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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 (CO₂) and to possible increases in solar activity.

Climate models, driven 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. 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.

Although the discussion of global warming often focuses on temperature, global warming or any climate change may cause changes in other things as well, including the sea level, precipitation, weather patterns, etc. These may impact on human activity via floods, droughts, heat waves, declining agricultural yields, etc.

Terminology

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.

Scientific opinion

Main article: scientific opinion on climate change

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". Naomi Oreskes 2004 survey of scientific literature concluded

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.

In 2004 Naomi Oreskes published a survey of 928 peer-reviewed scientific abstracts on climate change, concluding that there is a scientific consensus on the reality of anthropogenic climate change Beyond the Ivory Tower: The Scientific Consensus on Climate Change .

Temperature records

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:

General discussion of Temperature records see: Temperature record
Record of past century see: Instrumental temperature record
Record of recent millenia see: Temperature record of the past 1000 years
For attribution see: Anthropogenic climate change

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 attribute terrestrial temperature variations to variations in solar output.

In the IPCC 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 .

However, a number of studies have suggested that additional solar variation feedbacks may exist which have not been incorporated in the present models or that the relative importance of solar variation may be underestimated . Such claims are disputed (e.g. ) but form an active area of current research. The outcome of this debate may play a key role in determining how much climate change is attributed to human vs. natural factors.

Main article: Solar variation theory

Other theories

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 some 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 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 clathrate (a greenhouse gas) 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 to not be due to clathrate release.

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 CO₂ 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 CO₂ concentrations comparable to doubling preindustrial levels, only results in about 0.75 °C warming rather than the usual 1.5-4.5 °C found by climate models . In turn the Vezier's recent work has been criticised .

Climate models

Main article: General circulation model

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 "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.

Potential effects

Many public policy organizations, governments, and individuals are concerned that global warming could harm the environment in various ways. As well as the intrinsic value attached by them to the preservation of the environment, there are wide range of possible consequences for humans. These include rising sea levels, declining output of global agriculture, increased extreme weather, and the spread of disease.

The extent and likelihood of these consequences 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.

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.

Sea level rise

Main article: sea level rise

Another effect of great concern is a 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. For historical reasons to do with trade, many of the world's largest and most prosperous cities are on the coast, and the cost of building better sea defences is likely to be considerable. Some countries will be more affected than others - low-lying countries such as Bangladesh and the Netherlands will be worst hit by any sea level rise, in terms of floods or the cost of preventing them.

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" .

Decline of agriculture

Main article: Global warming and agriculture

For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of CO2 in photosynthesis. This may still be true in some regions (such as Siberia), but recent evidence is that global yields will be negatively affected. "Rising atmospheric temperatures, longer droughts and side-effects of both, such as higher levels of ground-level ozone gas, are likely to bring about a substantial reduction in crop yields in the coming decades, large-scale experiments have shown."

Spread of disease

It has been claimed that global warming will probably extend the favourable zones for vectors conveying infectious disease such as malaria. However, it has been pointed out that despite the disappearance of infectious disease 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 variables are of equal or greater importance.

Destabilisation of local climates

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.

Reducing ozone layer

One of the lesser-known effects of global warming is a reduction in the thickness of the ozone layer, which protects life on Earth from harmful radiation. Global warming appears to be partially countering the improvements in the ozone layer caused by the reduction in the use of ozone-destroying chemicals achieved through the Montreal Protocol. "What appears to have caused the further loss of ozone is the increasing number of stratospheric clouds in the winter, 15 miles above the earth. These clouds, in the middle of the ozone layer, provide a platform which makes it easier for rapid chemical reactions which destroy ozone to take place." A reduced ozone layer has negative impacts on human health (notably skin cancer and eye problems such as cataracts) and on ecosystems. The ecological impact may have further knock-on effects, as it reduces photosynthesis in plants (with potential impacts on agriculture) and damages the DNA of plankton, which play a significant role in the world's carbon cycle.

Possible positive effects

However, global warming may also have positive effects, since higher temperatures and higher CO₂ concentrations may improve 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

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 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 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 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.

Attempts to combat global warming

Mitigating temperature rise

Mitigating the effects of global warming

References

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

External links

Bill Moyers: "Every credible scientific study in the world says human activity is creating global warming" (Grist Magazine, 2003).

Global Warming Media Analysis — US Election 2004 Web Monitor
Dispute of the "Hockey Stick" extrapolation — MIT Technology Review
Major climage change 5,200 years ago due to solar fluctuations — Space News
Earth's permafrost starts to squelch — BBC (29 December 2004)
Climate Change - News and information on climate change, alternative energy, global warming and energy efficiency
Rebuttal of the McIntyre and McKitrick dispute of the "Hockey Stick" extrapolation — Real Climate
Does CO₂ really drive global warming? by Robert Essenhigh.

Educational

New Scientist: Climate Change — Latest news, articles and FAQ on climate change from New Scientist magazine.
Open Democracy: Climate Change debate — substantial article series by scientists, activists and others, and global debate about the politics of climate change.
Discovery of Global Warming — An extensive introduction to the topic and the history of its discovery
Science of Climate Change — Meteorological Service of Canada, a guide to understanding the science behind climate change with a thorough FAQ section.
Global warming - a briefing document, summarising current arguments concerning global warming. They present various points of view and some of the basic problems in following the science.
http://www.msc-smc.ec.gc.ca/cd/brochures/humidex_table_e.cfm

 Humidex Table - Brochures - Meteorological Service of Canada - The Green Lane

United Nations

http://www.ipcc.ch — the Intergovernmental Panel on Climate Change (IPCC), established by WMO and UNEP (below) in 1988
http://www.wmo.ch — the World Meteorological Organization (WMO)
http://www.unep.org/ — the United Nations Environment Programme
http://unfccc.int/ — the United Nations Framework Convention for Climate Change (UNFCCC)
IPCC report: summary for policy makers, (2001) (pdf file)
IPCC report: Climate Change 2001: The Scientific Basis — Technical Summaries (pdf file)

Environmentalist

http://www.greenpeace.org/ — Greenpeace
http://www.panda.org/climate/ — the Worldwide Wildlife Fund (WWF)
http://www.worldwatch.org/about — Worldwatch Institute
http://www.greens.org/s-r/24/24-26.html John Hickman and Sarah Bartlett explain the "Global Tragedy of the Commons at COP 6" in the Green Party journal Synthesis/Regeneration.
"Global warming is real and underway" — Union of Concerned Scientists (UCS)
National Resources Defense Council (NRDC)
Climate Alliance of European Cities
http://www.undoit.org Global Warming: Undo It (Environmental Defense)
BBC: 2004 Climate Change Feature — Major feature from the BBC including climate change animations, articles, Q&A, video, analysis, charts and quizes.
Global Warming — National Wildlife Federation Includes maps of impact on birds and Senate voting records.

Global warming-skeptical organizations

Junk Science Website
http://www.sepp.org/ — the Science and Environmental Policy Project (SEPP)
The George Marshall Institute (GMI)

Industry-sponsored

— Carbon mitigation initiative — Information on carbon capturing, storage, science and policy
http://www.cei.org — the Competitive Enterprise Institute. Funded by petrol oil corporations (Source: )
Access to Energy
CO₂ science magazine
The Greening Earth Society
Global Warming Information Page sponsored by the Cooler Heads Coalition, a subgroup of the National Consumer Coalition.
The Global Climate Coalition

Independent (or receives too little support to constitute "sponsorship")

David Suzuki Foundation— : Impacts and Solutions,on Climate Change.
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 18 July 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

Other

Global Warming FAQ — A comprehensive FAQ.
A large compendium of links to sites with information on global warming
Ross McKitrick's commented list of resources on global warming
What is the ‘Hockey Stick’ Debate About? McKitrick reviews the criticisms of the Mann hockey stick for a general scientific audience.
Science and Technology Librarianship: Global Warming and Climate Change Science — Extensive commented list of Internet resources — Science and Technology Sources on the Internet.
John-Daly.com: Still Waiting for Greenhouse: A Lukewarm View of Global Warming
Activist Magazine: Carbon Activism for Beginners
Global Warming and the Effects on Skiing
Global warming - briefing document — Summary of arguments concerning global warming. They present various points of view and some of the basic problems in following the science.
ICR information against Global Warming — Institute for Creation Research information about global warming.
"What If All the Ice Melts?" Myths and Realities — includes a depiction of what the Earth would look like if all the icecaps melted.
Environmental Defence - Global Warming Skeptics: A Primer — How ExxonMobil funds scientists and media pundits to "de-bunk" the science of climate change
a contrary link:
"A Global Warming Bombshell" by Richard A. Muller, Technology Review , Oct. 2004; calls into question famous graph by Michael Mann
Global warming - Greenpeace pictures in Spitsbergen
Global Warming Consensus?