Retreat of glaciers since 1850

• 1. Introduction

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  The retreat of glaciers since 1850 is tracked with stakes, GPS surveys, aerial photographs and satellite altimetry across mountain ranges on every continent except mainland Australia. Between 1993 and 2018, the world's glaciers lost roughly 5,500 gigatons of ice, an average of 210 gigatons a year. That loss did not happen at a steady pace. Retreat sped up, slowed, and even reversed in different decades before accelerating again. What turns a glacier from a growing mass of ice into a shrinking remnant? Why did the pace of melt ease for a few decades only to surge again? And what does the loss of that ice mean for people who rely on it for drinking water, crops, and protection from sudden floods?

• 2. The Anatomy Of Retreat

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  In Washington state, the Easton Glacier is expected to shrink to roughly half its current size. The pace of loss will then slow until the glacier settles into a smaller, stable size. The reason lies in a balance sheet glaciologists call mass balance. It weighs snow gained in a glacier's accumulation zone against ice lost to melting in its ablation zone. When accumulation outweighs ablation, a glacier advances. When ablation wins, it retreats, and nearly every glacier studied today runs a negative balance.

  Montana's Grinnell Glacier shows the harsher version of that arithmetic. Its upper section is bare, melting and thinned, unlike Easton's, which stays healthy and snow covered. Researchers expect Grinnell to keep shrinking at an increasing rate until it disappears entirely. Thinning along a glacier's entire length is the clearest warning sign of that fate. It shows the accumulation zone itself is shrinking, not only the terminus at the glacier's foot.

  On Mount Stanley, a team from Project Pressure built the first three dimensional model of a glacier using drone photography and GNSS positioning. Their survey recorded a 29.5 percent decline in surface area between 2020 and 2024 alone. Glaciologists elsewhere track the same slow collapse using staked terminus markers, GPS mapping, aerial photography and laser altimetry. Small glaciers with a narrow range of elevation are the most likely to fall out of equilibrium and vanish for good.

• 3. A Global Timeline

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  Between about 1550 and 1850, a stretch of cooler climate known as the Little Ice Age settled over many regions. Glaciers there stayed larger than in the centuries before or after. Once that cool period ended, glaciers worldwide retreated as the climate warmed, a trend that continued until roughly 1940. Retreat then slowed, and in many cases reversed for a few decades, as global temperatures cooled slightly between 1950 and 1980. Since 1980, the pace and reach of glacier retreat have both increased sharply. Some glaciers have vanished completely, and many that remain are threatened with the same fate.

  In the Alps, researchers have traced part of the region's retreat to a specific cause. Soot from industrial black carbon dimmed the ice's reflectivity there after 1850. That darkening may have cut short an expansion of Alpine glaciers that could otherwise have continued until around 1910. More broadly, human activity since the start of the industrial era has driven up carbon dioxide and other heat-trapping gases in the atmosphere. That buildup is the principal driver of today's global warming. It also drives broader changes across the cryosphere, the glaciers, ice sheets and sea ice that make up Earth's frozen regions.

  The polar ice sheets are already following that same pattern of loss. If they were to melt completely, scientists estimate the world's oceans would rise by about 70 metres.

• 4. Rising Seas

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  The IPCC, the United Nations' climate panel, calls the ocean effect of glacial runoff a slow onset event. That means it builds gradually rather than arriving all at once. For years, the IPCC's 2007 assessment left rapid ice sheet decay out of its sea level predictions. That omission made the true scale of the risk difficult to pin down. A 2008 study still concluded that sea level would rise by at least 0.8 meters by 2100, even under conservative assumptions. More recent research confirms that Antarctica and Greenland are each now adding about 0.5 millimeters a year to global sea levels.

  The Thwaites Glacier in West Antarctica alone accounts for about 4 percent of global sea level rise today. It holds enough ice on its own to raise the oceans a little over 2 feet, or 65 centimeters. Thwaites also backstops neighboring glaciers holding an additional 8 feet, or 2.4 meters, of potential rise if it were to fail. Satellite data recorded its ice shelf melting at 207 meters a year between 2014 and 2017, the fastest rate ever recorded in Antarctica.

  Greenland's ice sheet lost mass at a quickening pace between 1996 and 2005. Its net yearly loss roughly doubled, from about 90 cubic kilometers to 220 cubic kilometers. By 2005 that acceleration had spread to almost every glacier south of 70 degrees north.

  For roughly half a century, Jakobshavn Isbrae in west Greenland was the fastest moving glacier on Earth. It advanced more than 24 meters a day with a stable front since at least 1950. In 2002 its floating terminus began a rapid retreat, accelerating past 30 meters a day as the ice front broke apart. Researchers say the glacier has since "slammed the brakes" and is now thickening by about 20 meters in height every year.

  A 2019 study found that Antarctica is now losing ice six times faster than it was 40 years earlier. Pine Island and Thwaites glaciers are melting five times faster than they were in the early 1990s.

• 5. Water Under Threat

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  In the Himalayas, retreating glaciers could cut summer water flows by as much as two thirds. In the Ganges basin alone, that kind of drop would leave roughly 500 million people short of water. Across the wider Hindu Kush Himalaya region, some 1.4 billion people depend on the five major rivers that the mountains feed. Meltwater volumes are likely to rise at first as glaciers retreat, then fall as the remaining ice mass shrinks.

  In South America, numerous artificial lakes used for irrigation are filled almost entirely by glacial melt. Central Asian countries have long relied on seasonal glacier melt for both irrigation and drinking water. In Norway, the Alps and the Pacific Northwest of North America, glacier runoff also feeds hydropower generation. The same meltwater that irrigates crops and drives turbines also keeps rivers cold enough for the species that depend on them.

• 6. Fragile Ecosystems

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  Salmon and cutthroat trout are among the freshwater fish that need cold water to survive and reproduce. When glacial runoff drops, stream flow can become too weak to sustain them. Freshwater entering the ocean from melting glaciers can also alter ocean currents. That includes thermohaline circulation, the deep ocean system that helps drive global water movement. Changes there carry knock-on effects for fisheries that people depend on.

  Retreating glaciers also leave behind unstable dams of rock and debris called moraines, holding back meltwater that has nowhere else to go.

• 7. Sudden And Deadly Floods

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  In 1892, a glacial lake outburst flood released about 200,000 cubic meters of water from the Tete Rousse Glacier. The flood killed 200 people in the French town of Saint-Gervais-les-Bains. Moraines like the one that failed there can give way if shaken by an earthquake, struck by a landslide, or otherwise breached. A breach releases a sudden, often deadly flood into the valley below. Towns built in steep, narrow valleys beneath glacial lakes face the greatest danger. Outburst floods of this kind have occurred in every glaciated region of the world.

  In Bhutan, the glacial lake Raphstreng Tsho measured 1.6 kilometers long, 0.96 kilometers wide and 80 meters deep in 1986. By 1995 it had grown to 1.94 kilometers long, 1.13 kilometers wide and 107 meters deep. In 1994, an outburst flood from the neighboring Luggye Tsho lake killed 23 people downstream. Researchers have identified 21 glacial lakes in Nepal and 24 in Bhutan whose failing moraines could pose similar hazards to people living below them.

  Continued glacier retreat is expected to create and expand glacial lakes of this kind, increasing the danger of future outburst floods.

• 8. Vanishing Peaks

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  On Kilimanjaro, glacier cover shrank by 75 percent between 1912 and 2006, and total ice volume fell by 80 percent over the same period. A March 2005 report found that the summit had been exposed for the first time in 11,000 years. In 2006, Al Gore predicted that within a decade there would be no more snows of Kilimanjaro. By 2006, a growing hole through the Furtwaengler Glacier's full 6 meter thickness was expected to split it in two within a year.

  On New Guinea's Puncak Jaya, the ice cap has lost about 80 percent of its area since 1936. Two thirds of that loss happened after a scientific expedition surveyed the peak in the 1970s. Nearby, the ice cap atop Wilhelmina Peak vanished completely between 1939 and 1963. The Mandala ice cap disappeared in the 1990s, and the Idenburg glacier dried up in 2003.

  Switzerland's Grosser Aletsch Glacier, the largest in the country, retreated 2.8 kilometers between 1880 and 2009. Nearly a third of that total retreat, some 800 meters, occurred in the final fifth of that period, after 1980. Tajikistan's Fedchenko Glacier stretches 70 kilometers, making it the largest non-polar glacier on Earth. It retreated 1 kilometer between 1933 and 2006, and lost 44 square kilometers of surface area from 1966 to 2000.

  Argentina's Perito Moreno Glacier tells a different story. It has advanced since 1947 and has stayed essentially stable since 1992. It is one of only three glaciers in Patagonia to advance, while several hundred others across the region continue to retreat.