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 glacial budget describes how ice accumulates and melts on a glacier which ultimately determines whether a glacier advances or retreats. The balance of accumulating ice (zone of accumulation) is weighed against melting ice (zone of melting or zone of ablation), 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 (or snowline, also called the firnline) 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 .
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 due to human burning of fossil fuels , 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 . This means that as time goes on, the glaciers are melting faster and contributing more to rising sea-level than expected based on the previous history.
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. In cases like these, the end of the glacier in the fjord may retreat but it will also lose thickness or deflate [8; 9]. A fjord is a narrow ocean-flooded valley with steep walls that were carved by a recent glacier. The retreating glacier or glaciers may add to sea level, and this increased sea level can also add to the flooding of the former glacially-carved valleys. Glacial retreat and deflation are well-illustrated in the 2009 TED Talk by James Balog.
6. Stocker, T. F. Climate change 2013: the physical science basis: Working Group I contribution to the Fifth assessment report of the Intergovernmental Panel on Climate Change. (Cambridge University Press, 2014).