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 attribute this retreat to the unintended effects of the Industrial Revolution and modern energy use; burning fossil fuels and other economic activities release greenhouse gases such as carbon dioxide and methane into the atmosphere and affect our environment.

Over long periods, glacial response to climate change becomes obvious.

1941-2004 comparison: Glacier Bay National Park and Reserve'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.

1928-2000 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 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 glaciers going back to some of the earliest photographs available (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. 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 as well as moving, the advance of the terminus expands the overall glacier area. Because glaciers move slowly, however, a significant time lag occurs 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.

More direct 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). 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.

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).

Global 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.

Last updated: 12 November 2014