Glaciers

Because they are so sensitive to temperature fluctuations, glaciers provide clues about the effects of global warming (Oerlemans, J. 2001). The 1991 discovery of the 5,000 year-old "ice man" preserved in a glacier in the European Alps fascinated the world, yet the discovery meant that this glacier had reached a 5,000-year minimum. With few exceptions, glaciers around the world have retreated at unprecedented rates over the last century. Some ice caps, glaciers, and even an ice shelf have disappeared altogether. Many more are retreating so rapidly that they may vanish within decades. Some scientists attribute this retreat to the Industrial Revolution; burning fossil fuels releases greenhouse gases into the atmosphere and affects our environment in ways we did not understand before.

Mountain Glacier Fluctuations:
Changes in terminus location and mass balance

Over long periods, glacial response to climate change becomes obvious.

1941-2004 comparison: Glacier Bay National Park and Reserve's White Thunder Ridge as seen on August 13, 1941 (left) and August 31, 2004 (right). Muir Glacier has retreated out of the field of view, Riggs Glacier has thinned and retreated significantly, and dense new vegetation has appeared. Muir Glacier was more than 2,000 feet thick in 1941. 2004 USGS photo by B. F. Molnia; 1941 photo by W. O. Field. See the Repeat Photography of Glaciers Special Collection in the Glacier Photograph Collection to access this and other photograph pairs.

1928-2000 comparison: These photos of the South Cascade Glacier in the Washington Cascade Mountains show dramatic retreat between 1928 and 2000. Photos courtesy of the USGS.

Glaciers differ from snow cover and sea ice extent in that scientists cannot use short-term changes in the areal extent of small glaciers as an index of current climatic conditions. Glaciers continually move, transporting mass from higher to lower elevations, somewhat like a conveyer belt. If the combination of climate and ice dynamics determines that the glacier is also advancing, the effect of the advance of the terminus is to increase the overall glacier area; however, because glaciers move slowly, a significant time lag occurs between the climatic conditions that caused the advance or retreat, and the actual advance or retreat. This time lag may last several years or longer, and is determined by the complicated and sometimes uncertain processes that control how fast the glacier moves.

More direct methods have been developed to determine the year-to-year "health" or mass balance of a glacier. During winter, a glacier gains mass from accumulating snow. During the following summer, some or all of that winter accumulation is lost to ablation. The difference between the accumulation and ablation for a given year describes the annual net mass balance, which corresponds to the change in glacier volume.

Explosive volcanic eruptions, which contribute dust to the stratosphere and affect Earth's climate, can also affect glacier mass balance.

Glacier mass balance: Image courtesy of Mark Dyurgerov, Institute of Arctic and Alpine Research, University of Colorado, Boulder.

For glaciers outside Antarctica or Greenland—referred to here as subpolar and mountain glaciers—considerable compilation and analysis of existing mass balance measurements have occurred (Cogley and Adams 1998; Dyurgerov and Meier 1997; Dyurgerov 2002; Cogley 2002). Glaciers involved in mass balance studies are sparsely distributed over all mountain and subpolar regions, with about 70 percent of the observations coming from the mountains of Europe, North America, and the former Soviet Union.

At one time or another, researchers have measured mass balance on more than 300 glaciers since 1946, although we only have a continuous record from about 40 glaciers since the early 1960s. These results indicate that in most regions of the world, glaciers are shrinking in mass. From 1961 to 2003, the thickness of "small" glaciers decreased approximately 8 meters, or the equivalent of more than 6,000 cubic kilometers of water. The Global Glacier Mass Balance graph shows average volume change data each year from 1961 to 2003, and a plot of the cumulative change in volume, expressed in cubic kilometers of water, for this period.

Last updated: 5 December 2007