Global warming is an increase in the average temperature of the earth's surface. Since the late 1800's, the average temperature has increased about 0.5 to 1.5 Fahrenheit degrees (0.3 to 0.8 Celsius degree). Scientists disagree about the causes and potential impact of this warming trend.

Many scientists believe that human activities have caused the trend by enhancing the earth's natural greenhouse effect. This effect warms the lower atmosphere and surface of the earth through a complex process involving sunlight, gases, and particles in the atmosphere.

Since the mid-1800's, human activities--chiefly the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land--have increased the amounts of heat-trapping atmospheric gases, called greenhouse gases. The burning of fossil fuels produces the greenhouse gas carbon dioxide. Most of this burning takes place in automobiles, electric power plants, and industrial facilities. The clearing of land reduces the amount of carbon dioxide that trees and other plants remove from the atmosphere in a process called photosynthesis.

Although researchers have not yet fully proved that the increase in greenhouse gases has raised the surface temperature, many scientists consider such a relationship likely. Continued global warming could have a beneficial impact in some areas and a harmful impact in others. For example, people could begin to farm in regions where it is currently too cold. At the same time, global warming could cause sea levels to rise and thereby increase the threat of flooding in low-lying coastal areas, many of which are densely populated.

Excerpt from the "Global warming" article, The World Book Encyclopedia © 1999