Climate change

Global Mean Temperatures 1880-2005

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) [1] [2] 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.

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; these are the senses in which the terms are used in Wikipedia. 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 [3]. Sometimes the term "anthropogenic climate change" is used to indicate the presumption of human influence.

Global warming theories attempt to assess the extent to which increases in carbon dioxide levels affect the global climate though both direct and indirect means.

If the only variable considered is the emission of greenhouse gases related to human activity, then climate models predict that temperatures will increase in the future; however, the precise magnitude of these increases is still uncertain [4], with a range of +1.4°C to +5.8°C for the temperature 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 and it is not clear if they under- or overpredict future climate change.

Since there is no method currently available to determine exactly what percentage of atmospheric carbon dioxide is anthropogenic, this comparison is of questionable value.

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.

Possible explanations for observations of global warming are discussed in attribution of recent climate change.

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. Current estimates predict that even if successfully and completely implemented, the Kyoto Accord will reduce global temperature by between 0.02°C and 0.28°C by the year 2050 - a tiny fraction of the projected increase. Because of this, critics question the value of the Kyoto Accords.

Scientific opinion

The current scientific consensus is that most of the warming observed over the last 50 years is attributable to human activities (see Attribution of recent climate change) and the extent of this consensus is discussed at scientific opinion on climate change.

Global warming is being studied by the Intergovernmental Panel on Climate Change (IPCC). The IPCC does not commission or carry out research itself, but rather disseminates the body of published research. The reports reflect the consensus of the published science. 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".

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. See scientific opinion of global warming for further discussion of this and other opinion surveys of scientists. 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.

Temperature records

2nd millenium of NH temperatures according to different articles.

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 historical 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 0.4–0.8 °C [5]. For details over the last century see the article historical temperature record; for the longer term see Temperature record of the past 1000 years; for attribution see anthropogenic climate change.

Over the past 20,000 years the dominant temperature signal has been the end of the last ice age, approximately 12,000 years ago [6]. Since then the temperature has been relatively stable, though with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age. The graph, based on Mann et al. 1999, entitled '10 centuries of NH temperature' did not show either of these anomalies. For further discussion see Temperature record of the past 1000 years.

The temperature increase has not been uniform over the globe or over time [7]. 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 [8].

One version of the satellite temperature record of the lower atmosphere for the 25 years; others show up to three times as much warming

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 [9].

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 anamolies (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, a climate modeller at the GKSS research centre in Geesthacht, Germany. "But it does not weaken in any way the hypothesis that recent observed warming is a result mainly of human activity."[10] According to D. M. Anderson and C. A. Woodhouse of the Paleoclimatology Branch, National Climatic Data Center, Boulder, Colorado, Mobergs 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 [11]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" [12].

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. [13]

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. [14] 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. [15]

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Annually averaged CO₂ at Mauna Loa since 1958
See-also [16] [17]

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 [18].

"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 [19]. The monthly measurements display small seasonal oscillations.

Solar variation theory

Various hypotheses have been proposed to link terrestrial temperature variations to solar variations. The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century [20]. The solar variations theory also has counter evidence in the form of measured global dimming which shows that the intensity of sunlight at the Earth's surface actually fell since the 1950's. To reconcile theories of an increase in solar radiation with the measurements would require other changes either in the spectrum of the sun (which has not been observed) or in the absorption profile of the atmosphere, which would probably imply some kind of climate change.

Sami Solanki, the director of the Max Planck Institute for Solar System Research in Göttingen, Germany said:

The sun has been at its strongest over the past 60 years and may now be affecting global temperatures... the brighter sun and higher levels of so-called "greenhouse gases" both contributed to the change in the Earth's temperature, but it was impossible to say which had the greater impact. [21]

Willie Soon and Sallie Baliunas of the Harvard Observatory correlated historical sunspot counts with temperature proxies. They report that when there are fewer sunspots, the earth cooled (see Maunder Minimum, Little Ice Age) — and that when there are more sunspots the earth warmed (see Medieval Warm Period, though since sunspot numbers were only counted from 1700 the link to the MWP warmth is speculative).

The theories have usually represented one of three types:

• Solar irradiance changes directly affecting the climate. This is generally considered unlikely, as the variations seem to be too small.
• Variations in the ultraviolet component having an effect. The UV component varies by more than the total.
• Effects mediated by changes in cosmic rays (which are affected by the solar wind, which is affected by the solar output) such as changes in cloud cover.

Although correlations often can be found, the mechanism behind these correlations is a matter of speculation. Many of these speculative accounts have fared badly over time, and in a paper "Solar activity and terrestrial climate: an analysis of some purported correlations" (J. Atmos. and Solar-Terr. Phy., 2003 p801–812) Peter Laut demonstrates problems with some of the most popular, notably those by Svensmark and by Lassen (below).

In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and temperature changes throughout the northern hemisphere. Initially, they used sunspot and temperature measurements from 1861 to 1989, but later found that climate records dating back four centuries supported their findings. This relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period (see graph). Sallie Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has been among the supporters of the theory that changes in the sun "can account for major climate changes on Earth for the past 300 years, including part of the recent surge of global warming." [22]

On May 6, 2000, however, New Scientist magazine reported that Lassen and astrophysicist Peter Thejll had updated Lassen's 1991 research and found that while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4 °C since 1980. "The curves diverge after 1980," Thejll said, "and it's a startlingly large deviation. Something else is acting on the climate.... It has the fingerprints of the greenhouse effect."[23]

Later that same year, Peter Stott and other researchers at the Hadley Centre in the United Kingdom published a paper in which they reported on the most comprehensive model simulations to date of the climate of the 20th century. Their study looked at both "natural forcing agents" (solar variations and volcanic emissions) as well as "anthropogenic forcing" (greenhouse gases and sulphate aerosols). They found that "solar effects may have contributed significantly to the warming in the first half of the century although this result is dependent on the reconstruction of total solar irradiance that is used. In the latter half of the century, we find that anthropogenic increases in greenhouses gases are largely responsible for the observed warming, balanced by some cooling due to anthropogenic sulphate aerosols, with no evidence for significant solar effects." Stott's team found that combining all of these factors enabled them to closely simulate global temperature changes throughout the 20th century. They predicted that continued greenhouse gas emissions would cause additional future temperature increases "at a rate similar to that observed in recent decades". It should be noted that their solar forcing included "spectrally-resolved changes in solar irradiance" and not the indirect effects mediated through cosmic rays for which there is still no accepted mechanism — these ideas are still being fleshed out.Cosmic Rays, Clouds, and Climate, K. S. Carslaw, R. G. Harrison, and J. Kirkby Science Nov 29 2002: 1732-1737 and those articles citing it In addition, the study notes "uncertainties in historical forcing" — in other words, past natural forcing may still be having a delayed warming effect, most likely due to the oceans.[24] A graphical representation of the relationship between natural and anthropogenic factors contributing to climate change appears in "Climate Change 2001: The Scientific Basis", a report by the Intergovernmental Panel on Climate Change (IPCC). [25]

Other theories

Various other hypotheses have been proposed, including but not limited to:

1. The warming is within the range of natural variation and needs no particular explanation
2. The warming is a consequence of coming out of a prior cool period — the Little Ice Age — and needs no other explanation.

Some skeptics claim that the warming trend itself has not been clearly established, and therefore does not need any explanation.

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 [26]) 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. [27]

Climate models

The most recent climate models produce a good simulation of the global temperature change over the last century. Climate simulations do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or to anthropogenic forcing (see anthropogenic global warming). Climate models currently attribute the warming occurring from approximately 1975 to 2000 to anthropogenic causes. These conclusions depend on the accuracy of the models used and on the estimation of the external factors, such as direct and indirect solar forcing.

According to the IPCC, the majority of climatologists agree that important climate processes are incorrectly accounted for by the climate models but don't think that better models would change the conclusion.

Critics point out that there are unspecified flaws in the models and unspecified external factors not taken into consideration that could change the conclusion above. Some critics say that the climate simulations are unable to fit the water vapor feedback or handle clouds. Some indirect solar effects may be very important and are not accounted for by the models (Source: The Skeptical Environmentalist). Skeptics also argue that if larger solar coupling exists than can be accounted for by direct insolation, models that don't account for this effect's role in past warming will attribute too much of that warming to greenhouse gases and will inevitably be biased to predict greater warming from future greenhouse gas increases. See also climate models.

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 [28]).

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.

Potential effects

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. [29]

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 working out the details of their expected eventual evacuation. There is significant variance of measured sea level rise depending on which land mass the rise is measured from.

According to IPCC reports, global warming causes sea level rise because sea water expands as it warms, and because portions of glaciers will melt if warmed. The IPCC TAR says: "Global mean sea level is projected to rise by 0.09 to 0.88 metres between 1990 and 2100, for the full range of SRES scenarios. This is due primarily to thermal expansion and loss of mass from glaciers and ice caps" [30]. Additionally, the introduction of freshwater into the ocean from glaciers can have an effect on the thermohaline conveyor, an oceanic circulatory system which plays a large part in regulating Earth's climate.

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" [31] [32].

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. [33]

Spread of disease

Global warming will probably extend the favourable zones for vectors conveying infectious disease, such as Malaria, Dengue fever, 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.

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. See ozone depletion for more.

• 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 [34]

• 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 phenomena 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.

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

Data

• NOAA CMDL CCGG - Interactive Atmospheric Data Visualization NOAA CO2 data
• UNEP - The present carbon cycle - Climate Change carbon levels and flows
• Temperature data Climatic Research Unit, University of East Anglia, Norwich, UK. Philip D. Jones and other.
• Hadley Centre: Global temperature data

Carbon dioxide "production"

• International Energy Annual: Reserves
• IEA Publications Bookshop - Key World Energy Statistics
• International Energy Annual 2002: Carbon Dioxide Emissions
• International Energy Annual 2002: Notes and Sources for Table H.1co2 (Metric tons of carbon dioxide can be converted to metric tons of carbon equivalent by multiplying by 12/44)
• DOE - EIA - Alternatives to Traditional Transportation Fuels 1994 - Volume 2, Greenhouse Gas Emissions (includes "Greenhouse Gas Spectral Overlaps and Their Significance")
• The Marshall Institute - Increasing Carbon Dioxide and Global Climate Change - January 1, 2000 (temp. data)
• NOAA Paleoclimatology Program - Vostok Ice Core

Scientific

• NASA's Global Hydrology and Climate Center
• NASA's Goddard Institute for Space Studies (GISS) — Global Temperature Trends.
• NASA's GISS paleoclimate site
• National Oceanic and Atmospheric Administration — NOAA, United States Department of Commerce — Global Warming FAQ
• NOAA's paleoclimate site
• MIT Joint Program on the Science and Policy of Global Change
• An Overview of Results from the Coupled Model Intercomparison Project (CMIP)
• Arctic Climate Impact Assessment
• ACIA report: Impacts of a Warming Arctic, (2004) Arctic Climate Impact Assessment (ACIA)
• Carbon Dioxide Information Analysis Centre FAQ Includes links to lots of useful Carbon Dioxide statistics.
• Some Coolness Concerning Global Warming by Richard Lindzen.

Newspapers, magazines, and broadcasts

• Hotter-burning sun warming the planet — Michael Leidig, LONDON SUNDAY TELEGRAPH
• Ode to Kyoto: The energy industry's stealth campaign to confuse the public and stop Kyoto — Broadcast January 23, 2004 on PBS' NOW with Bill Moyers (RealVideo format).

• 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.
• 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.

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.

Conservative-affiliated

• 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: [35])
• 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")

• 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

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
• 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
• theClimate.info — Global warming news aggregator.
• 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"b y Richard Muller, Technology Review , Oct. 2004; calls into question famous graph by Michael Mann
• Global warming - Greenpeace pictures in Spitsbergen
• Global Warming Consensus?

See also

• Carbon sequestration
• Carbon tax
• Climate change
• Earth's atmosphere
• global cooling
• Global dimming
• Global warming controversy — socio-political issues surrounding the theory of global warming.
• Global warming potential
• Glossary of climate change
• Greenhouse effect
• Impact of global climate changes on agriculture
• Intergovernmental Panel on Climate Change
• Kyoto Protocol
• National Assessment on Climate Change
• phenology
• Scientific opinion on climate change
• United Kingdom Climate Change Programme