A glacier is ice made of compacted and recrystallized snow. One of its defining characteristics is that it flows. Thus, an expanse of snow that lasts for years is not a glacier because it does not flow. Nor are those floating out at sea. Glaciers change, accumulating or losing area annually. The loss is called ablation.
Glacier Formation
Glacial ice is considered to be rock, albeit easily deformed rock. It forms in stages, starting with a lot of snowfall---more than can melt during spring and summer. As snow accumulates, it compacts, thaws and refreezes. The snow changes from fluffy snow, which is about 80 to 90 percent air, to granular snow and then granular ice, which is called firn. Firn is compacted some more, which changes it to glacial ice. Glacial ice is quite solid compared to the snow that it came from---it's about 90 percent solid.
Glacial Budget
A glacier's budget refers to how much a glacier has gained or lost in a year. If, on the glacier, snow accumulation and ablation---loss---are about equal, the budget is balanced. If there has been a net accumulation, the budget is positive. If there has been more ablation than accumulation, the budget is negative. If there has been more ablation than accumulation over time, the glacier's boundary recedes and the glacier's size decreases. If the budget is positive, the glacier expands and increases in thickness.
Zones
All glaciers have zones of loss and gain: the zone of accumulation, the zone of ablation---sometimes called a zone of wastage---and an equilibrium line, which is the border between the zones. The zone of accumulation is the area of the glacier where snowfall survives the spring and summer melting. Since on other areas of the glacier the accumulation doesn't survive, the zone of ablation in summer becomes bare ice.
How Ablation Occurs
Ablation happens in several ways. First cause, of course, is through melting, in the middle and lower latitudes. In high latitudes, the temperatures in summer might not rise above freezing. In high altitudes, ablation occurs mainly through sublimation; that is, the snow converts to water vapor directly, without a liquid phase. Some glaciers flow into water. Those that terminate that way can lose ice through "calving." In calving, ice breaks off from a glacier and becomes an iceberg.
Climate Change and Ablation
There is a lot of water locked up in glaciers. In Earth's two current continental ice sheets, Greenland and Antarctica, so much water is frozen that if it melted, the Earth's sea level would rise about 260 feet or about 80 meters. A rise of 10 meters would flood the U.S., affecting about 25 percent of its population. In a September 2008 joint report, the United Nations Environment Programme and the World Glacier Monitoring Service found mounting evidence that climate change is causing glacial ablation. In the current climate change scenarios, there will be ongoing, worldwide, rapid and perhaps accelerating melt, the result being the disappearance of mountain glaciers during the 21st century. Glaciers have economic importance to population centers; for instance, they provide water in the summer for irrigation and electricity production.
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