SOTC: Mountain Glaciers

Because they are so sensitive to temperature fluctuations, glaciers provide clues about the effects of global warming (Oerlemans, J. 2001). The 1991 discovery of the 5,000 year-old "ice man" preserved in a glacier in the European Alps fascinated the world, yet the discovery meant that this glacier had reached a 5,000-year minimum. With few exceptions, glaciers around the world have retreated at unprecedented rates over the last century. Some ice caps, glaciers, and ice shelves have disappeared altogether. Many more are retreating so rapidly that they may vanish within decades. Scientists have confirmed that this retreat is an unintended effect of the Industrial Revolution and modern energy use (Roe et al. 2016). Burning fossil fuels and other economic activities release greenhouse gases such as carbon dioxide and methane into the atmosphere, and warm the ocean and atmosphere. Warming air and ocean temperatures increase glacier ice melt.

Over long periods, glacial response to climate change becomes obvious as glaciers retreat and, in some cases, disappear.

White Thunder Ridge comparison1941 to 2004 comparison: Glacier Bay National Park and Preserve's White Thunder Ridge as seen on August 13, 1941 (left) and August 31, 2004 (right). Muir Glacier has retreated out of the field of view, Riggs Glacier has thinned and retreated significantly, and dense new vegetation has appeared. Muir Glacier was more than 2,000 feet thick in 1941. 2004 USGS photo by B.F. Molnia; 1941 photo by W.O. Field. See Repeat Photography of Glaciers in the Glacier Photograph Collection to access this and other photograph pairs.

South Cascade Glacier1928 to 2000 comparison: These photos of the South Cascade Glacier in the Washington Cascade Mountains show dramatic retreat between 1928 and 2000. Photos courtesy USGS.

Mountain glaciers occur on all continents except Australia. In this text, mountain glaciers exclude the large ice sheets of Greenland and Antarctica and the surrounding smaller ice caps. The world's glaciers have an estimated total area of about 525,000 square kilometers. Large collections of aerial and ground photographs can be used to study historic changes in some glaciers (see Repeat Photography of Glaciers in the Glacier Photograph Collection). In the last few decades, satellite imagery has provided a means to monitor glacier extent changes worldwide, greatly expanding the areas of ice that scientists can examine. The Global Land Ice Measurements from Space (GLIMS) project, with participation from more than 60 institutions in 28 nations, has assembled a baseline study to quantify the areal extent of existing glaciers (Raup et al. 2007, Kargel et al. 2014).

Scientists can study short-term changes in the extent of snow cover and sea ice to gauge climatic conditions, but glaciers are different. Glaciers continually move, carrying mass downhill somewhat like a conveyor belt. If the combination of climate and ice dynamics determines that the glacier is advancing, the advance of the glacier terminus expands the overall glacier area. Because glaciers move slowly, however, a time lag can occur between the changing climatic conditions and the resulting glacier advance or retreat. This time lag may last several decades, and is determined by complicated processes that control how fast the glacier moves. Scientists have confirmed, however, that the widespread retreat of glaciers across the world today is a result of our warming global temperature (Roe et al. 2016).

Direct measurement methods have been developed to determine the year-to-year mass balance, or "health," of a glacier. During winter, a glacier gains mass from accumulating snow. During the following summer, some or all of that winter accumulation is lost to ablation. The difference between the accumulation and ablation for a given year describes the annual net mass balance, which corresponds to the change in glacier thickness and volume.

For glaciers outside Antarctica or Greenland—referred to here as subpolar and mountain glaciers—researchers have compiled and analyzed numerous measurements of existing mass balance (Dyurgerov and Meier 1997, Cogley and Adams 1998, Dyurgerov 2002, Cogley 2002, Dyurgerov and Meier 2005, Kaser et al. 2006, Meier et al. 2007, Gardner et al. 2013, Zemp et al. 2015). Glaciers involved in mass balance studies are sparsely distributed over all mountain and subpolar regions, with about 70 percent of the observations coming from the mountains of Europe, North America, and the former Soviet Union. The Randolph Glacier Inventory (RGI) provides a globally complete inventory of glacier outlines. The Global Land Ice Measurements from Space (GLIMS) project released Version 5.0 of the RGI in 2015 (Arendt et al. 2015).

Continuous mass balance records have been kept for about 40 glaciers since the early 1960s. These results indicate that, in most regions of the world, glaciers are shrinking in mass. From 1961 to 2005, the thickness of "small" glaciers decreased approximately 12 meters, or the equivalent of more than 9,000 cubic kilometers of water.

New research published in 2014 indicated that, starting in the late 1970s, glaciers "crossed an invisible line" into a declining state that cannot easily be attributed to natural causes (Marshall 2014). The study relied on multiple global climate models to simulate mass balance of glaciers worldwide, excluding Antarctica, from 1851 to 2010. The authors concluded that, for the entire period, only 25 ± 35 percent of the glacier mass loss could be attributed to anthropogenic causes, but from 1991 to 2010, the glacier mass loss increased to 69 ± 24 percent. The study found that the anthropogenic signal from 1991 to 2010 was detectable with high confidence (Marzeion et al. 2014). Another recent study determined that centennial glacier retreat is categorical evidence of regional climate change (Roe et al. 2016).

A study of observational data sets from the World Glacier Monitoring Service (WGMS) concluded that "rates of early 21st-century mass loss are without precedent on a global scale, at least for the time period observed and probably also for recorded history" (Zemp et al. 2015). The Bulletin of the American Meteorological Society State of the Climate in 2015 reported that the cumulative mass balance loss between 1980 and 2015 is "the equivalent of cutting a 20.5 m thick slice off the average glacier" (Pelto 2016).

Graph of glacier balance, 1980-2015Glacier balance, 1980-2015: This graph shows mass balance for 41 reference glaciers monitored by the WGMS. The bars indicate mean annual glacier balance, and the line indicates cumulative annual balance. Graph from Pelto 2016.

Glacier thickness change graphGlobal Glacier Thickness Change: This shows average annual and cumulative glacier thickness change, measured in vertical meters, for the period 1961 to 2005. Explosive volcanic eruptions, which contribute dust to the stratosphere and cool the Earth's climate, can also affect glacier mass balance. Four significant eruptions with worldwide impacts are shown on this graph and are generally associated with periods of increased mass balance due to lowered temperatures. Image courtesy of Mark Dyurgerov, Institute of Arctic and Alpine Research, University of Colorado, Boulder.

In its Fourth Assessment Report released in 2007, the Intergovernmental Panel on Climate Change (IPCC) erroneously stated that Himalayan glaciers were likely to melt completely as soon as 2035. The miscalculation resulted from a lapse in the IPCC's review process. A report published by UNESCO in the 1990s predicted a catastrophic retreat by the year 2350 (Kotlyakov 1996), and Banerjee and Collins (2010) reported that the IPCC report introduced a typographic error that changed the number to 2035. Although the 2035 statement was wrong, peer-reviewed science does show that many Himalayan glaciers are retreating at an accelerated pace, and over 80 percent of the glaciers in western China have retreated over the past several decades (Bagla 2009).

A climate-and-cryosphere project of the World Climate Research Project is the Glacier Model Intercomparison Project (GlacierMIP). GlacierMIP issued an open call for data and received six published global glacier models in response. The models collectively produced 214 projections forced with 25 General Circulation Models (GCMs) and four Representative Concentration Pathways (emission scenarios): RCP2.6, RCP4.5, RCP6.0, and RCP8.5 (Hock et al. 2019). The study projected mass changes for glaciers outside the Antarctic and Greenland Ice Sheets (but including glaciers along the margins of those ice sheets) for the year 2100, relative to 2015. Global mass losses varied by model and emission scenario, though all models found significant mass loss. Averaged over the model runs, the losses ranged from 18 ± 7 percent for RCP2.6, to 36 ± 11 percent for RCP8.5.

An exception to the worldwide trend of glacier retreat occurs in the Karakoram region of the western Himalaya. Known as the Karakoram anomaly, glaciers in this this region have remained stable or even expanded over the past 150 years. The explanation for this unusual behavior may be meteorological. Other Himalayan glaciers receive most of their precipitation during the summer monsoon seasons, and have experienced reduced snowfall. The Karakoram, however, receives most of its precipitation from non-monsoonal winter storms, and the region stays mostly cool and dry during the summer (Kapnick et al. 2014). A subsequent study likewise found that the western portion of South Asia is the one region in the world where glaciers are slightly gaining mass (Zemp et al. 2019).

Glaciologists have long understood that greenhouse-gas emissions triggered by the Industrial Revolution contributed to glacier retreat. But in the Alps, the Industrial Revolution has driven glacier retreat by an additional mechanism: soot.

From roughly 1300 to 1850, glaciers in the European Alps were substantially bigger than they are today, a defining characteristic of the Little Ice Age. Around 1865, Alpine glaciers began retreating. In general, glaciers retreat due to a combination of rising temperatures and declining precipitation, but the temperature and precipitation records from the Alps indicated that glaciers should have continued advancing until around 1910. A more plausible explanation for the mid-19th-century retreat involves black carbon. Black-carbon emissions rose significantly after the mid-19th century, and didn't abate until well into the 20th century. By lowering albedo, soot from the Industrial Revolution raised the amount of solar radiation absorbed by snow, leading to Alpine glacier retreat. Glacier retreat starting in the mid-19th century was not a global phenomenon; glaciers in Argentina did not begin retreating until the early 20th century (Painter et al. 2013).

Last updated: 18 June 2019