Glaciers form when snow accumulates on a patch of land over tens to hundreds of years. The snow eventually becomes so thick that it collapses under its own weight and forms dense glacial ice. When enough of the ice is compacted together it succumbs to gravity and begins to flow downhill or spread out across flat lands (Williams and Hall, 1993). There are many different types of glaciers, and not all of them are good indicators of climate change. "Glaciers that do tend to be good climate indicators are small land-based, non-surge type glaciers. They respond directly to both regional temperature and precipitation [snow]," said Dorothy Hall, a hydrospheric scientist at NASA’s Goddard Flight Space Center. She and a team of scientists from around the globe have used satellites over the past 25 years to measure changes in glaciers in Europe, Iceland and Alaska.

More than 90 percent of the 33 million cubic kilometers of glacier ice in the world is locked up in the gigantic Greenland and Antarctic ice sheets. Because they are so massive and exist in such frigid latitudes, large-scale changes are very difficult to track and verify, said Hall.

In addition to the ice sheets, there are two types of small glaciers that make for bad climate gauges. "Both surge glaciers and tidewater glaciers have their own cycles of advance and retreat. These cycles are certainly related to climate, but we are not exactly sure how," said Hall. Even if the climate changes in the region, these glaciers would most likely maintain their distinctive patterns of behavior.
 

The Muir tidewater glacier in Alaska’s Glacier Bay has been observed for nearly 200 years. Explorers in the mid-1700s recorded observations of Muir at its peak. They noticed that about two hundred years ago it began to recede. Recent measurements show that the glacier has withdrawn more than 90 kilometers (Hall et. al., 1994).

Challenges to detecting glacial change
At the Edge: Monitoring Glaciers to Watch Global Change