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 2019/2020, 2020 was the 33rd year in a row that glaciers tracked by the World Glacier Monitoring Service lost rather than gained ice.
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. Continent-scale ice sheets still cover Greenland and Antarctica, while smaller ice caps and glaciers retreated to the world's high latitudes and mountains.
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.
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.
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 40 glaciers as "climate reference" glaciers.
In the 2020 update to their annual Global Glacier Change Bulletin, experts at the World Glacier Monitoring Service reported that glaciers in the reference network lost more than 1.2 meters (3.9 feet) water equivalent in both the 2017/18 and 2018/19 water years. The preliminary estimate for 2019/20 was just over 0.6 meters (2 feet) water equivalent. Tallying up each year's losses or gains, reference network glaciers have lost the equivalent of nearly 25 meters (82 feet) water equivalent relative to 1970—roughly the same as slicing an average of 27.5 meters (90 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 has 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.
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
WGMS (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.