Retreat of glaciers since 1850
The retreat of glaciers since 1850 is a well-documented effect of climate change. The retreat of mountain glaciers provides 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.
On Earth, 99% of glacial ice is contained within vast ice sheets 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 are called ice sheets. 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. Human influence is the principal driver of changes to the cryosphere, of which glaciers are a part.
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. 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. Melting glaciers also leads to sea level rise.
Scale at the global level
The Little Ice Age was a period from about 1550 to 1850 when certain regions experienced relatively cooler temperatures compared to the time before and after. Subsequently, until about 1940, glaciers around the world retreated as the climate warmed substantially. Glacial retreat slowed and even reversed temporarily, in many cases, between 1950 and 1980 as global temperatures cooled slightly.Since 1980, climate change has led to glacier retreat becoming increasingly rapid and ubiquitous, so much so that some glaciers have disappeared altogether, and the existence of many of the remaining glaciers is threatened. 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 yr.
Causes
The mass balance, or difference between accumulation and ablation, of a glacier is crucial to its survival. Climate change may cause variations in both temperature and snowfall, resulting in changes in mass balance. A glacier with a sustained negative balance loses equilibrium and retreats. A sustained positive balance is also out of equilibrium and will advance to reestablish equilibrium. Currently, nearly all glaciers have a negative mass balance and are retreating.Glacier retreat results in the loss of the low-elevation region of the glacier. Since higher elevations are cooler, the disappearance of the lowest portion decreases overall ablation, thereby increasing mass balance and potentially reestablishing equilibrium. If the mass balance of a significant portion of the accumulation zone of the glacier is negative, it is in disequilibrium with the climate and will melt away without a colder climate and/or an increase in frozen precipitation.
For example, Easton Glacier in Washington state, U.S. will likely shrink to half its size but at a slowing rate of reduction and stabilize at that size despite the warmer temperature over a few decades. However, the Grinnell Glacier in Montana, U.S. will shrink at an increasing rate until it disappears. The difference is that the upper section of Easton Glacier remains healthy and snow-covered, while even the upper section of the Grinnell Glacier is bare, is melting and has thinned. Small glaciers with minimal altitude range are most likely to fall into disequilibrium with the climate.
Measurement techniques
Methods for measuring retreat include staking terminus location, global positioning mapping, aerial mapping and laser altimetry. The key symptom of disequilibrium is thinning along the entire length of the glacier. This indicates a diminishment of the accumulation zone. The result is marginal recession of the accumulation zone margin, not just of the terminus. In effect, the glacier no longer has a consistent accumulation zone and without an accumulation zone cannot survive. On Mount Stanley, an exhibition team sent out by Project Pressure created the first ever 3D model of the glacier using drone photography and GNSS technology showing a surface area decline of 29.5% between 2020 and 2024.Impacts
Sea level rise
Water runoff from melting glaciers causes global sea level to rise, a phenomenon the IPCC terms a "slow onset" event.The potential for major sea level rise depends mostly on a significant melting of the polar ice caps of Greenland and Antarctica, as this is where the vast majority of glacial ice is located. If all the ice on the polar ice caps were to melt away, the oceans of the world would rise an estimated. Although previously it was thought that the polar ice caps were not contributing heavily to sea level rise, recent studies have confirmed that both Antarctica and Greenland are contributing a year each to global sea level rise. The Thwaites Glacier alone, in Western Antarctica is "currently responsible for approximately 4 percent of global sea level rise. It holds enough ice to raise the world ocean a little over 2 feet and backstops neighboring glaciers that would raise sea levels an additional 8 feet if all the ice were lost." The fact that the IPCC estimates did not include rapid ice sheet decay into their sea level predictions makes it difficult to ascertain a plausible estimate for sea level rise but a 2008 study found that the minimum sea level rise will be around by 2100.
Water supply
The continued retreat of glaciers will have a number of different quantitative effects. In areas that are heavily dependent on water runoff from glaciers that melt during the warmer summer months, a continuation of the current retreat will eventually deplete the glacial ice and substantially reduce or eliminate runoff. A reduction in runoff will affect the ability to irrigate crops and will reduce summer stream flows necessary to keep dams and reservoirs replenished. This situation is particularly acute for irrigation in South America, where numerous artificial lakes are filled almost exclusively by glacial melt. Central Asian countries have also been historically dependent on the seasonal glacier melt water for irrigation and drinking supplies. In Norway, the Alps, and the Pacific Northwest of North America, glacier runoff is important for hydropower.In the Himalayas, retreating glaciers could reduce summer water flows by up to two thirds. In the Ganges area, this would cause a water shortage for 500 million people. In the Hindu Kush Himalaya area, around 1.4 billion people are dependent on the five main rivers of the Himalaya mountains. Although the impact will vary from place to place, the amount of meltwater is likely to increase at first as glaciers retreat. Then it will gradually decrease because of the fall in glacier mass.