The well-documented retreat of the world's glaciers is widely viewed as a direct result of global warming. Though scientists cannot be sure whether or when these frozen repositories will add their entire freshwater contents to the world's seas, they do know that their melt water would raise global sea level. Moreover, a rapid rise in sea level -- which we are more and more certain is occurring through accelerated atmospheric warming -- would have consequences that are not immediately apparent.

Today, glaciers are found on all of Earth's continents except Australia. The two largest -- the Antarctic and Greenland ice sheets -- contain about 90 percent of the planet's freshwater supply, which is enough to raise global sea levels 60 meters (200 feet). Glaciers can form at either at an altitude or at sea level anywhere that snow and ice remain year-round. They grow slowly over thousands of years, as snowflakes from one snowfall are compacted and eventually turn to ice under the weight of subsequent snowfalls. Glaciers grow only if new snow exceeds what is lost through melting or evaporation.

One reason glaciologists study glaciers is to learn more about their past so that they can better predict how glaciers will respond to climate change in the future. In addition, glacial deposits archive data that can be used to reconstruct environmental conditions in the past. For example, bands of light and dark snow in ice core samples indicate seasonal changes. Light layers are deposited in summer, and dark layers in winter. The layers vary in color because summer and winter snows have different densities and crystal shape. Taken together with other historical climate data concerning solar radiation, the presence of atmospheric dust, and the atmosphere's chemical composition, glacial deposits provide a timeline record of past environmental change that includes changes in air temperature as well as tectonic events such as volcanic eruptions.

A scientific report endorsed by the United Nations states that unless greenhouse gas emissions are curtailed, average global temperatures may rise between one and three degrees Celsius (two and six degrees Fahrenheit) in the next hundred years. The ramifications of a temperature change at just the low end of this range would be severe. A one-degree Celsius (two-degree-Fahrenheit) change in temperature is predicted to result in a one-meter (three-foot) rise in sea level, which would displace millions of people in coastal cities and low-lying islands. For example, virtually all of the agricultural land in Bangladesh would be covered in seawater and rendered unusable. Another consequence of increased global temperatures would be an acceleration of glacial melt in mountain valleys, which would in turn result in massive flooding in their drainage basins. Rivers that are currently glacier-fed would dry up, impacting agriculture and other economic activities.

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