What is global warming?

Although our own climate records cover only about 100 years, we can infer climatic conditions from the pollens found in layers of mud and from the gas bubbles in ice cores bored through major ice sheets (including an ice core from the Antarctic that gives a record going back well over 200 000 years). These ice cores enable scientists to identify correlations between the composition of the atmosphere and regional climatic conditions in the past. Recent and predicted increases in concentrations of greenhouse gases in the atmosphere are expected to cause additional warming of average surface air temperatures.

These increases in greenhouse gas concentrations have been related to human activities such as the burning of fossil fuels and the reduction of forests. Combustion changes the composition of the atmosphere by adding carbon dioxide and other gases. The large-scale destruction of forests releases large amounts of carbon stored in trees and forest soils.

These human-induced changes to the greenhouse effect of the earth's atmosphere are expected to result in global warming and other changes in climate. Most scientists agree that the threat of climate change is real: what is debatable is the extent of change and how it will vary from place to place.

The models used by climatologists generally agree that the temperature increase, as a global annual average, might be from 1° to 4 °C by the year 2100. They also agree that the effect would be greater in the high latitudes, especially during the winter months and over large land masses. The warmer temperature would trigger other changes, such as a change in global precipitation patterns, a decrease in snow and ice coverage, and a rise in sea levels.

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