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| Based on the belief that most recent warming is man-made, several steps have been taken to mitigate global warming, such as the [[Kyoto Protocol]], an international treaty intended to reduce [[greenhouse gas]] emissions. Further policy changes are widely, but not universally, recommended. See "[[#The politics of global warming|The politics of global warming]]," below. | | Based on the belief that most recent warming is man-made, several steps have been taken to mitigate global warming, such as the [[Kyoto Protocol]], an international treaty intended to reduce [[greenhouse gas]] emissions. Further policy changes are widely, but not universally, recommended. See "[[#The politics of global warming|The politics of global warming]]," below. |
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| ==Methods for past temperature reconstruction==
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| Instrumental measurements of air temperature have been taken for centuries, but data are difficult to intercalibrate and are available for few localities. Instrumental data can thus be used for global reconstructions of temperature only from 1850, and global coverage is achieved only from 1957, when meteorological stations were established in Antarctica. Since 1980, satellite data are available.<ref name="IPCC07ch3">Trenberth, K.E., P.D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A. Klein Tank, D. Parker, F. Rahimzadeh, J.A. Renwick, M. Rusticucci, B. Soden and P. Zhai, 2007: Observations: Surface and Atmospheric Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref>
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| Past temperatures before the instrumental period are recontructed by proxies, i.e., parameters measured in geologic records as sediments, ice cores, tree rings and stalagmites, and that are influenced by temperature. The most important of such proxies is the [[isotope|isotopic]] composition of [[oxygen]] in ice, or in [[carbonate]] precipitates: in fact, all phase transitions as the condensation of water to ice or the precipitation of carbonate from waters imply [[isotopic fractionation]], which is in turn proportional to temperature. The concentration of the heavy isotope of oxygen, <sup>18</sup>O, augments in the fractionation product if fractionation occurs at lower temperatures. Measuring the isotope ratio in carbonates and waters, provided that all other parameters are known, permit to measure the temperature at which the isotopic fractionation occurred.<ref name="Emiliani55">Emiliani, C., 1955. Pleistocene temperatures. J. Geology 63, 538-578</ref>
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| The use of calibrated instrumental records and proxies allowed to reconstruct, with various degrees of confidence, the average air temperatures and atmospheric CO<sub>2</sub> concentrations throughout the [[Geologic time scale|Phanerozoic]].<ref>Citation needed for temperature</ref><ref name="Berner06">Berner RA, 2006 - GEOCARBSULF: A combined model for Phanerozoic atmospheric O<sub>2</sub> and CO<sub>2</sub>. Geochimica et Cosmochimica Acta, v. 70, pp. 5653-5664</ref><ref name="Retallack01">Retallack GJ, 2001 - A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles. Nature, v. 411, pp. 287-290</ref>
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| The causes of climate change are studied mostly through modelling. Scientists build models of the planet, e.g., mathematical representations of the world's oceans and atmosphere (Global Circulation Models or [[GCM]]), and explore the response of climate parameters (as temperature) to various initial conditions. The result of model studies are routinely checked by modeling known time intervals, and comparing model results with actual observations.
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| ==Past temperature and CO<sub>2</sub> variations==
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| Earth's average air temperature changed drastically over the Phanerozoic. A well known example of extreme cooling is the [[Precambrian]] [[snowball Earth]], when most of the planet was covered by ice<ref>Citation needed for snowball Earth</ref>. More recently, during [[Ice age]]s a great portion of Europe and North America were covered by extensive ice caps, and temperatures were substantially lower than today.
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| On the contrary, [[Mesozoic]] climates were generally warmer than today with a virtual absence of ice caps.<ref name="Price99">Price G. D., 1999 - The evidence and implication of polar ice during the Mesozoic. Earth Science Reviews, Vol. 48, pp. 183-210</ref>
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| Earth's average air temperature has increased hundreds of times over the last 900,000 years and then decreased again each time. These cycles of approximately 1,500 years can be seen in proxy records such as the Vostok Ice Core.[http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok.html]
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| Since the beginning of written historical records in Ancient Rome, there has been a warm period, followed by the cool period of the Dark Ages, followed by the Medieval Climate Optimum (when Greenland was colonized), a Little Ice Age (when European settlers abandoned Greenland), and since around 1850 a warming trend.
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| In the historical ice-core records, variations in [[carbon dioxide]] (CO<sub>2</sub>) levels correlate closely with the ups and downs of air temperature, lagging behind by about 800 ± 200 years. Most scientists believe that the variations in CO<sub>2</sub> are driven by the variations in air temperature in the historical record because of natural variations in Earth's axis tilt and orbit around the Sun, called [[Milankovitch cycles]]. These slight changes in Earth's movement cause the onset of the warming. This warming leads to higher CO<sub>2</sub> level, which in turn cause further warming (positive feedback). Measurements of present-day warming show CO<sub>2</sub> leads temperature.
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| ==Historical observations of temperature==
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| (Under construction}
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| ==Causes of global warming==
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| ===The link between carbon dioxide and climate===
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| Warm weather causes carbonate dissolved in the ocean to evaporate into the Earth's atmosphere. Changes in CO<sub>2</sub> levels have historically lagged behind changes in air temperature by several hundred years. Scientists generally think that this means temperature drives carbon dioxide, but a small number of climatologists have advanced a theory of [[anthropogenic global warming]] which says it is the other way around.
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| ===The Sun's role on climate===
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| When the Sun boasts a maximum of spots, cycle after cycle, Earth tends to be warmer than when its face is clear. [http://www.hno.harvard.edu/gazette/1997/11.06/BrighteningSuni.html] A lengthy period of cold weather coincided with the [[Maunder Minimum]] when hardly any sunspots were observed. While the Sun has played a role in climate change, recent observations show it is not a major cause of recent warming trends since the 1980s.[http://www.springerlink.com/content/x7l9752682l836p2/][http://www.nature.com/nature/journal/v443/n7108/abs/nature05072.html]
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| ==Claims of a "consensus" about anthropogenic global warming== | | ==Claims of a "consensus" about anthropogenic global warming== |
The global warming controversy is about claims that the rise of average atmospheric and ocean temperatures of the last several decades is largely (or mostly) man-made. The term "global warming" thus has two comman meanings. One refers to this political issue. The other refers to any period in which global temperature is higher than average or is rising (see climate cycles).
Climate change has been a natural phenomenon that has occurred hundreds of times through geologic time, but the term "global warming" is common used to refer to warming since the end of the Little Ice Age in the mid-1800s. Many people also use the term "global warming" as an abbreviaton for "anthropogenic global warming", both because it is shorter and because the term anthropogenic is used chiefly by specialist.
Many people consider the Modern Warm Period to be mostly attributable to human activity, with registered US voters split evenly on the issue (73% of Republicans say it's natural, 75% of Democrats say it's man-made). "Anthropogenic climate change" is also sometimes used to refer how humans are impacting the climate.
The causes of the Modern Warming Period have been strongly debated in the last decades, but in the last few years, many scientists, journalists, and politicians have reported an strong consensus on the anthropogenic origin of this warming. This view is advanced, prominently, by the reports of the United Nations' International Panel on Climate Change.[1] See "The consensus about anthropogenic global warming," below.
Based on the belief that most recent warming is man-made, several steps have been taken to mitigate global warming, such as the Kyoto Protocol, an international treaty intended to reduce greenhouse gas emissions. Further policy changes are widely, but not universally, recommended. See "The politics of global warming," below.
Claims of a "consensus" about anthropogenic global warming
Politicians and other partisans who urge "action" to "combat" global warming have asserted that there is a "scientific" consensus about AGW. They cite organization positions, statements signed by groups of scientists, polls, and a literature search.
Others say that "the science is not settled" and that far from there being a scientific consensus, most scientists either disagree with AGW or are undecided: i.e., that support for AGW is decidedly in the minority.
The politics of global warming
The origin of global warming, and the extent of the consensus about it, have been subject to considerable political fighting, primarily because an anthropogenic origin is widely thought to require policy changes, such as the adoption of the Kyoto Protocol, necessary. Most Democrats and Greens assert that most scientists have reached a consensus on its anthropogenic origin. Some Republicans, notably the conservative Jim Inhofe, U.S. state climatologists, and several prominent individual scientists remain skeptical.
References and notes