Retreat of glaciers since 1850

• 1. The Long Winter's End

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  Alaska's McCarty Glacier retreated about 10 kilometers in less than a century, and dense, diverse vegetation has become established on the slopes. The retreat of glaciers since 1850 is a well-documented effect of climate change. This phenomenon 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 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.

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• 2. The Balance Of Ice And Water

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  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. 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.

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• 3. Mountains In Decline

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  The Trift Glacier had the greatest recorded retreat, losing 100 meters of its length between 2003 and 2005. The Grosser Aletsch Glacier is the largest glacier in Switzerland and has been studied since the late 19th century. Aletsch Glacier retreated from 1880 to 2009. This rate of retreat has also increased since 1980, with 30%, or 4 kilometers, of the total retreat occurring in the last 20% of the time period. The Morteratsch Glacier in Switzerland has had one of one of the longest periods of scientific study with yearly measurements of the glacier's length commencing in 1878. The overall retreat from 1878 to 1998 has been 1.6 kilometers with a mean annual retreat rate of approximately 10 meters per year. This long-term average was markedly surpassed in recent years with the glacier receding 20 meters per year during the period between 1999 and 2005. In the Italian Alps, only about a third were in retreat in 1980, while by 1999, 89% of these glaciers were retreating. In 2005, the Italian Glacier Commission found that 123 glaciers in Lombardy were retreating. A random study of the Sforzellina Glacier in the Italian Alps indicated that the rate of retreat from 2002 to 2006 was much higher than in the preceding 35 years.

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• 4. The Rising Tide

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  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. 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 70 meters. Although previously it was thought that the polar ice caps were not contributing heavily to sea level rise (IPCC 2007), recent studies have confirmed that both Antarctica and Greenland are contributing about 1 millimeter 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 (65 centimeters) and backstops neighboring glaciers that would raise sea levels an additional 8 feet (2.4 meters) if all the ice were lost.

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• 5. The Drying Of Rivers

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  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.

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• 6. Floods From The Ice

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  Glacier meltwater left behind by the retreating glacier is often held back by moraines that can be unstable and have been known to collapse if breached or displaced by earthquakes, landslides or avalanches. If the terminal moraine is not strong enough to hold the rising water behind it, it can burst, leading to a massive localized flood. The likelihood of such events is rising due to the creation and expansion of glacial lakes resulting from glacier retreat. Past floods have been deadly and have resulted in enormous property damage. Towns and villages in steep, narrow valleys that are downstream from glacial lakes are at the greatest risk. In 1892 a Glacial Lake Outburst Flood released some of 300,000 cubic meters of water from the lake of the Tête Rousse Glacier, resulting in the deaths of 200 people in the French town of Saint-Gervais-les-Bains. GLOFs have been known to occur in every region of the world where glaciers are located. Continued glacier retreat is expected to create and expand glacial lakes, increasing the danger of future GLOFs. A growing concern is the potential for GLOFs researchers estimate 21 glacial lakes in Nepal and 24 in Bhutan pose hazards to human populations should their terminal moraines fail.

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• 7. The Vanishing Future

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  Based on current pledges, global mean temperature is projected to increase by +2.7 °C, which would cause loss of about half of Earth's glaciers by 2100 with a sea level rise of 115±40 millimeters. Research, published in 2019 by ETH Zurich, says that two-thirds of the ice in the glaciers of the Alps is doomed to melt by the end of the century due to climate change. In the most pessimistic scenario, the Alps will be almost completely ice-free by 2100, with only isolated ice patches remaining at high elevation. On the 13th of August 2005, the Ayles Ice Shelf broke free from the north coast of Ellesmere Island. The ice shelf drifted into the Arctic Ocean. This followed the splitting of the Ward Hunt Ice Shelf in 2002. The Ward Hunt has lost 90% of its area in the last century. A study published in 2015 concluded that Totten Glacier, has the largest contribution of ice thinning rate on the East Antarctic continent, and that the thinning is driven by enhanced basal melting, because of ocean processes, and affected by polynya activity.

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