Glacier Mass Balance: equilibrium or disequilibrium response?

Guest Commentary from Mauri S. Pelto

I get asked at least once a day about the future prognosis for alpine glaciers and whether they have a future. I will focus here on North American glaciers whose mass balance measurements in the West from 1984-2005 indicate a declining trend. The trend suggests that all of the glaciers are out of balance and that some will disappear. The question is determining which glaciers are merely out of equilibrium and can retreat to a position of equilibrium, and which are in complete disequilibrium and will melt away? Let me explain.

Glaciers have long been recognised as sensitive and reliable indicators of climate. This together with their wide distribution has made them a key marker of current climate change. The World Glacier Monitoring Service (WGMS) collects from alpine glaciers around the world annual mass balance data, which is the difference between annual accumulation and annual ablation (from melting and sublimation). A glacier is divided into an ablation zone where all accumulated snow is lost from the winter and an accumulation zone where snowpack is retained to the end of the summer. The alpine glaciers discussed here are not calving, nor do they have superimposed ice, two factors that complicate measurement. For an alpine glacier to have an equilibrium balance it typically must have upwards of 60% of its area in the accumulation zone at the end of the summer melt season.

Glaciers respond to climate change in an attempt to achieve a new equilibrium. A glacier advances due to a climate cooling/snowfall increase that causes positive mass balance. The advance increases the glaciers area at low elevation where ablation is highest, returning the glacier to equilibrium. A climate warming/snowfall decrease leads to negative mass balances and glacier retreat. An equilibrium response occurs when a new equilibrium is reached by a retreating glacier losing enough of its high ablating sections, usually at its lowest elevations, so that accumulation once again balances ablation. This is a glacier equivalent to jettisoning bad debt. If a glacier cannot retreat to a point where equilibrium is established, it is in disequilibrium with the current climate. A glacier that is in disequilibrium will melt away with a continuation of the current climate.

Therefore, it is important to note whether glaciers are undergoing an equilibrium or disequilibrium response to climate change. It was noted in the Tropical Glacier Retreat post that if the annual equilibrium line rises above the mountain top this will lead to glacier loss. The equilibrium line is the elevation on a glacier at which annual accumulation equals ablation. This is not the only means to an end for a glacier, nor is it a robust means for identifying glacier disequilibrium. A glacier that is approaching equilibrium during retreat will thin mostly near the terminus, and at some elevation above the terminus, usually near the equilibrium line or in the lower accumulation zone, there will be comparatively little thinning (Schwitter and Raymond, 1993). This indicates that at least a portion of the glacier is healthy. A glacier that is not approaching equilibrium will thin appreciably along the entire length of the glacier (Pelto, 2006). There are two means to identify glacier disequilibrium. The first is substantial thinning along entire longitudinal profile of the glacier. The second is frequent years with no accumulation area at all – that is, the equilibrium line is above the glacier.

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