Generally, mass balance refers to the net sum between the input and loss of mass to a reservoir. In glaciology, the term refers to the balance between the loss and gain of ice from a glacier, ice cap, or ice sheet. When more ice is gained than lost, the result is a positive mass balance and the overall growth of the ice body. A negative mass balance occurs when more ice is lost than is gained and the ice body shrinks.

Several other terms are used to specify the gain or loss of glacial ice. "Total mass balance" refers to the total net gain or loss of ice from an entire glacier over a single hydrologic cycle (Axelrod, 2004). "Specific mass balance," by contrast, refers to such loss or gain at a particular point on the surface of a glacier. Total and specific mass balances can diverge in extremely large ice bodies, such as the Antarctic ice sheets, whose mass balances vary in different geographic locations. The average mass balance per unit area of an ice body is called the "mean specific mass balance."

It can be used to compare the amount of ice gained or lost by ice bodies of different sizes. Glaciers and other ice bodies are highly sensitive to climate change and are viewed by many scientists as early warning detectors for global warming or global cooling. Ice bodies do not form in a single year or in response to a single snow event. Instead, they form over a sustained period of time, as temperatures remain cool enough that not all of the snow that falls in a given winter melts in the subsequent summer (Botterill, 2003). It is this long-term accumulation of snow that leads to the development of ice and, eventually, a glacier or other ice body. Ice bodies are dynamic, much like rivers. Ice can accumulate at the head of a glacier at the same time that it is being lost from the toe of the same glacier through ablation or iceberg calving. Also, ice bodies can change in size and shape throughout the course of a hydrologic cycle. Thus, to determine whether a glacier is growing or shrinking in size, it is necessary to consider the mass balance, or net sum, of ice that accumulates and is lost throughout an entire hydrologic cycle, rather than simply examining the size of a glacier at two points in time.

References

1. Axelrod, Regina S., David Leonard Downie, and Norman J. Vig, eds. The Global Environment: Institutions, Law, and Policy. 2d ed. Washington, D.C.: CQ Press, 2004.

2. Botterill, Linda C., and Melanie Fisher, eds. Beyond Drought: People, Policy, and Perspectives. Collingwood, Vic.: CSIRO, 2003.

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