Retreat of glaciers since 1850

Example of a mountain glacier retreat: White Chuck Glacier, Washington

White Chuck Glacier in the United States in 1973

Same vantage point in 2006, at the same time of the year. The glacier retreated 1.9 kilometres (1.2 mi) in 33 years.

The retreat of glaciers since 1850 is a well-documented effect of climate change. The retreat of mountain glaciers provide evidence for the rise in global temperatures since the late 19th century. Examples include mountain glaciers in western North America, Asia, the Alps in central Europe, and tropical and subtropical regions of South America and Africa. Since glacial mass is affected by long-term climatic changes, e.g. precipitation, mean temperature, and cloud cover, glacial mass changes are one of the most sensitive indicators of climate change. The retreat of glaciers is also a major reason for sea level rise. Excluding peripheral glaciers of ice sheets, the total cumulated global glacial losses over the 26 years from 1993 to 2018 were likely 5500 gigatons, or 210 gigatons per year.^[1]: 1275

On Earth, 99% of glacial ice is contained within vast ice sheets (also known as "continental glaciers") in the polar regions. Glaciers also exist in mountain ranges on every continent other than the Australian mainland, including Oceania's high-latitude oceanic island countries such as New Zealand. Glacial bodies larger than 50,000 km² (19,000 sq mi) are called ice sheets.^[2] They are several kilometers deep and obscure the underlying topography.

Deglaciation occurs naturally at the end of ice ages. But the current glacier retreat is accelerated by global warming due to human-caused greenhouse gas emissions. Human activities since the start of the industrial era have increased the concentration of carbon dioxide and other heat-trapping greenhouse gases in the air, causing current global warming.^[3] Human influence is the principal driver of changes to the cryosphere, of which glaciers are a part.^[3]

The glacier mass balance is the key determinant of the health of a glacier. If the amount of frozen precipitation in the accumulation zone exceeds the quantity of glacial ice the ablation zone lost due to melting, a glacier will advance. If the accumulation is less than the ablation, the glacier will retreat. Glaciers in retreat will have negative mass balances. They will eventually disappear if they do not reach an equilibrium between accumulation and ablation.

Mid-latitude mountain ranges show some of the largest proportionate glacial losses. Examples of such mountain ranges are the Himalayas in Asia, the Rocky Mountains and the Cascade Range in North America, the Alps in Europe, the Southern Alps in New Zealand, the southern Andes in South America, as well as isolated tropical summits such as Mount Kilimanjaro in Africa.

Glacial ice is the largest reservoir of fresh water on Earth, holding with ice sheets about 69 percent of the world's freshwater.^[4][5] The retreat of glaciers has near term impacts on the availability of fresh water for drinking water and irrigation. For example, in the Andes and Himalayas the demise of glaciers will affect water supplies for people in that region.^[6] Melting glaciers also lead the sea level rise.