11.3: Glacial Budget
- Page ID
- 28297
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Glaciers gain mass during the winter as snow accumulates. During summer the snow melts. The glacier is like a bank account, if more money is coming in (snow accumulating in winter) than going out (snow melting in summer), then the bank account grows. The glacial budget works in a similar way. The balance of accumulating ice (zone of accumulation) is weighed against melting ice (zone of ablation or zone of melting), and whichever is greater determines whether the glacier will advance or retreat. In the zone of accumulation, the rate of annual snowfall is greater than the rate of melting. In other words, not all of the snow that falls each winter melts during the following summer, and the ice surface is always covered with snow. In the zone of melting or ablation, more ice melts then accumulates as snow during the year. The equilibrium line marks the boundary between the zones of accumulation and ablation. Below the equilibrium line, in the zone of melting, bare ice is exposed because last winter’s snow has all melted; above that line, the ice is still mostly covered with snow from last winter. The position of the equilibrium line changes from year to year as a function of the balance between snow accumulation in the winter and snowmelt during the summer. More winter snow and less summer melting obviously favors the advance of the equilibrium line (and of the glacier’s leading-edge or terminus), but of these two variables, it is the summer melt that matters most to a glacier’s budget. Cool summers promote glacial advance and warm summers promote glacial retreat [5].
If warmer summers promote glacial retreat, then overall climate warming over many decades and centuries causes the glacier to melt and retreat significantly. Since the global climate has been warming [6], this warming is likely causing the ice sheets to melt (or lose mass) at an increasing rate over years and decades rather than over centuries [7].
When ice sheets start to melt, such as those in Antarctica and Greenland, their flow into the ocean speeds up eventually creating floating ice sheets. The edges of the glacier or its extension as floating ice break off in a process called calving.