Global warming is a term used to describe an increase over time of the average temperature of Earth's atmosphere and oceans. Global warming theories attempt to account for the rise in average global temperatures since the late 19th century (0.6 ± 0.2°C)   and assess the extent to which the effects are due to human causes. The most common global warming theories attribute temperature increases to increases in the greenhouse effect caused primarily by anthropogenic (human-generated) carbon dioxide and to possible increases in solar activity.
Use of the term "global warming" generally implies a human influence — the more neutral term climate change is usually used for a change in climate with no presumption as to cause and no characterization of the kind of change involved, such as the Ice Ages. Note, however, that there is one important exception to this: the UNFCCC uses "climate change" for human caused change and "climate variability" for non-human caused change . Sometimes the term "anthropogenic climate change" is used to indicate the presumption of human influence.
Climate models, forced by estimates of increasing CO2 and to a lesser extent, by generally decreasing sulfate aerosols, predict that temperatures will increase (with a range of 1.4°C to 5.8°C for change between 1990 and 2100 []). Much of this uncertainty results from not knowing future CO2 emissions, but there is also uncertainty about the accuracy of climate models, especially the representation of clouds and aerosols and the failure to correctly model the vertical temperature profile over the tropical oceans.
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 over the next one hundred years due to the lag in warming caused by the oceans. The models show that 10cm of sea level rise is also committed for the next century, but that while the temperature lag will have been caught up after 100 years, there are several more centuries of sea level rise already committed even without accounting for any glacier or ice cap melting.
Although the discussion of global warming often focuses on temperature, global warming or any climate change causes changes in other things as well: overall precipitation and its patterns, cloud cover, weather, and all the other elements of our atmospheric system will be impacted by the increase in radiative forcing due to human changes in greenhouse gas concentrations in the atmosphere.
The Kyoto Protocol proposes binding greenhouse gas limits for developed countries, but there are considerable disagreements about the extent to which the Kyoto Protocol will be able to address the issue of greenhouse gases and global warming even if it is successfully implemented.
The current scientific consensus on global warming might be summarized by the findings of the Intergovernmental Panel on Climate Change (IPCC). In their Third Assessment Report, they concluded that "most of the warming observed over the last 50 years is attributable to human activities".
The IPCC does not commission or carry out research itself, but rather brings together hundreds of scientists to review and interpret previously published research. This group then issues reports to summarize the state of climate change science and, when possible, expresses the consensus of IPCC participants. The 1995 IPCC report concluded that "The balance of evidence suggests a discernible human influence on global climate"; this was strengthened in the 2001 TAR to "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities". The Fourth Assessment Report of the IPCC is scheduled for November 2007.
A survey in 1996 by Dennis Bray and Hans von Storch of the Meteorologisches Institut der Universität Hamburg generated responses from over 400 German, American and Canadian climate researchers and was reported in the United Nations Climate Change Bulletin . The survey reported the response of scientists in this field to the statement that it is "certain that, without change in human behavior, global warming will definitely occur sometime in the future". Scientists polled gave this statement an average score of 2.6 on a scale from 1 to 7, where 1 indicated complete agreement and 7 indicated complete disagreement.
The period of time over which the change has been observed may vary according to the focus of the user of the term: sometime since the Industrial Revolution, or since the beginning of an approximately global instrumental temperature record in about 1860; or over the past century; or the most recent 50 years.
Over the past century or so the global (land and sea) temperature has increased by approximately 0.6 ± 0.2°C . Over the past 1-2 thousand years the temperature has been relatively stable, with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age.
For details of changes during various periods:
The temperature increase has not been uniform over the globe or over time . Recent research (Peterson 2003; Parker 2004) indicates that estimates of temperature trends may not be much influenced by the urban heat island effect. While the accuracy of collected station data is not in dispute, the records suffer from incomplete coverage, geographically and historically, making the conclusions drawn from the data subject to disagreement. 
Temperatures in the lower troposphere have increased at somewhere between 0.08 and 0.22 °C per decade since 1979 (see Satellite temperature measurements). Just like the surface record, the average temperature rise is not linear, but has rises and falls superimposed on it due to natural variability, most notably El Niño's. Over the same period the surface record shows a warming of approximately 0.15 °C/decade. 
A new reconstruction by Moberg, et al, published in Nature 433, 613 - 617 (10 February 2005) shows both the Medieval Warm Period and Little Ice Age anomalies (although not by name) and concludes that the temperatures around 1000 and 1100 AD were comparable to those of the 20th century before 1990. "Moberg's reconstruction will help to put the record straight in one of the most contested issues in palaeoclimatology," says Hans von Storch. "But it does not weaken in any way the hypothesis that recent observed warming is a result mainly of human activity.". Moberg's results are consistent with those of Von Storch, et al, who conducted a modeling analysis that showed the variability to be about twice as great as previously published Science 306, 679 - 682 (2004).
Mann concedes that past climate variations may be larger than thought, but that "The contrarians would have us believe that the entire argument of anthropogenic climate change rests on our hockey-stick construction. But in fact some of the most compelling evidence has absolutely nothing to do with it, and has been around much longer than our curve" .
Theories to explain temperature change
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.
Greenhouse gas theory
In the late 19th century the Swedish chemist and 1903 Nobel Laureate Svante Arrhenius used the measured infrared absorption of carbon dioxide to calculate that increases in greenhouse gas concentration would lead to higher global mean temperatures while decreases would lead to colder global mean temperatures. The idea arose largely as Arrhenius' attempt to explain ice ages. At the time his peers largely rejected this theory.
Arrhenius' colleague Arvid Högbom was one of the first to study the carbon cycle. Through him Arrhenius was aware that in 1890 emission and absorption of CO2 were roughly in balance. Their best estimates were that burning of fossil fuels would not be a future problem, but this was based on coal consumption at the end of the 19th century. 
The longest sustained measurement of CO2 in the atmosphere is reflected in the Keeling Curve. Measurements conducted by Dr. Charles David Keeling atop Mauna Loa in Hawaii show an increase of more than 50 parts per million by volume from 1958 to the present.
The theory that human greenhouse gas emissions are contributing to the warming of the Earth's atmosphere has gained many adherents and some opponents in the scientific community within the past 25 years. The IPCC, which was established to assess the risk of human-induced climate change, attributes most of the recent warming to human activities. The United States National Academy of Sciences also endorsed the theory. Atmospheric physicist Richard Lindzen and other skeptics oppose aspects of the theory.
There are many subtle aspects to the question. Atmospheric scientists know that adding carbon dioxide (CO2) to an atmosphere, with no other changes, will tend to make a planet's surface warmer. But there is a significant amount of water vapor in the Earth's atmosphere in the form of humidity and clouds, and water vapor is a strong greenhouse gas. If adding CO2 to the atmosphere changes processes that regulate the amount of water vapor in the Earth's atmosphere, that could have a profound effect on the climate: more water vapor means more warmth.
The effect of clouds is also critical. Clouds have competing effects on the climate; everyone has noticed that surface temperature drops when a cloud passes overhead on an otherwise hot, sunny summer day. So clouds cool the surface by reflecting sunlight back into space. However, many people have also noticed that clear winter nights tend to be colder than cloudy winter nights. That is because clouds block the radiation of heat away from the surface (and eventually into space), and radiate it back to the surface of the Earth.  If CO2 changes the amount or distribution of clouds, it could have various complex effects on the climate. In the 2001 IPCC report on climate change, the possible changes in cloud cover were highlighted as one of the dominant uncertainties in predicting future climate change.
Given this, it is not correct to imagine that there is a debate between those who "believe in" and "oppose" the theory that adding CO2 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 CO2 will be, and whether changes in water vapor, clouds, and so on will cancel out its warming effect. The observed warming of the Earth over the past 50 years appears to be at odds with the skeptics' theory that climate feedbacks will cancel out the warming.
Scientists have also 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 CO2 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. Skeptics point to the growing evidence that variation in cosmic ray flux represents an indirect effect of changes in solar activity that increases the warming response to increases in solar activity. Climate models that pass the above tests while only modeling the direct effects of increases in solar activity will have attributed too much of the historical warming to greenhouse gas forcing, and will predict larger increases in temperature in the future. Skeptics of global warming point to potential feedbacks that current models poorly understand, such as changes in vegetation and cloud cover, and suggest that these processes reduce the sensitivity of the climate to greenhouse gas forcing. Proponents of established global warming theory argue that the uncertainty could just as easily extend to increases in warming as reductions in it. For example, it has been hypothesized that an increase in average temperature would cause volatilization of methane clathrates, which would increase the amount of methane in the atmosphere, causing further warming and hence further volatilization. 
Coal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the human environment contribute about 22 billion tons of carbon dioxide (corresponding to 6 billion tons of pure carbon) and other greenhouse gases into the earth's atmosphere each year. The atmospheric concentration of CO2 has increased by 31% above pre-industrial levels since 1750. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 40 million years ago. About three-quarters of the anthropogenic emissions of CO2 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 .
"Greenhouse gases" get their name because they trap radiant energy from the sun that would otherwise be re-radiated back into space, by analogy with the glass panes in a greenhouse. The analogy, however, is a false one, as the effects are different — see greenhouse effect. The natural greenhouse effect that tempers the earth's climate is not at issue in the debate over global warming. Without it, temperatures would drop by approximately 30°C, the oceans would freeze and life as we know it would be impossible. What climatologists are concerned about, rather, is that increased levels of greenhouse gases in the atmosphere might cause more heat to be trapped.
"The longest continuous instrumental measurement of CO2 mixing ratios began in 1958 at Mauna Loa. Since then, the annually averaged value has increased monotonically from 315 ppm. In fact, it is clear that the increase is faster than linear. On March 21, 2004, it was reported that the concentration reached 376ppm in 2003. South Pole records show similar growth . The monthly measurements display small seasonal oscillations.
Solar variation theory
Various hypotheses have been proposed to link terrestrial temperature variations to solar variations.
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. See also: urban heat island.
Other global warmings
It is thought by geologists that the Earth experienced global warming in the early Jurassic period, with average temperatures rising by 5° Celsius (9° Fahrenheit). Research by the Open University published in Geology (32, 157–160, 2004 ) indicates that this caused the rate of weathering of rocks to increase by 400%, which in turn reduced carbon dioxide levels back to normal over roughly the next 150,000 years.
Sudden release of methane clathrate has been hypothesized as a cause of past global warming, because methane is a greenhouse gas. 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 to not be due to clathrate release. 
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.
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" .
Theories and criticisms
Leaving the realm of scientific journals, the debate has spilled out into the public arena, with some politicians making the issue a component of their campaigns for high office. One example of this is 2000 U.S. presidential candidate Al Gore, author of Earth in the Balance. Global warming is a more central and sustained issue, however, for the EU.
Much about global warming theories is controversial, particularly whether there exists a scientific consensus sufficient to justify concerted international action to ameliorate its effects (see Kyoto Protocol).
Proponents of global warming theory express a wide range of opinions. Some merely recognize the validity of the observed increases in temperature. Others support measures such as the Kyoto Protocol that are intended to have some near-future climate effects and to lead eventually to further measures. Others believe that the environmental damage will have such severe impact that immediate steps must be taken to reduce CO2 emissions, regardless of the economic costs to advanced nations such as the United States (which has the largest emissions of greenhouse gases of any country in absolute terms, and the second largest emissions per capita after Australia ).
Critics of the global warming theory similarly offer a wide spectrum of opinions. Some, such as Patrick Michaels, propose that human influence has warmed the atmosphere yet dispute the conclusion of the IPCC TAR, which says "[t]here is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities". Others point out that observations of global temperatures over much larger time spans, thousands of years rather than decades, show global temperatures fluctuated wildly in the past long before the introduction of human industrial activity such as the industrial revolution. An additional assertion of many critics is that it cannot be possible to ascertain any definitive global temperature trend from the limited temperature record having often been cited — the Earth is much older than that, they affirm. Other scientists theorize global temperature change may in fact be induced by natural causes, such as volcanism and solar activity.
The above paragraphs might give the impression that belief in the course of past climate change correlates strongly with advocacy for future actions: this is not necessarily so. It is possible, perhaps common, to study the past record and give no counsel on the future.
Controversial subjects are discussed further in the article Global warming controversy.
Many public policy organizations, governments, and individuals are concerned that global warming could potentially harm the environment and global agriculture.
This is a matter of considerable controversy, with environmentalist groups typically emphasizing the possible dangers and groups close to industry questioning the climate models and consequences of global warming — and funding scientists to do so. A summary of possible effects and our current understanding can be found in the report of the IPCC Working Group II. Much remains to be learned, however. 
Due to potential effects on human health and economy due to the impact on the environment, global warming is a cause of great concern. Some important environmental changes have been observed and linked to global warming.
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.
Sea level rise
Another effect of great concern to some is sea level rise. Sea levels appear to be rising 1 to 2 mm/y this century, although satellite data show a rate of 3 mm/y since 1992. Some Pacific Ocean island nations, such as Tuvalu, are concerned about the possibility of an eventual evacuation.
Main article: sea level rise
More extreme weather
As the climate grows warmer, evaporation will increase. This will cause heavier rainfall and more erosion. Many scientists think that it could result in more extreme weather as global warming progresses. The IPCC TAR says: "...global average water vapour concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected"  .
Stop of drift
Global warming might also have other, less obvious effects. The North Atlantic drift, for instance, is affected by temperature changes. It seems that it is diminishing as the climate grows warmer, and there has been speculation that areas like Scandinavia and Britain that are warmed by the drift might face a colder climate in spite of the general global warming. Some even fear that global warming may be able to trigger the type of abrupt massive temperature shifts which bracketed the Younger Dryas period. (See the discussion of chaos theory for related ideas.). However, in coupled AOGCMs the warming effects outweigh the cooling, even locally: the IPCC TAR notes that even in models where the THC weakens, there is still a warming over Europe. 
Spread of disease
Global warming will probably extend the favourable zones for vectors conveying infectious disease, such as Dengue fever, Malaria, West Nile virus, St. Louis encephalitis, or Yellow fever.
Possible positive effects
However, global warming may also have positive effects, since higher temperatures and higher CO2 concentrations may improve the ecosystems' productivity. Satellite data shows that the productivity of the Northern Hemisphere has 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. Similarly, from the human economic viewpoint, an increase in total biomass but a decrease in crop harvests would be a net disadvantage. In addition, IPCC models predict that higher CO2 concentrations would only spur growth of flora up to a point, because in many regions the limiting factors are water or nutrients, not temperature or CO2; after that, though greenhouse effects and warming would continue there would be no compensatory increase in growth.
A possible counter-argument to this is the claim that suppression of plant growth is caused by a shortage of atmospheric carbon dioxide, which is rare in comparison to oxygen (21%). This carbon dioxide starvation becomes apparent in photorespiration, where there is so little carbon dioxide, that oxygen can enter a plant's chloroplasts and takes the place where carbon dioxide normally would be in the Calvin Cycle. This causes the sugars being made to be destroyed, badly suppressing growth.
The relation between global warming and ozone depletion
Although they are often interlinked in the popular press, the connection between global warming and ozone depletion is not strong. There are three areas of linkage:
- Global warming from CO2 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 
- 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.
The relation between global warming and 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 CO2 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.
Main article: global dimming
Carbon dioxide "production"
Newspapers, magazines, and broadcasts
Bill Moyers: "Every credible scientific study in the world says human activity is creating global warming" (Grist Magazine, 2003).
Global warming-skeptical organizations
Independent (or receives too little support to constitute "sponsorship")
Possible solutions — Introduce interesting theories like "sequestering" or forcing carbon dioxide deep into the oceans
GreenFacts— A faithful summary of the Third Assessment Report of the IPCC, a leading scientific consensus document on Climate Change and Global Warming produced in 2001 by a large international panel of scientists
- http://www.lomborg.com, personal site of Bjørn Lomborg, author of The Skeptical Environmentalist
The PR Plot to Overheat the Earth, analysis of industry efforts to discredit global warming science, by Bob Burton and Sheldon Rampton, published in the Earth Island Journal.
Testimony before the Senate Environment and Public Works Committee on July 18, 2001 of Thomas R. Karl
Testimony, on 2 May 2001 of Richard S. Lindzen
BBC News summary of climate change
Sleepwalking to Extinction, by George Monbiot (Z Magazine)
Climate change (global warming) : a couple of answers to some elementary questions by Jean-Marc Jancovici
Global Warming FAQ by Tom Rees
Greenhouse gas emissions in industrialised countries: Where does Australia stand? by The Australia Institute
EnviroSpin Watch — A skeptical blog; often addresses global warming, doubts of its existence by Philip Stott