Global warming is an increase over time of the average temperature of Earth's atmosphere and oceans. Global warming theories attempt to account for the documented rise in average global temperatures since the late 19th century (0.6 ± 0.2°C) [1] and assess the extent to which the effects are due to human causes, principally emissions of carbon dioxide (CO2) increasing the "greenhouse effect." All climate models currently predict that if the only variable considered is human emission of greenhouse gases, then this will cause temperatures to increase in the future; however the precise magnitude of these increases is still uncertain.

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; these are the senses in which the terms are used in wikipedia. Note, however, that there is one important exception to this: the UNFCCC defines "climate change" as anthropogenic [2]. Sometimes the term "anthropogenic climate change" is used to indicate the presumption of human influence. The Kyoto Protocol proposes binding greenhouse gas limits for developed countries.

Note that although the discussion often focusses on temperature, global warming or any climate change implies changes in other variables: 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.

Scientific opinion Edit

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.

Since it is such an important issue, governments need predictions of future trends in global change so they can take political decisions to avoid undesired impacts. 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 Universitat 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 recordsEdit

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 20,000 years the dominant temperature signal has been the end of the last ice age, approximately 12,000 years ago [3]. Since then the temperature has been quite stable, though with various fluctuations, e.g. Medieval Warm Period or Little Ice Age. Over the past century or so the global (land + sea) temperature has increased by approximately 0.4–0.8 °C [4]. 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.

File:Lower troposphere trend 2004.png

Temperatures in the lower troposphere have increased at somewhere between 0.08 and 0.22 °/decade since 1979 (see Satellite temperature measurements). Just like the surface record, the average temperature rise is not linear, but has superimposed on it rises and falls due to natural variability, most notably El nino's. Over the same period the surface record shows a warming of approximately 0.15 °C/decade [5].

Theories to explain temperature changeEdit

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 theoryEdit

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, however, this was based on coal consumption at the end of the 19th century. [6]

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 an important amount of water vapor (humidity, clouds) in the Earth's atmosphere, 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. But many people have also noticed that clear winter nights tend to be colder than cloudy winter nights. That is because clouds also radiate heat 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 is 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 CO2 warming, unless other environmental factors have contributed to that 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 represent an indirect effect of changes in solar activity that increase the warming response to increases in solar activity. Climate models that pass the above tests while modeling the only 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 which 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; although the uncertainty could just as easily be in the other direction. Skeptics have been unable to produce a model of the climate that does not predict that temperatures will increase in the future. Thus, the skeptics' theory that climate feedbacks will eliminate any CO2 warming effect has not been substantiated by either observations or modeling.

CO2 increases at Mauna Loa since 1958.
CO2 at Mauna Loa since 1958
See-also [7] [8]

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 [9].

"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: 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.

Increases in CO2 measured since 1958 at Mauna Loa show a monotonically increasing atmospheric concentration of CO2. In fact, it is clear that the increase is faster than linear. On March 21, 2004, it was reported that the concentration in ppm reached 376ppm in 2003. South Pole records show similar growth [10].

Solar variation theoryEdit

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 [11].

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.... He continued: 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. [12]

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 at best 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 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." [13]

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."[14]

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", i.e., past natural forcing may still be having a delayed warming effect, most likely due to the oceans.[15] 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). [16]


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 would claim that the warming trend itself is not valid, and therefore does not need any explanation.

Jurassic global warmingEdit

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 [17]) 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. See also the Paleocene-Eocene Thermal Maximum for another episode of global warming.

Climate modelsEdit

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). All models show that the warming occurring from approximately 1975 to 2000 is largely anthropogenic. These conclusions depend on the accuracy of the models used and on the correct estimation of the external factors.

The majority of scientists agree that important climate processes are incorrectly accounted for by the climate models but don't think that better models would change the conclusion. (Source: IPCC)

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 unidentified critics say that the climate simulations are unable to fit the water vapor feedback, and handle clouds. Some indirect solar effects may be very important and are not accounted for by the models. Or then again, they might not be important at all. (Source: The Skeptical Environmentalist)

See also: climate models

Theories and criticisms Edit

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, such as 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 (GWT) 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 (the United States has the largest emissions of greenhouse gases of any country in absolute terms, and the second largest per capita emissions after Australia [18]). There are no known climatologists supporting this viewpoint.

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 conclude observations of global temperatures over much larger time spans, spans of thousands of years rather than decades – epochs rather than centuries, 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 effectsEdit

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.

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.

Examples of secondary evidence of global warming— lessened snow cover, rising sea levels, weather changes— are 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 spread. Few of the terrestrial ecoregions on Earth could expect to be unaffected.

Another effect of great concern to some is sea level rise. Sea levels are rising 1 to 2 centimetres (around half an inch) per decade, and some Pacific Ocean island nations, such as Tuvalu are working out the details of their expected eventual evacuation.

Global warming causes the sea level to rise because sea water expands as it warms and through thinning of the polar ice caps and sea ice and reduction of glaciers. 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" [19].

As the climate gets hotter, evaporation will increase. This will cause heavier rainfall and more erosion. Many people think that it could result in more extreme weather as global warming progresses. The IPCC TAR says: " 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" [20].

Global warming can also have other, less obvious effects. The North Atlantic drift, for instance, is driven by temperature changes. It seems that it is diminishing as the climate grows warmer, and this means that areas like Scandinavia and Britain that are warmed by the drift might face a colder climate in spite of the general global warming. It is now feared 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.)

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

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 Edit

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 [21]
  • 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.


External linksEdit


Newspapers, magazines, and broadcastsEdit

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


United NationsEdit




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


See alsoEdit

ca:Escalfament global da:Global opvarmning de:Globale Erwärmung et:Globaalne soojenemine es:Calentamiento global fr:Réchauffement global he:התחממות גלובלית ja:地球温暖化 pt:Aquecimento global zh:全球变暖

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