Global warming is an increase in the average temperature of Earth's surface. Since the late 1800's, the global average temperature has increased about 0.7 to 1.4 °F (0.4 to 0.8 °C). Many experts estimate that the average temperature will rise an additional 2.5 to 10.4 °F (1.4 to 5.8 °C) by 2100. That rate of increase would be much larger than most rates of past increases.

Scientists worry that human societies and natural ecosystems might not adapt to rapid climate changes. An ecosystem consists of the living organisms and physical environment in a particular area. Global warming could cause much harm, so countries throughout the world have drafted an agreement to limit it. However, that agreement, known as the Kyoto Protocol, has not yet been ratified (formally approved).

Causes of global warming

Climatologists (scientists who study climate) have analyzed the global warming that has occurred since the late 1800's. A majority of climatologists have concluded that human activities are responsible for most of the warming. Human activities contribute to global warming by enhancing Earth's natural greenhouse effect. The greenhouse effect warms Earth's surface through a complex process involving sunlight, gases, and particles in the atmosphere. Gases that trap heat in the atmosphere are known as greenhouse gases.

The main human activities that contribute to global warming are the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in automobiles, in factories, and in electric power plants that provide energy for houses and office buildings. The burning of fossil fuels creates carbon dioxide, whose chemical formula is CO2. CO2 is a greenhouse gas that slows the escape of heat into space. Trees and other plants remove CO2 from the air during photosynthesis, the process they use to produce food. The clearing of land contributes to the buildup of CO2 by reducing the rate at which the gas is removed from the atmosphere or by decomposition of dead vegetation.

A small number of scientists argue that the increase in greenhouse gases has not made a measurable difference in the temperature. They say that natural processes could have caused global warming. Those processes include increases in the energy emitted (given off) by the sun. But the vast majority of climatologists believe that increases in the sun's energy have contributed only slightly to recent warming.

Contributors: Michael D. Mastrandrea, B.S., Graduate Fellow, School of Earth Sciences, Geological and Environmental Sciences, Stanford University. Stephen H. Schneider, Ph.D., Professor of Biological Sciences, Stanford University.

Additional resources

Christianson, Gale E.Greenhouse: The 200-Year Story of Global Warming. Walker, 1999. Godrej, Dinyar.The No-Nonsense Guide to Climate Change. Verso, 2001. Johansen, Bruce E.The Global Warming Desk Reference. Greenwood, 2002. Stein, Paul.Global Warming. Rosen Pub. Group, 2001. Younger readers. -

Source : World Book 2005.