Climate Change: Glacier Mass Balance
In Earth’s highest mountains and across the land in both polar regions, brief summers were long overpowered by snowy winters. Over many centuries, layers of snow accumulated to form thick glaciers. Present since the last ice age, most of the world’s glaciers are now shrinking or disappearing altogether.
Explore this interactive graph: Click and drag either axis to view different parts of the graph. To squeeze or stretch the graph in either direction, hold your Shift key down, then click and drag. The graph shows cumulative mass loss in "meters of water equivalent," which is the depth of the meltwater spread out over the glacier's surface area. Data* are from the World Glacier Monitoring Service, and provided by Mauri Pelto.
Measuring glacier change
Glaciers gain mass through snowfall and lose mass through melting and sublimation (when water evaporates directly from solid ice). Glaciers that terminate in a lake or the ocean also lose mass through iceberg calving.
To see if a glacier is growing or shrinking, glaciologists check the condition of snow and ice at several locations on the glacier at the end of the melt season. The scientists check snow levels against stakes they’ve inserted in the glacier, dig snow pits in the surface to examine the sequence of seasonal layers, and insert long poles into the glacier to probe characteristics of the snow and ice.
Generally, the difference in thickness of snow from the previous measurement indicates the glacier’s mass balance—whether the glacier has grown or shrunk.
Change over time
Scientists have described more than one hundred thousand glaciers in the World Glacier Inventory, but only a small fraction of these have been consistently monitored for long enough to measure climate-related changes in their size or mass. Scientists refer to this collection of 40 glaciers as "reference" glaciers. Since 1980, the average thickness of the reference glaciers has decreased by an amount equivalent to just over 57 feet (17 meters) of water.
In the 2014 State of the Climate report (edited by NOAA NCEI scientists and published by the American Meterological Society), scientists reported that reference glaciers lost the equivalent of 853 millimeters of water, meaning the equivalent depth of water (spread out over the entire glacier area) that would be produced from the amount of melted snow or ice. This loss was not quite as severe the loss from 2013 (887 millimeters), but it still counted among the larger losses recorded since 1980.
Glaciers that exist today are remnants of the last ice age. Thick sheets of ice advanced and retreated across most continents several times before withdrawing to the polar regions about 10,000 years ago; continental ice sheets still cover Greenland and Antarctica. Originally, scientists began studying glaciers only for the clues they offered about Earth’s climate during past ice ages.
Today, many glaciologists are more concerned with predicting when various glaciers will disappear. In many parts of the world—including the western United States, South America, China, and India—glaciers are frozen reservoirs that provide a reliable water supply each summer to hundreds of millions of people and the natural ecosystems on which they depend.
*Data from the most recent year are preliminary and may not include observations from all reference glaciers.
M.S. Pelto, 2015: [Global Climate] Alpine Glaciers [in “State of the Climate in 2014”]. Bulletin of the American Meteorological Society (BAMS), 96 (7), S19-S20.
Zemp, M., Roer, I., Kääb, A., Hoelzle, M., Paul, F., and Haeberli, W. 2008. Global Glacier Changes: facts and figures. World Glacier Monitoring Service, UNEP, 88 p. ISBN:978-92-807-2898-9. Accessed August 29, 2011.
World Glacier Monitoring Service Glacier Mass Balance Bulletin Accessed August 29, 2011.