Greenland ice sheet

• 1. Introduction

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  On the 22nd of December 1991, an automatic weather station near the topographic summit of the Greenland ice sheet logged a temperature of -69.6 C. That reading was the lowest ever recorded in the Northern Hemisphere. It went unnoticed for more than 28 years, and was finally recognized only in 2020. The ice that produced such cold is the second-largest body of ice on Earth. It averages 1673 m thick, reaches over 3488 m at its deepest, and covers 1710000 km2, roughly 80% of the surface of Greenland. Beneath it lie mountains and lake basins. This is a place where a fighter plane that crashed early in World War II was found buried under 268 ft of ice. How did a single sheet of ice come to swallow an island, what is it now releasing as it melts, and what would the world look like if all 2900000 km3 of it were gone?

• 2. Birth Of An Island Of Ice

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  For at least 18 million years Greenland has carried major glaciers and ice caps, yet for most of that span these were scattered bodies, not one sheet. Conditions began to change around 10 million years ago, during the middle Miocene, when the passive continental margins that now form the uplands of West and East Greenland were lifted upward. That uplift created an upper planation surface at a height of 2000 to 3000 m above sea level. Later uplift during the Pliocene formed a lower planation surface at 500 to 1000 m, and a third stage carved valleys and fjords below them. As recently as 3 million years ago, during the Pliocene warm period, Greenland's ice clung only to the highest peaks in the east and south. The separate ice caps finally merged once atmospheric carbon dioxide fell to between 280 and 320 ppm, around 2.7 to 2.6 million years ago, when a single ice sheet first covered most of the island some 2.6 million years ago. Ice forms through glaciation, as yearly snow layers pile up and their weight compresses the deeper snow into firn and then solid glacier ice over hundreds of years. The weight makes the ice flow, unless a large obstacle such as a mountain stops it. Greenland's coastal mountains usually hold the ice back, but in 11 periods of its history the sheet grew large enough to flow over them, extending up to 120 km beyond its current boundaries, with the last such episode around 1 million years ago.

• 3. What The Ice Remembers

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  A 3 km deep ice core drilled from the summit has revealed ice that is about 1,000,000 years old, the oldest known ice on Greenland. Near the summit, the sheet slides over a basal layer that froze solid to the ground, preserving ancient soil that drilling can recover. The oldest such soil had been continuously covered by ice for around 2.7 million years. Subtle differences in the oxygen isotope composition of water molecules in a core reveal the water cycle of the time, while air bubbles frozen inside hold a snapshot of the atmosphere's gases and particles. Properly read, cores reconstruct past temperature, precipitation, volcanic eruptions, solar variation, ocean primary production, and even soil vegetation and wildfire frequency. They also record human impact, including lead production during the time of Ancient Greece and the Roman Empire. Sediment samples from the Labrador Sea show that nearly all of the south Greenland ice had melted around 400,000 years ago, during Marine Isotope Stage 11. Cores from Camp Century in the northwest reveal that the ice there melted at least once during the past 1.4 million years, during the Pleistocene, and did not return for at least 280,000 years. Those periods, when less than 10% of the current ice volume remained, came with temperatures less than 2.5 C-change warmer than preindustrial conditions, which contradicts how climate models usually simulate continuous solid ice. The roughly 100,000-year records from cores drilled between 1989 and 1993 into the summit gave evidence of geologically rapid climate change, and informed research on tipping points such as the Atlantic meridional overturning circulation.

• 4. The Warming That Broke The Record

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  From the 1960s to the 1980s, an area of the North Atlantic that included southern Greenland was one of the few places on Earth that cooled rather than warmed. That same location had been relatively warmer in the 1930s and 1940s than in the decades around it. Fuller data sets later traced warming and ice loss back to 1900, and a phase of strong warming starting around 1979 that matched the decline of Arctic sea ice. In the years 1995 to 1999, central Greenland was already 2 C-change warmer than in the 1950s, and between 1991 and 2004 the average winter temperature at Swiss Camp rose almost 6 C-change. The 1970s were the last decade the sheet grew, gaining about 47 gigatonnes a year. From 1980 to 1990 it lost about 51 Gt a year on average, and 1996 was the last year it saw a net mass gain. By 2022 it had been losing ice for 26 years in a row, with temperatures the highest of the past millennium, about 1.5 C-change above the 20th century average. Losses accelerated through the 2000s, reaching about 187 Gt a year in 2000 to 2010 and 286 Gt a year from 2010 to 2018. Half of the net loss of 3,902 gigatons between 1992 and 2018, roughly 0.13% of the sheet's total mass, happened in those eight years. The sheet now loses more mass each year than the Antarctic ice sheet, because its Arctic position exposes it to intense regional amplification of warming.

• 5. The Glaciers At The Edge

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  Kangerlussuaq Glacier, in the southeast, is over 20 mi long, 4.5 mi wide and around 1 km thick, the third largest in Greenland. Between 1993 and 1998, parts within 5 km of the coast lost 50 m in height, and its flow accelerated to 8.7 mi per year by 2005, then the fastest known flow of any glacier. Retreat across the ice sheet's edges traces a larger collapse, with losses from glaciers explaining between 49% and 66.8% of observed ice loss since the 1980s. Net loss was already seen across 70% of the margins by the 1990s. Between 1998 and 2006, coastal glaciers thinned four times faster than in the early 1990s, falling between 1 m and 10 m a year, while landlocked glaciers barely changed. Jacobshavn Isbrae, also called Sermeq Kujalleq, in west Greenland is the single largest outlet glacier, historically shedding ice from 6.5% of the sheet at speeds of about 20 m per day. It retreated about 30 km between 1850 and 1964, held in balance for 35 years, then switched to rapid loss after 1997, shedding 94 km2 of ice between 2001 and 2005 and reaching 45 m per day in 2012 before slowing. Petermann Glacier, in the north, lost 85 km2 of floating ice in 2000 to 2001, a 28 km2 iceberg in 2008, and a 260 km2 iceberg in August 2010, the largest Arctic iceberg since 1962. In July 2012 it lost another iceberg of 120 km2, twice the area of Manhattan, and by 2023 its ice shelf had lost around 40% of its pre-2010 state. Helheim Glacier, the second largest, retreated rapidly in 2005 alongside a sharp rise in glacial earthquakes between 1993 and 2005, then held near that position, a reminder that glacier dynamics are hard to predict.

• 6. Why The Retreat Accelerates

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  Since the early 2000s, glaciologists have concluded that Greenland's glacier retreat is too fast to be explained by warmer surface air alone. The rapid calving at the largest glaciers matches what was first called the Jacobshavn effect in 1986, where thinning makes a glacier more buoyant, cuts the friction that resists retreat, and spreads a force imbalance across its mass. The acceleration from 1997 onward was tied to warming North Atlantic waters melting glacier fronts from below, and to a 1997 shift in circulation that pushed warmer currents from the Irminger Sea against West Greenland. By 2016, waters along much of that coast had warmed by 1.6 C-change relative to the 1990s. Meltwater at the surface offers another path, flowing down to bedrock through moulins where it lubricates the base, raises basal pressure, and speeds the ice, a mechanism seen at Sermeq Kujalleq in 1998 and 1999 when flow rose by up to 20% for two to three months. Once in the ocean, meltwater can create turbulent plumes that damage the calving front even without ocean warming, and observations of 13 glaciers found plumes matter most where grounding lines are shallow. Meltwater can also slip through cracks only 2 cm wide that reach several hundred meters down, weakening the ice and letting it flow faster. Alun Hubbard, one of the scientists behind these findings, described moulins where current scientific understanding does not accommodate their presence, because it disregards how they may form from hairline cracks without the large crevasses once thought necessary.

• 7. Slush, Algae, And Rain

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  In July 2012, the melt zone extended to 97% of the ice sheet's cover, the first directly observed massive melting event, when melting struck almost the entire surface at once. The sheet lost about 0.1% of its total mass that year, with a net loss of 464 Gt setting a record. That event revealed that cloud cover, which usually cools through albedo, can instead interfere with meltwater refreezing in the firn at night, raising runoff by over 30%. Ice cores showed the previous event of that magnitude was in 1889, prompting hope of a 150-year cycle, but summer 2019 disproved it with an even larger event covering over 300,000 mi2 and a record net loss of 586 Gt. The melt zone below the snow line has expanded at an accelerating rate since detailed measurements began in 1979, growing 16% by 2002. In July 2021 another record event peaked at 340,000 mi2 with daily losses of 88 Gt, and rain fell for 13 hours at Summit Station, at 10,551 ft elevation. Researchers had no rain gauges, because temperatures there have risen above freezing only three times since 1989 and it had never rained there before. Warmer temperatures also let darker algae grow on the surface, absorbing more radiation, and in 2020 algae were shown to be raising annual melting by 10 to 13%, an effect ice sheet models do not capture. As the surface lowers through melting, it warms further and struggles to rebuild ice, a process called surface-elevation feedback.

• 8. What The Meltwater Carries

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  Even in 1993, Greenland's melt sent 300 cubic kilometers of fresh meltwater into the seas each year, equivalent to 0.7% of the freshwater entering the oceans from all the world's rivers. That water carries iron, about half of it, around 0.3 million tons a year, bioavailable as a nutrient for phytoplankton. In the Labrador Sea, 40% of total primary production has been attributed to nutrients from this meltwater. The water also holds dissolved organic carbon from microbes on the surface and from ancient soil below, with about 0.5 to 27 billion tonnes of pure carbon beneath the sheet, far less than the 1400 to 1650 billion tonnes in Arctic permafrost. At Russell Glacier, meltwater carbon escapes as methane, though methanotrophic bacteria there limit those emissions. A 2021 claim of vast mercury deposits beneath the southwest was overturned in 2024, when a study of 21 locations found only very low concentrations and blamed the earlier result on contamination with mercury(II) chloride used as a reagent. A real hazard remains at Camp Century, a former United States military site built to carry nuclear weapons for Project Iceworm, which was never cleaned up and threatens to release nuclear waste, 20,000 liters of chemical waste, and 24 million liters of untreated sewage as the melt advances. The extra fresh water also reaches ocean circulation, linked to the cold blob in the North Atlantic and the apparent slowdown of the Atlantic meridional overturning circulation. A 2016 study projected the AMOC weakening around 18% by 2090 to 2100 under one scenario and 37% under a higher-emissions one, with a 44% chance of outright collapse in the worst case if extended toward 2290 to 2300.

  If all 2900000 km3 of the ice were to melt, global sea levels would rise by about 7.4 m. The sheet has already given up 1.4 cm of sea level rise since 1972, and warming between 1.7 C-change and 2.3 C-change would likely make full melting inevitable, though even 1.5 C-change would cost ice equal to 1.4 m of rise. A 2022 paper found that the 2000 to 2019 climate alone commits the sheet to about 27 cm of sea level rise, and that if the 2012 melt became normal the commitment would reach around 78 cm. The damage is uneven. The south is far more vulnerable, and the loss of ice is subtle enough to change the deformation of Earth's crust and its rotation, already making the East Coast of the United States rise faster than the global average. Around Greenland itself, isostatic rebound and weaker gravitational pull would cause local sea levels to fall even as they climb elsewhere. The reverse happened during the Little Ice Age, when added ice mass drew in more water and flooded certain Viking settlements, likely driving the Viking abandonment soon afterward. Polar amplification warms the Arctic three to four times faster than the global average, so during the Eemian interglacial 130,000 to 115,000 years ago the sheet was 8 C-change warmer and its northwest part 130 m lower than today. In 2015 the glaciologist Eric Rignot said even the most conservative people in the field agree that Greenland's ice is gone after 2 C-change or 3 C-change of warming. The fastest plausible disintegration takes 1000 years, set against a worst-case future of temperatures exceeding 10 C-change by 2500, while the longest estimate runs to 15,000 years, and the sheet would not begin to regrow until temperatures fall below preindustrial levels.