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Climate Change: Glacier Mass Balance
Updated September 23, 2014
In Earth’s highest mountains and across the land in both polar regions, brief summers were long overpowered by snowy winters. Over many centuries, layers of snow accumulated to form thick glaciers. Present since the last ice age, most of the world’s glaciers are now shrinking or disappearing altogether.
Scientists have described more than one hundred thousand glaciers in the World Glacier Inventory, but only a small fraction of these have been monitored for long enough to measure climate-related changes in their size or mass. The status of a set of "reference" glaciers located in mountain ranges around the world has been carefully documented over time. On average, from 2001 to 2010, the glaciers lost ice at a rate equivalent to 28 inches (0.7 meters) of water per year over the total glacier area. The 2013 State of the Climate Report indicated that the glaciers with the longest, most reliable records had lost the equivalent of 50 feet (15.1 meters) of water since 1980.
Explore this interactive graph: Click and drag either axis to view different parts of the graph. To squeeze or stretch the graph in either direction, hold your Shift key down, then click and drag. The graph (source data) shows cumulative average mass balance of reference glaciers around the world as measured by the World Glacier Monitoring Service.
Glaciers gain mass through snowfall and lose mass through melting and sublimation (when water evaporates directly from solid ice). Glaciers that terminate in a lake or the ocean also lose mass through iceberg calving. To see if a glacier is growing or shrinking, glaciologists check the condition of snow and ice at several locations on the glacier at the end of the melt season. The scientists check snow levels against stakes they’ve inserted in the glacier, dig snow pits in the surface to examine the sequence of seasonal layers, and insert long poles into the glacier to probe characteristics of the snow and ice.
Generally, the difference in thickness of snow from the previous measurement indicates the glacier’s mass balance—whether the glacier has grown or shrunk. Since 1980, the average thickness of the reference glaciers has decreased by an amount equivalent to 50 feet (15 meters) of water.
Glaciers that exist today are remnants of the last ice age. Thick sheets of ice advanced and retreated across most continents several times before withdrawing to the polar regions about 10,000 years ago; continental ice sheets still cover Greenland and Antarctica. Originally, scientists began studying glaciers only for the clues they offered about Earth’s climate during past ice ages. Today, many glaciologists are more concerned with predicting when various glaciers will disappear. In many parts of the world—including the western United States, South America, China, and India—glaciers are frozen reservoirs that provide a reliable water supply each summer to hundreds of millions of people and the natural ecosystems on which they depend.
Zemp, M., Roer, I., Kääb, A., Hoelzle, M., Paul, F., and Haeberli, W. 2008. Global Glacier Changes: facts and figures. World Glacier Monitoring Service, UNEP, 88 p. ISBN:978-92-807-2898-9. Accessed August 29, 2011.
World Glacier Monitoring Service Glacier Mass Balance Bulletin Accessed August 29, 2011.
Since 1976, the average thickness of a set of well-studied reference glaciers has decreased by an amount equivalent to more than 50 feet (16 meters) of water.