global warming

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Increase in average global temperature, amounting to approximately 0.74°C/1.3°F from 1906 to 2005. Much of this is thought to be related to human activity. Global temperature has been highly variable in Earth history and many fluctuations in global temperature have occurred in historical times, but this most recent episode of warming coincides with the spread of industrialization, prompting the suggestion that it is the result of an accelerated greenhouse effect caused by atmospheric pollutants, especially carbon dioxide gas. The melting and collapse of parts of the Larsen Ice Shelf, Antarctica, in two distinct events in January 1995 and February 2000, have been attributed to global warming, as has the reduction in the Arctic polar ice cap. Melting of land-based ice is expected to raise the sea level in the coming decades (melting of floating ice does not alter sea level).

In 1988, the World Meteorological Organization (WMO) and the United Nations (UN) set up the Intergovernmental Panel on Climate Change (IPCC), a body of more than two thousand scientists, to investigate the causes of and issue predictions regarding climate change.

In 2007, the IPCC's Fourth Assessment Report stated that: most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increased in anthropogenic (human) greenhouse gas concentrations. The IPCC predicts that a doubling of carbon dioxide concentrations, expected before the end of the 21st century, would probably increase the average global temperature by 2–4.5°C/3.6–8.1°F.

Assessing the impact of humankind on the global climate is complicated by the natural variability on both geological and human time scales. The present episode of global warming has thus far still left England approximately 1°C/1.8°F cooler than during the peak of the so-called Medieval Warm Period from 1000 to 1400. The latter was part of a purely natural climatic fluctuation. The consensus view of climatologists is that the Medieval Warm Period was not on a global scale, but this is still a contentious topic.

In addition to a rise in average global temperature, global warming has caused seasonal variations to be more pronounced in recent decades. Examples are the most severe winter on record in the eastern USA 1976–77, and the record heat waves in the Netherlands and Denmark the following year. Mountain glaciers have shrunk, late summer Arctic sea-ice has thinned by 40%, and sea levels have risen by 10–20 cm/4–8 in. Scientists have predicted a greater number of extreme weather events, and sea levels are expected to rise by 9–88 cm/4–35 in by 2100. 1998 was the warmest year globally of the last millennium, according to US researchers who used tree rings and ice cores to determine temperatures over the past 1,000 years.