Difference between accumulation and melting on a glacier
See also: Retreat of glaciers since 1850
Seasonal glacier melt contributes to runoff; the annual balance (net change of glacier mass) contributes to sea level rise.[1]From 1970 to 2004, mountain glaciers thinned (yellow and red) in some regions and thickened (blue) in others.
Crucial to the survival of a glacier is its mass balance of which surface mass balance (SMB), the difference between accumulation and ablation (sublimation and melting). Climate change may cause variations in both temperature and snowfall, causing changes in the surface mass balance.[2] Changes in mass balance control a glacier's long-term behavior and are the most sensitive climate indicators on a glacier.[3] From 1980 to 2012 the mean cumulative mass loss of glaciers reporting mass balance to the World Glacier Monitoring Service is −16 m. This includes 23 consecutive years of negative mass balances.[3]
A glacier with a sustained negative balance is out of equilibrium and will retreat, while one with a sustained positive balance is out of equilibrium and will advance. Glacier retreat results in the loss of the low elevation region of the glacier. Since higher elevations are cooler than lower ones, the disappearance of the lowest portion of the glacier reduces overall ablation, thereby increasing mass balance and potentially reestablishing equilibrium. However, if the mass balance of a significant portion of the accumulation zone of the glacier is negative, it is in disequilibrium with the local climate. Such a glacier will melt away with a continuation of this local climate.[4]
The key symptom of a glacier in disequilibrium is thinning along the entire length of the glacier.[5] For example, Easton Glacier (pictured below) will likely shrink to half its size, but at a slowing rate of reduction, and stabilize at that size, despite the warmer temperature, over a few decades. However, the Grinnell Glacier (pictured below) will shrink at an increasing rate until it disappears. The difference is that the upper section of Easton Glacier remains healthy and snow-covered, while even the upper section of the Grinnell Glacier is bare, melting and has thinned. Small glaciers with shallow slopes such as Grinnell Glacier are most likely to fall into disequilibrium if there is a change in the local climate.
In the case of positive mass balance, the glacier will continue to advance expanding its low elevation area, resulting in more melting. If this still does not create an equilibrium balance the glacier will continue to advance. If a glacier is near a large body of water, especially an ocean, the glacier may advance until iceberg calving losses bring about equilibrium.
^"Global Glacier State". WGMS.ch. World Glacier Monitoring Service under the auspices of ISC (WDS), IUGG (IACS), UN environment, UNESCO, WMO. 2020. Archived from the original on 4 August 2022.
^Mauri S. Pelto (Nichols College). "Glacier Mass Balance of North Cascade, Washington Glaciers 1984–2004". In "Hydrologic Processes". Archived from the original on December 25, 2007. Retrieved February 27, 2008.
^ abMichael Zemp, WGMS (September 9, 2008). "Glacier Mass Balance". World Glacier Monitoring Service. Archived from the original on March 7, 2008.
^Mauri S. Pelto (Nichols College). "The Disequilibrium of North Cascade, Washington Glaciers 1984–2004". In "Hydrologic Processes". Retrieved February 14, 2006.
^Pelto, M.S. (2010). "Forecasting temperate alpine glacier survival from accumulation zone observations" (PDF). The Cryosphere. 4 (1): 67–75. Bibcode:2010TCry....4...67P. doi:10.5194/tc-4-67-2010. Retrieved February 9, 2010.
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