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 hundreds of times through geologic time, so the term "anthropogenic global warming" is used to refer to warming which is the result of human activity.

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. 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.

Historical observations of temperature

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.[1]

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.

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.

The Sun's role on climate

When the Sun boasts a maximum of spots, cycle after cycle, Earth tends to be warmer than when its face is clear. [2] 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.[3][4]

The politics of global warming

The origin of global warming, and the extent of this consensus, however, have been subject to considerable political fighting, primarily because an anthropogenic origin is widely thought to make policy changes, such as the Kyoto Protocol, necessary. Most Democrats and Greens assert that most scientists have reached a consensus on its anthropogenic origin. Some Republicans, U.S. state climatologists, and several prominent individual scientists remain skeptical.