety of secondary effects, including sea level rise, impacts on agriculture, reductions in the ozone layer (see below), 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  . Three new papers correlating climate change with increased hurricane intensity seem to be making the case that the two phenomena are linked. 
The extent and likelihood of these consequences is a matter of considerable controversy. A summary of possible effects and our current understanding can be found in the report of the IPCC Working Group II .
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 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.
Destabilisation of ocean currents
Main article: Destabilisation of ocean currents
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 world-wide 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 millenia. 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.   
Spread of disease
It has been claimed that global warming will probably extend the favourable zones for vectors conveying infectious diseases such as malaria. An example of this may be the recent extension to the north Mediterranean region of bluetongue disease in domesticated ruminants associated with mite bites. Another is the increase of hantavirus infection, Crimean-Congo hemorrhagic fever, tularemia and rabies in wide areas of Russia during 2004–2005 associated with a population explosion of rodents and their predators. Some of this, however is 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 (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.
The United Nation’s Environmental Program recently announced that severe weather around the world this year has made 2005 the most costly year on record.
Possible beneficial effects
Global warming may also have positive effects. Plants form the basis of the biosphere. Using photosynthesis, they can use the sun's energy to convert water, nutrients, and carbon dioxide into usable biomass. Plant growth can be limited by a number of factors, including soil fertility, water, temperature, and carbon dioxide concentration. Lack of carbon dioxide can induce photorespiration, which can destroy existing sugars. Thus, an increase in temperature and atmospheric carbon dioxide can stimulate plant growth in places where these are the limiting factors. IPCC models predict that higher carbon dioxide concentrations would only spur growth of flora up to a point however, because in many regions the limiting factors are water or nutrients, not temperature or carbon dioxide. Despite the limiting factor of water, an increase in carbon dioxide concentration has the direct effect of increasing the transpiration efficiency of most plants so that they actually produce more net biomass per unit of water used by the plant. Satellite data shows that the productivity of the northern hemisphere has indeed increased since 1982.
On the other hand, an increase in the total amount of biomass produced is not necessarily all good, since biodiversity can still decrease even though a smaller number of species are flourishing.
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 percent between 1969 and 2004 .
Mitigating and adapting to global warming
- Main article: Mitigation of global warming
"Mitigation of global warming" covers all actions aimed at reducing the extent or likelihood of global warming. The world's primary international agreement on combating climate change is the Kyoto Protocol. Various other strategies include development of new technologies, wind power, nuclear power, renewable energy, biodiesel, electric cars (and hybrids), and fuel cells, Energy conservation, carbon taxes and carbon sequestration schemes.
- Naomi Oreskes, 2004 Beyond the Ivory Tower: The Scientific Consensus on Climate Change - The author discussed her survey of 928 peer-reviewed scientific abstracts on climate change. Retrieved December 8, 2004. Also available as a 1 page pdf file
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- Template:Journal reference "Shafts of ancient ice pulled from Antarctica's frozen depths show that for at least 650,000 years three important heat-trapping greenhouse gases never reached recent atmospheric levels caused by human activities, scientists are reporting today." (November 25, 2005) 
- Climate change
- Effects of global warming
- Global Atmosphere Watch
- Iris Hypothesis
- National Assessment on Climate Change
- United Kingdom Climate Change Programme
- Wind power
- NASA's Global Hydrology and Climate Center
- National Center for Atmospheric Research - Overview of NCAR research on climate change
- Discovery of Global Warming — An extensive introduction to the topic and the history of its discovery
- Climate change (global warming) : a couple of answers to some elementary questions by Jean-Marc Jancovici
- The Science & Environmental Policy Project
- Global Warming FAQ by Tom Rees
- A large compendium of links to sites with information on global warming
- Science and Technology Librarianship: Global Warming and Climate Change Science — Extensive commented list of Internet resources — Science and Technology Sources on the Internet.
- RealClimate - A group blog of climate scientists
- Climate Indymedia - An independent, open publishing, news media website about Climate Chaos. (A topical Indymedia )ca:Escalfament global
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