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Global warming describes an increase in the average temperature of the Earth's atmosphere and oceans. The terms global warming or anthropogenic global warming are also used to describe the theory that increasing temperatures are the result of a strengthening greenhouse effect caused primarily by man-made increases in carbon dioxide and other greenhouse gases.

The scientific opinion on climate change, as expressed by the UN Intergovernmental Panel on Climate Change (IPCC) and explicitly endorsed by the national science academies of the G8 nations, is that the average global temperature has risen 0.6 ± 0.2 °C since the late 19th century, and that "most of the warming observed over the last 50 years is attributable to human activities", most prominently the emission of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄). A small minority of qualified scientists contest the view that humanity's actions have played a significant role in increasing recent temperatures. Uncertainties do exist regarding how much climate change should be expected in the future, and a hotly contested political and public debate exists over what actions, if any, should be taken in light of global warming.

Based on the climate models referenced by the IPCC, temperatures may increase by 1.4 to 5.8 °C between 1990 and 2100 [1]. This is expected to result in other climate changes including rises in sea level and changes in the amount and pattern of precipitation. Such changes may increase extreme weather events such as floods, droughts, heat waves, and hurricanes, change agricultural yields, or contribute to biological extinctions. Although warming is expected to affect the frequency and magnitude of these events, it is very difficult to connect any particular event to global warming.

Overview

The scientific consensus on global warming is that the Earth is warming, and that humanity's greenhouse gas emissions are making a significant contribution. This consensus is summarized by the findings of the Intergovernmental Panel on Climate Change (IPCC). In the Third Assessment Report, the IPCC concluded that "most of the warming observed over the last 50 years is attributable to human activities". This position was recently supported by an international group of science academies from the G8 countries and Brazil, China and India [2].

Over the past century or so the global (land and sea) temperature has increased by 0.6 ± 0.2 °C [3]. The effects of global warming are increasingly visible. At the same time, atmospheric carbon dioxide has increased from around 280 parts per million (by volume) in 1800 to around 315 in 1958 and 367 in 2000, a 31% increase over 200 years. Other greenhouse gas emissions have also increased. Future CO₂ levels cannot be predicted with any precision, since they depend on uncertain economic, sociological and technological developments. The IPCC SRES gives a wide range of future CO₂ scenarios [4], ranging from about 0.04 to 0.1 % by volume by 2100.

Climate models, driven by estimates of increasing carbon dioxide and to a lesser extent by generally decreasing sulphate aerosols, predict that temperatures will increase (with a range of 1.4 to 5.8 °C for change between 1990 and 2100 [5]). Much of this uncertainty results from not knowing future CO₂ emissions, but there is also uncertainty about the accuracy of climate models. Climate commitment studies predict that even if levels of greenhouse gases and solar activity were to remain constant, the global climate is committed to 0.5 °C of warming (some model results are as high as 1.0 °C) over the next one hundred years due to the lag in warming caused by the oceans.

Although the combination of scientific consensus and economic incentives (especially for Russia) were enough to persuade the governments of more than 150 countries to ratify the Kyoto Protocol - there are issues about just how much greenhouse gas emissions warm the planet. Uncertainties remain and have been emphasized by some politicians, and others questioning the costs needed to reduce future global warming; however, the business position on climate change is increasingly changing to accept global warming as both real and anthropogenic, and that action such as carbon emissions trading and carbon taxes is needed. The scientific consensus is questioned by a small minority of scientists.

Warming of the Earth

Over the past century or so the global (land and sea) temperature has increased by 0.6 ± 0.2°C [6]. Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C per decade since 1979. Over the past 1-2 thousand years before 1850 the temperature has been relatively stable, with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age.

The period of time over which one is interested in change may vary according to the focus of the user of the term and the datasets available for investigation. Temperature record holds a discussion of the various records. An approximately global instrumental temperature record begins in about 1860; contamination from the urban heat island is believed to be small. A longer-term perspective is available from various proxy records for recent millenia; see Temperature record of the past 1000 years for a discussion of these records and their differences. Attribution of recent climate change is clearest for the most recent period (the last 50 years) for which the most detailed data is available. Satellite temperature measurements of the tropospheric temperature date from 1979.

Causes of global warming

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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.

Atmospheric scientists know that adding carbon dioxide (CO₂) or methane (CH₄) to an atmosphere, with no other changes, will tend to make a planet's surface warmer. Indeed, greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be 30°C lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the theory that adding CO₂ or CH₄ 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 and CH₄ will be, and whether changes in water vapor, clouds, the biosphere and various other climate factors 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.

Greenhouse gas emissions

Coal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the human environment contribute about 22 billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. Animal agriculture, manure, natural gas, rice paddies, landfills, coal, and other sources contribute about 250 million tons of methane each year. About half of human emissions have remained in the atmosphere. The atmospheric concentrations of CO₂ and CH₄ have increased by 31% and 149% respectively 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 [7].

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 ppmv (see the Keeling Curve). The concentration reached 376 ppmv in 2003. South Pole records show similar growth [8]. The monthly measurements display small seasonal oscillations.

Alternative theories

Solar variation theory

Various hypotheses have been proposed to attribute terrestrial temperature variations to variations in solar output.

In the IPCC TAR, it was reported that volcanic and solar forcings might account for half of the temperature variations prior to 1950, but that the net effect of such natural forcings was roughly neutral since then [9]. In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be 8 times larger than the change in forcing due to increasing solar activity over the same period [10].

Work published since the IPCC TAR, has revised the assessment of solar contribution to the post 1950 warming. "The best estimate of the warming from solar forcing is estimated to be 16% or 36% of greenhouse warming depending on the solar reconstruction."."Do Models Underestimate the Solar Contribution to Recent Climate Change" Peter A. Stott, et al, Journal of Climate, 15 DECEMBER 2003

However, a number of studies have suggested that additional solar variation feedbacks may exist which have not been incorporated in the present models or that the relative importance of solar variation may be underestimated [11] [12]. Such claims are disputed (e.g. [13] [14]) but form an active area of current research. The outcome of this debate may play a key role in determining how much climate change is attributed to human vs. natural factors.

Other theories

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

The warming is within the range of natural variation and needs no particular explanation.
The warming is a consequence of coming out of a prior cool period — the Little Ice Age — and needs no other explanation.
The warming trend itself has not been clearly established, and therefore does not need any explanation.

At present, none of these has more than a small number of supporters within the climate science community.

Climate models

Scientists have 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 greenhouse gases 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.

As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4°C to 5.8°C between 1990 and 2100 [15]. They have also been used to help determine the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors.

The most recent climate models can produce a good match to observations of global temperature changes over the last century. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions. Adding simulation of the ability of the environment to sink carbon dioxide suggested that rising fossil fuel emissions would decrease absorption from the atmosphere, amplifying climate warming beyond previous predictions, although "Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses" [16].

Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release the potent greenhouse gas, methane, that is trapped in large quantities in permafrost and ice clathrates [17].

Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds [18]. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considers climate models "to be suitable tools to provide useful projections of future climates" [19].

Issues

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 four areas of linkage:

Global warming from CO₂ radiative forcing is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes.

Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposed effects: reduced ozone allows more solar radiation to penetrate, thus warming the troposphere. But a colder stratosphere emits less long-wave radiation, tending to cool the troposphere. Overall, the cooling dominates: the IPCC concludes that observed stratospheric O₃ losses over the past two decades have caused a negative forcing of the surface-troposphere system [20] of about –0.15 ± 0.10 W m–2 [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.

Ozone depleting chemicals are also greenhouse gases, representing 0.34 ± 0.03 W/m2, or about 14% of the total radiative forcing from well-mixed GHG's [22].

The relation between global warming and global dimming

Some scientists now consider that the effects of the recently recognized phenomenon of global dimming (the reduction in sunlight reaching the surface of the planet, possibly due to aerosols) may have masked some of the effect of global warming. If this is so, the indirect aerosol effect is stronger than previously believed, which would imply that the climate sensitivity to greenhouse gases 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.

Pre-human global warming

It is thought by some geologists that the Earth experienced global warming in the early Jurassic period, with average temperatures rising by 5 °C (9 °F). Research by the Open University published in Geology (32: 157–160, 2004 [23]) indicates that this caused the rate of rock weathering to increase by 400%. As a result of this, carbon dioxide levels dropped back to normal over roughly the next 150,000 years.

Sudden release of methane clathrate (a greenhouse gas) has been hypothesized as a cause of past global warming. Two events possibly linked in this way are the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last ice age is thought to not be due to clathrate release [24].

The greenhouse effect has also been invoked to explain how the Earth made it out of the Snowball Earth period. During this period all silicate rocks were covered by ice, thereby preventing them to combine with atmospheric carbon dioxide. The atmospheric carbon dioxide level gradually increased until it reached about 350 times current levels. At this point temperatures were raised to an average of 50 °C, hot enough to melt the ice. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere. Thick layers of abiotic carbonate sediment which can be found on top the glacial rocks from this period are believed to be formed by this rapid carbon dioxide removal process.

Using paleoclimate data for the last 500 million years (Veizer et al. 2000, Nature 408, pp. 698-701) concluded that long-term temperature variations are only weakly coupled to CO₂ variations. Shaviv and Veizer (2003, [25]) extended this by arguing that the biggest long-term influence on temperature is actually the solar system's motion around the galaxy. Afterwards, they argued that over geologic time a change in CO₂ concentrations comparable to doubling preindustrial levels, only results in about 0.75 °C warming rather than the usual 1.5-4.5 °C reported by climate models [26]. In turn Veizer's recent work has been discussed and criticised on RealClimate.org [27].

Leading palaeoclimatologist William Ruddiman has argued (eg Scientific American, March 2005) that human influence on the global climate began around 8000 years ago with the development of agriculture. This prevented CO₂ (and later methane) levels falling as rapidly as they would have done otherwise. Ruddiman argues that without this effect, the Earth would be entering, or already have entered, a new ice age. However other work in this area (Nature 2004) argues that the present interglacial is most analogous to the interglacial 400,000 years ago that lasted approximately 28,000 years, in which case there is no need to invoke the spread of agriculture for having delayed the next ice age.

Public controversy

Leaving the realm of scientific journals, the debate has spilled out into the public arena. In the US, some politicians have made the issue a component of their campaigns for high office. Global warming is a more central and sustained issue, however, for the EU.

Effects

The predicted effects of global warming are many and various, both for the environment and for human life. The primary effect of global warming is increasing carbon dioxide and increasing global average temperature. From this flow a variety of secondary effects, including sea level rise, impacts on agriculture, reductions in the ozone layer (see below), increased extreme weather, and the spread of disease. In some cases, the effects may already be being experienced, although it is generally difficult to attribute specific natural phenomena to long-term global warming.

The extent and likelihood of these consequences is a matter of considerable controversy. A summary of possible effects and our current understanding can be found in the report of the IPCC Working Group II [28].

Effects on ecosystems

Secondary evidence of global warming — lessened snow cover, rising sea levels, weather changes — provides examples of consequences of global warming that may influence not only human activities but also the ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may flourish. Few of the terrestrial ecoregions on Earth could expect to be unaffected.

Destabilisation of ocean currents

Main article: Destabilisation of ocean currents

Environmental refugees

Even a relatively small rise in sea level would make some densely settled coastal plains uninhabitable and create a significant refugee problem. If the sea level were to rise in excess of 4 metres almost every coastal city in the world would be severely affected, with the potential for major impacts on world-wide trade and economy. Presently, the IPCC predicts sea level rise of less than 1 meter through 2100, but they also warn that global warming during that time may lead to irreversible changes in the Earth's glacial system and ultimately melt enough ice to raise sea level many meters over the next millenia. It is estimated that around 200 million people could be affected by sea level rise, especially in Vietnam, Bangladesh, China, India, Thailand, Philippines, Indonesia and Egypt. [29] [30] [31]

Spread of disease

It has been claimed that global warming will probably extend the favourable zones for vectors conveying infectious diseases such as malaria. An example of this may be the recent extension to the north Mediterranean region of bluetongue disease in domesticated ruminants associated with mite bites. Another is the increase of hantavirus infection, Crimean-Congo hemorrhagic fever, tularemia and rabies in wide areas of Russia during 2004–2005 associated with a population explosion of rodents and their predators. Some of this, however is blamed on breakdowns in governmental vaccination and rodent control programs.[32] Similarly, despite the disappearance of malaria in most temperate regions, the indigenous mosquitoes that transmitted it were never eliminated and remain common in some areas. Thus, although temperature is important in the transmission dynamics of malaria, many other factors are influential [33].

Financial effects

Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warn in a joint study (summary) that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost 150 billion US dollars each year in the next decade. These costs would, through increased costs related to insurance and disaster relief, burden customers, tax payers, and industry alike.

Possible positive effects

Global warming may also have positive effects, since higher temperatures and higher CO₂ concentrations may improve ecosystems' productivity. Satellite data shows that the productivity of the Northern Hemisphere has increased since 1982. On the other hand, an increase in the total amount of biomass produced is not necessarily all good, since biodiversity can still decrease even though a smaller number of species are flourishing. Similarly, from the human economic viewpoint, an increase in total biomass but a decrease in crop harvests would be a net disadvantage. In addition, IPCC models predict that higher CO₂ concentrations would only spur growth of flora up to a point, because in many regions the limiting factors are water or nutrients, not temperature or CO₂; after that, though greenhouse effects and warming would continue there would be no compensatory increase in growth.

A possible counter-argument to this is the claim that suppression of plant growth is caused by a shortage of atmospheric carbon dioxide, which is rare in comparison to oxygen (21%). This carbon dioxide starvation becomes apparent in photorespiration, where there is so little carbon dioxide, that oxygen can enter a plant's chloroplasts and takes the place where carbon dioxide normally would be in the Calvin Cycle. This causes the sugars being made to be destroyed, badly suppressing growth.

Melting Arctic ice may open the Northwest Passage in summer, which would cut 5,000 nautical miles from shipping routes between Europe and Asia. This would be of particular relevance for supertankers which are too big to fit through the Panama Canal and currently have to go around the tip of South America. According the Canadian Ice Service, the amount of ice in Canada's eastern Arctic Archipelago decreased by 15 percent between 1969 and 2004 [34].

Mitigating and adapting to global warming

"Mitigation of global warming" covers all actions aimed at reducing the extent or likelihood of global warming. The world's primary international agreement on combating climate change is the Kyoto Protocol. Various other strategies include development of new technologies, renewable energy, biodiesel, electric cars (and hybrids), and fuel cells, Energy conservation, carbon taxes and carbon sequestration schemes.

Adaptation stategies accept some warming as a given and focus on preventing or reducing undesirable consequences: for example defending against rising sea levels or ensuring food security.

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
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External links

Educational

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

Environmentalist

"Global warming is real and underway" — Union of Concerned Scientists (UCS)
Natural Resources Defense Council (NRDC): overviews, news and reports on global warming — NRDC
Climate Alliance of European Cities
http://www.undoit.org Global Warming: Undo It (Environmental Defense)
BBC: 2004 Climate Change Feature — Major feature from the BBC including climate change animations, articles, Q&A, video, analysis, charts and quizes.
Global Warming — National Wildlife Federation Includes maps of impact on birds and Senate voting records.
Global Warming & Hurricanes - Review based on latest articles in Science and Nature. Graph of trends in 6 hurricane basins.

Global warming-skeptical

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

Sites which are considered to be independent or receive too little support to be considered influential sponsorship

Greenhouse Effect by Thomas C. Schelling, winner of the 2005 Nobel Prize for economics
David Suzuki Foundation— : Impacts and Solutions,on Climate Change.
Possible solutions — Introduce interesting theories like "sequestering" or forcing carbon dioxide deep into the oceans
GreenFacts— A summary of the Third Assessment Report of the IPCC
The PR Plot to Overheat the Earth, analysis of industry efforts to discredit global warming science, by Bob Burton and Sheldon Rampton, published in the Earth Island Journal.
Testimony before the Senate Environment and Public Works Committee on 18 July 2001 of Thomas R. Karl
Testimony, on 2 May 2001 of Richard S. Lindzen
BBC News summary of climate change
Sleepwalking to Extinction, by George Monbiot (Z Magazine)
Climate change (global warming) : a couple of answers to some elementary questions by Jean-Marc Jancovici
Global Warming FAQ by Tom Rees
Greenhouse gas emissions in industrialised countries: Where does Australia stand? by The Australia Institute
Global Warming - a collection of articles from globalissues.org which also has developing country perspective

Other

Global Warming FAQ — A comprehensive FAQ.
A large compendium of links to sites with information 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.
Activist Magazine: Carbon Activism for Beginners
Global Warming and the Effects on Skiing
Global warming - briefing document
Environmental Defence - Global Warming Skeptics: A Primer — How ExxonMobil funds scientists and media pundits to "de-bunk" the science of climate change
Global warming - Greenpeace pictures in Spitsbergen
NOVA scienceNOW - A 7 minute video of the NOVA broadcast that aired on PBS, July 26, 2005. Hosted by Robert Krulwich, the video is about the world's fastest glacier and how "moving way too fast reveals how unpredictable the effects of global warming can be".
Global Warming Related News
[36]Methane Burps: Ticking Time Bomb

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 {{main|Ozone depletion}}
-Although, they are often interlinked in the popular press, the connection between global warming and [[ozone depletion]] is not strong. There are four areas of linkage:
+Although they are often interlinked in the popular press, the connection between global warming and [[ozone depletion]] is not strong. There are four areas of linkage:
 * Global warming from CO<SUB>2</SUB> 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.