Climate Change: Mountain glaciers

Among the most dramatic evidence that Earth's climate is warming is the retreat and disappearance of mountain glaciers around the world. Based on preliminary data for 2022/23, 2023 was the 36^th year in a row that the reference glaciers tracked by the World Glacier Monitoring Service lost rather than gained ice.

Glaciers that exist today are mostly 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. Continent-scale ice sheets still cover Greenland and Antarctica, while smaller ice caps and glaciers retreated to the world's high latitudes and mountains.

Roughly 20,000 years ago, during the Last Glacial Maximum of the Pleistocene Ice Age, ice spread over much of North America and Eurasia. (High-resolution without annotations available.) Image by Climate.gov based on data from the University of Zurich Applied Sciences, provided by Science on a Sphere.

Originally, scientists began studying glaciers only for the clues they offered about Earth’s climate during past ice ages and the role they played in shaping the landscape. Today, they are also trying to understand how quickly human-caused climate change will cause them to disappear altogether. 

Pedersen Glacier, at Aialik Bay in Alaska’s Kenai Mountains, in 1917 (left) and 2005 (right). In the early 20th century, the glacier met the water and calved icebergs into a marginal lake near the bay. By 2005, the glacier had retreated, leaving behind sediment allowed the lake to be transformed into a small grassland. Photos courtesy of Louis H. Pedersen (1917) and Bruce F. Molina (2005), obtained from the Glacier Photograph Collection, Boulder, Colorado USA: National Snow and Ice Data Center/World Data Center for Glaciology. Large images: 1917 | 2005

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. Those that end in the ocean are called tidewater glaciers, and they have more complex cycles of advance and retreat than glaciers that terminate on land, at least on annual and decades-long time scales. Even in a stable climate, such glaciers can experience periods of rapid retreat that are more influenced by seafloor topography and ocean circulation at their terminus than recent climate conditions.

Retreat of southeastern Alaska’s Muir Glacier between August 1941 and August 2004. Historically, Muir Glacier was an iceberg-calving, tidewater glacier. Its terminus was grounded in the waters of Muir Inlet, a narrow opening to Glacier Bay. In the six decades between the two photos, it has retreated so far that it's terminus is now inland. Photos from the National Snow and Ice Data Center’s collection of repeat photography of glaciers.

To see if a glacier is growing or shrinking, glacier experts 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. Changes in the area and terminus of larger glaciers can also be tracked with satellite imagery.

Global 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 global collection of about 60 glaciers as "climate reference" glaciers. A given year's annual update will include 40 or so of the reference glaciers. (Not all glaciers are surveyed every year). 

Photos documenting the disintegration of Italy’s Careser Glacier between 1933 (top) and 2012 (bottom). After decades of negative mass balance, glaciers worldwide are shrinking, fragmenting, or disappearing. Photos courtesy of Luca Carturan, University of Padova

According to the WGMS, the most recent hydrological years (2021/22 and 2022/23) have each brought ice losses well above the average of even the past full decade (2010-2019). Observed reference glaciers experienced an ice loss of 1.1 meters of water equivalent and 1.2 meters of water equivalent, respectively. That's roughly 4 feet of ice off each glacier in the network each year. "With this," they report on their website, "seven out of the ten most negative mass-balance years have been recorded after 2010." 

Tallying up each year's losses or gains through 2023, reference network glaciers have lost just over 26 feet of water equivalent relative to 1970—roughly the same as slicing an average of 28.6 meters (94 feet) off the top of each glacier. 

In State of the Climate in 2019, glacier expert Mauri Pelto reported that the pace of glacier loss in the WGMS reference network accelerated from -171 millimeters (6.7 inches) per year in the 1980s, to -460 millimeters (11 inches) per year in the 1990s, to -500 millimeters (1.6 feet) per year in the 2000s, to -889 millimeters (2.9 feet) per year for the 2010s. 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. Their accelerating retreat poses major challenges for people and nature.

References

Pelto, M., WGMS Network. (2020). Alpine glaciers [in State of the Climate in 2019]. Bulletin of the American Meteorological Society, 101(8), S37–S38. doi.org/10.1175/2020BAMSStateoftheClimate.1.

Pfeffer, W. T. (2003). Tidewater glaciers move at their own pace. Nature, 426(6967), 602–602. https://doi.org/10.1038/426602b

World Glacier Monitoring Service (2020, updated, and earlier reports). Global Glacier Change Bulletin No. 3 (2016-2017). Zemp, M., Gärtner-Roer, I., Nussbaumer, S. U., Bannwart, J., Rastner, P., Paul, F., and Hoelzle, M. (eds.), ISC(WDS)/IUGG(IACS)/UNEP/UNESCO/WMO, World Glacier Monitoring Service, Zurich, Switzerland, 274 pp., publication based on database version: doi:10.5904/wgms-fog-2019-12.

World Glacier Monitoring Service. (n.d.) Global Glacier State. Accessed May 10, 2024.