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Global warming is an increase over time of the average temperature of Earth's atmosphere and oceans. The global warming theory states that the temperature has risen since the late 19th century due to human causes (anthropogenic global warming).

Use of the term "global warming" often implies a human influence. Climate change is a more neutral term which includes natural variability.

Terminology

The term global warming (GW) is associated with the following terms:

Quantitative warming event reference: "An increase over time of the average temperature of Earth's atmosphere and oceans." This usage refers to any increases in average atmospheric temperature which have occurred, regardless of time or cause.
Recent temperature changes: References to the world's average temperature increase since the beginning of the Industrial Revolution
- Recent century-scale change: In this sense, the term is often used to describe only the temperature rise over the past century, since 1901 or an earlier start of temperature records.
- Warming during presence of fossil fuels: References only to the half of that rise since 1945, the period during which most fossil fuels have been burned.
- Recent rate of change: Some references examine the recent rate of change and begin around 1975, when a cooling period ended.

Related terms:
- Drastic change inclusion: The term also is used to refer to a collection of crises that many say will arise due to warming.
- Global warming theory (GWT): Possible explanations for observations of global warming
- Climate change: A statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use.[1]

In the 1970s it was unclear whether warming or cooling were more likely in the near future (next 100 years). By the late 1980s and early 1990s, the prospect that the earth's surface might become dangerously overheated captured public attention, and it has been a vigorously debated topic.

For attribution of change, see Anthropogenic global warming.

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, such as Al Gore, author of Earth in the Balance.

Much about global warming theories is controversial, particularly whether there exists a scientific consensus sufficient to justify radical action to ameliorate its effects (see Kyoto Protocol).

Proponents of global warming theory (GWT) express a wide range of opinions. Some merely "believe in" the observed increase in temperature. Others support measures such as the Kyoto Protocol, intended to have minor climate effects and lead 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 [2]). 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 there is "There 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.

Controversial subjects are discussed further in the article Global warming controversy.

Temperature records

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 oC [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.

The current conflict results in professional disagreements as well as pressuring political forces. Presently this is particularly visible in various interpretations in topics such as the UNFCCC Kyoto Protocol (see also global warming controversy).

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.

Scientific opinion

"It is not possible to attribute all, or even a large part, of the observed global-mean warming to the enhanced greenhouse effect on the basis of observational data currently available."

The above statement is from the 1990 IPCC report, but "buried" on an inner page. [5]

A survey in 1996 by Dennis Bray and Hans von Storch of the Meteorologisches Institut der Universitat Hamburg and Hans von Storch of GKSS Forschungszentrum generated responses from over 400 German, American and Canadian climate researchers and was reported in the United Nations Climate Change Bulletin.

"We can say for certain that global warming is a process already underway."

Ten percent (10%) of the researchers surveyed "strongly agreed" with the statement.
Thirty-five percent(35%) disagreed with the statement or were undecided. [6]

The survey showed a tendency of some scientists in this field agree it is "certain that, without change in human behavior, global warming will definitely occur sometime in the future". Scientists gave this statement an average score of 2.6 on a scale from 1 to 7, where 1 indicated complete agreement and 7 indicated complete disagreement.

"The balance of evidence suggests a discernible human influence on global climate"

This is the politically negotiated statement of the IPCC in 1990. The extent to which scientists contributing to the report agree with this statement is disputed.

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

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.

Most climatologists accept that the earth has warmed recently. Somewhat more controversial is what may have caused this change. See Anthropogenic Climate Change for further discussion of "attribution" of change.

Greenhouse gas theory

The hypothesis that increases or decreases in greenhouse gas concentration would lead to higher or lower global mean temperature was first postulated in the late 19th century by Swedish chemist and 1903 Nobel Laureate Svante Arrhenius, largely as an attempt to explain ice ages. At the time his peers largely rejected his theory.

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 was established to assess the risk of human-induced climate change, and 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 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.

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. Bottom line, clouds have competing effects on the climate. If CO₂ changes the amount of distribution of clouds, it could have various complex effects on the climate.

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.

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 Nino. It is universally found that climate models that pass these tests always predict that the net effect of adding CO₂ will be a warmer climate in the future, when all the water vapor and cloud changes are taken into account. The amount of predicted warming varies by model, however, which probably reflects the way different models depict clouds differently. Skeptics of "global warming" say that the models are incorrect, but have been unable to produce a model of the climate that does not predict temperatures will increase in the future. Thus, the skeptics' theory that climate feedbacks will eliminate any CO₂ warming effect is not supported by either the observations or any credible model.

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

Coal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the 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 exists, from ice cores. From less direct geological evidence it is believed that 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]. They are called greenhouse gases because they trap radiant energy from the sun that would otherwise be re-radiated back into space. (The fact that a natural greenhouse effect occurs is well-known and 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 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 [11].

Sami Solanki, the director of the Max Planck Institute for Solar System Research in Gottingen, 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 pact. [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 has always cooled (see Maunder Minumum, Little Ice Age) -- and that when there are more sunspots the earth has always warmed (see Medieval Warm Period).

The theories have usually been 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.

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.

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). Like Lassen and Thejll, they found that the natural factors accounted for gradual warming to about 1960 followed by a return to late 19th-century temperatures, consistent with the gradual change in solar forcing throughout the 20th century and volcanic activity during the past few decades. These factors alone, however, could not account for the warming in recent decades. Similarly, anthropogenic forcing alone was insufficient to explain the 1910-1945 warming, but was necessary to simulate the warming since 1976. 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."[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]

Others

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

The current increase in temperature is predicted by the Milankovitch cycles theory, in which gradual changes in the Earth's orbit around the Sun and changes in the Earth's axial tilt affect the amount of solar energy reaching the Earth.
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.

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

Jurassic global warming

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 weathering of rocks to be speeded up by 400%, a process that took around 150,000 years to return carbon dioxide levels to normal.

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). 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)

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.

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.

The examples of secondary evidence cited above (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 cause of great concern is sea level rise. Sea levels are rising 1 to 2 centimetres (around half an inch) per decade, and some small countries in the Pacific Ocean are expressing concerns that if this rise in sea level continues, they soon will be entirely under water. Global warming causes the sea level to rise mainly because sea water expands as it warms, but some scientists are concerned that in the future, the polar ice caps and glaciers may melt. 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" [18]. Some researchers have found a negative correlation between sea level rise and average global temperature; water evaporates more quickly than it expands. (Source: Science and Environmental Policy Project)

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: "...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" [19].

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, 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 small 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 Intergovernmental Panel 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 Kyoto Protocol

The Kyoto Protocol proposes binding greenhouse gas limits for developed countries.

External links

Every source has a point of view or a sponsor that might be a source of bias. If you discover evidence for bias or a major source of its funding, please include it in the site's description.

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).

Educational

Discovery of Global Warming, an extensive introduction to the topic

Editor affiliated with the Libertarian Cato Institute
Liberal group PR Watch says, "Steven Milloy's website is actually a good example of junk science itself, heaping adolescent insults on any and all scientists (ranging from Samuel Epstein to the New England Journal of Medicine) who fail to defend the corporate, anti-environmentalist worldview." (Source: [20])

http://www.sepp.org/ -- the Science and Environmental Policy Project (SEPP)

SEPP was founded with funds by the Rev. Sun Myung Moon, the leader of the Unification Church It is currently funded by various conservative foundations/individuals, including Bradley Foundation, Smith Richardson, and Forbes.

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: [21] )
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 Bjorn 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 of Thomas R. Karl before the Senate Environment and Public Works Committee on July 18, 2001 -- Thomas R. Karl is director of the US National Climatic Data Center, National Environmental Satellite Data and Information Services, and National Oceanic and Atmospheric Administration
Testimony of Richard S. Lindzen, on 2 May 2001 -- Lindzen is a professor at MIT
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

http://www.globalwarming.org - "a project of the Cooler Heads Coalition, a sub-group of the National Consumer Coalition".
http://www.istl.org/01-fall/internet.html - Extensive commented list of Internet resources - Science and Technology Sources on the Internet
Carbon Activism for Beginners
http://www.uoguelph.ca/~rmckitri/cc.html - Ross McKitrick's commented list of resources - Competing views on global warming
http://www.theclimate.info
http://www.brighton73.freeserve.co.uk/gw/globalwarmingfaq.htm - An excellent FAQ
http://www.pistehors.com/articles/global-warming.htm - Global Warming and the Effects on Skiing
[Institute for creation research] information about Global Warming]

@@ Line 257: / Line 257: @@
 *http://www.globalwarming.org - "a project of the Cooler Heads Coalition, a sub-group of the National Consumer Coalition".
 *http://www.istl.org/01-fall/internet.html - Extensive commented list of Internet resources - Science and Technology Sources on the Internet
+*[http://www.activistmagazine.com/index.php?option=content&task=view&id=120  Carbon Activism for Beginners]
 *http://www.uoguelph.ca/~rmckitri/cc.html - Ross McKitrick's commented list of resources - Competing views on global warming
 *http://www.theclimate.info
@@ Line 262: / Line 263: @@
 *http://www.pistehors.com/articles/global-warming.htm - Global Warming and the Effects on Skiing
 *[http://icr.org/cgi-bin/search/search.cgi?Realm=Entire+ICR+Website&Terms=Global+warming [Institute for creation research]] information about Global Warming]
 ==See also==