Global warming

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 continually 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. "Anthropogenic global warming" is also sometimes used to distinguish current warming from previous natural cycles of change.

The causes of the current warming have been strongly debated in the last decades, but in the last few years, many scientists have reached a 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. See "The consensus about anthropogenic global warming," below.

Based on the belief that most recent warming is man-made, a wide variety of policy options have been recommended, primarily by climatologists, politicians, and environmental activists. The Kyoto Protocol, an international agreement aimed at reducing greenhouse gas emissions, was adopted by 169 nations. 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.

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.

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.

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

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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. 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. 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.

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), a Little Ice Age (when European settlers abandoned Greenland), and since around 1850 a warming trend. The latest warming trend, however, is by far the strongest climate anomaly recorded in instrumental series.

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

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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

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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. However, even among people who accept that global warming is man-made, there exists strong differences of opinion on the proper responses to it, and on the climatological and economic impacts of various proposed policies.