Global warming

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Global warming is usually understood to mean the rise of average atmospheric and ocean temperatures of the last several decades—which is the main subject of this article—although it can mean any such rise of temperature. Climate change has been a natural phenomenon that has occurred hundreds of times through geologic time, so the term "global warming" is commonly used to refer to warming since the mid-1800s, which is believed to be mostly attributable to human activity.[1] "Anthropogenic climate change" is also sometimes used to refer how humans are impacting the climate.

The causes of the current warming have been strongly debated in the last decades, but in the last few years, many scientists, journalists, and politicians have reported a 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.[2] 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 aimed at reducing greenhouse gas emissions. Further policy changes are widely, but not universally, recommended. See "The politics of global warming," below.

Methods for past temperature reconstruction

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 1979, satellite data are available.[3]

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 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, 18O, 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.[4]

The use of calibrated instrumental records and proxies allowed to reconstruct, with various degrees of confidence, the average air temperatures and atmospheric CO2 concentrations throughout the Phanerozoic.[5][6][7]

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.

Historical observations of temperature

(under construction)

Past temperature and CO2 variations

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[8]. More recently, during Ice ages a great portion of Europe and North America were covered by extensive ice caps, and temperatures were substantially lower than today. On the contrary, Mesozoic climates were generally warmer than today with a virtual absence of ice caps.[9] Climate change is thus a natural phenomenon that commonly occurred throughout geologic time. However, the youngest evidence of strong climate change dates to the late Pleistocene, ca. 10000 years ago, and consists of abrupt warming events (known as Dansgaard-Oeschger cycles) with a pacing of ca. 1500 years. These cycles can be seen in proxy records such as the oxygen isotope curve of the Vostok Ice Core[2], and are explained by abrupt switches in the circulation of the Atlantic Ocean.[10]


The subsequent time interval, the Holocene, was characterized by a substantial climate stability (at least with respect to older times). This is the climate stability we are still experiencing today. 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[11]), a Little Ice Age (when European settlers abandoned Greenland[12]), and since around 1850 a warming trend. The latest warming trend, however, is by far the strongest climate anomaly recorded in instrumental series [13]

In the historical ice-core records, variations in carbon dioxide (CO2) 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 CO2 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 CO2 level, which in turn cause further warming (positive feedback). Measurements of present-day warming show CO2 leads temperature.

Causes of global warming

Carbon dioxide and climate

(Under construction)

Solar variations and climate

When the Sun boasts a maximum of spots, cycle after cycle, Earth tends to be warmer than when its face is clear.[3]

A lengthy period of cold weather called the Little Ice Age (LIA) coincided with the Maunder Minimum, when hardly any sunspots were observed. There is some disagreement as to when the onset of the LIA occurred, but it is generally said to have begun around the 14th century and lasted until the mid-19th century. The Maunder Minimum last from about 1645 to 1715.[4] The Spörer Minimum, a period between 1400 and 1500 marked by low solar activity as measured by carbon-14 levels, also coincided with the LIA.[5] It is questioned, however, whether the LIA is a global phenomenon rather than a regional one.[6]

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.[7][8]

The consensus about anthropogenic global warming

(Under construction)

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, some U.S. state climatologists, and several prominent individual scientists remain skeptical.

References and notes

  1. Summary for Policymakers (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2007-02-05). Retrieved on 2007-02-02.
  2. An extended discussion of global warming is given in the Fourth Assessment Report by the International Panel on Climate Change (IPCC). [1]
  3. 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.
  4. Emiliani, C., 1955. Pleistocene temperatures. J. Geology 63, 538-578
  5. Citation needed for temperature
  6. Berner RA, 2006 - GEOCARBSULF: A combined model for Phanerozoic atmospheric O2 and CO2. Geochimica et Cosmochimica Acta, v. 70, pp. 5653-5664
  7. Retallack GJ, 2001 - A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles. Nature, v. 411, pp. 287-290
  8. Hoffman PF., Kaufman AJ., Halverson GP., Schrag DP., 1998 - A Neoproterozoic Snowball Earth. Science, v. 281, pp. 1342–1346
  9. Price G. D., 1999 - The evidence and implication of polar ice during the Mesozoic. Earth Science Reviews, Vol. 48, pp. 183-210
  10. Ganopolski A., Rahmstorf S., 2002 - Abrupt glacial climate changes due to stochastic resonance. Physical Review Letters, v. 88, DOI: 10.1103/PhysRevLett.88.038501
  11. ref. needed for colonization of Greenland
  12. ref. needed for abandonement of Greenland
  13. ref. needed for recent warming - MAnn et al 99 ok but more recent works exist