DThe Greenland ice sheet has lost around a fifth more mass in recent decades than previously thought. In a recent paper in the journal Nature, scientists from NASA's Jet Propulsion Laboratory come to the conclusion that more than 1,000 gigatons of additional ice have melted since 1985 due to the retreat of coastal glaciers.
Surprisingly, changes in the floating ends of glaciers on the coast have been largely ignored in previous estimates of ice loss. Glaciologist Chad Greene and his co-authors used satellite images to map the location of the glacier fronts in the period 1985 to 2022. The researchers were able to detect glacier retreat on almost all of the 250 glaciers examined.
On the coast of Greenland, numerous ice streams flow into the ocean in fjords. At the front end of the glaciers, the ice floats on the sea. On this so-called calving front, icebergs then break off, drift into the sea and finally melt.
Addition to previous estimates
“This is a completely normal process that, in addition to the meltwater runoff, forms a counterweight to the accumulation of snow inland,” says Ingo Sasgen from the Alfred Wegener Institute (AWI) in Bremerhaven. “However, the study addresses a parameter that is important for the dynamics of the Greenland ice sheet: the change in the position of the calving front.”
The American scientists were now able to prove that the icebergs breaking off had significantly intensified in the 2000s. The glaciers lost mass accordingly and retreated from the fjords inland.
Estimates of the mass balance of Greenland's glaciers have so far only taken into account the melting of the ice on the mainland. According to geodesy professor Martin Horwarth from the TU Dresden, there are good reasons for this. “Only changes in ice mass on land are relevant to sea level rise. Because if ice that is already floating on the ocean water melts, according to Archimedes’ principle, sea level will not change.”
Like an ice cube in a glass of water
The additional 1,000 gigatons of melted ice that have now been determined have not contributed to sea level rise. The glacier ice near the calving fronts was already below sea level in the deep fjords and displaced sea water accordingly. Ocean water has long since filled the now ice-free space again.
Even if the sea level does not change due to the melting of the floating ice portions, meaning that coastal residents do not have to fear any damage from the melting of the front glacier front, the accounting of the glacier ends represents a sensible new target for Martin Horwarth. “It is important to strengthen the floating glacier tongues to take the look. Their changes ultimately also control the flow behavior of the ice on land.”
The study authors also emphasize that the change in mass of the floating ends of glaciers is also important for the future assessment of freshwater input into the ocean.
Meltwater from the Greenland ice sheet influences the dynamics of the Atlantic Overturning Movement (AMOC) off Greenland. Differences in density allow cold, salty water to sink into the deep sea. This drives a global conveyor belt that transports large amounts of thermal energy from the southern to the northern hemisphere. The influx of fresh water disrupts this process. Researchers have been measuring a weakening of the AMOC for several years. According to the scientists, if the conveyor belt fails completely, global effects are to be feared. Like the Greenland ice sheet, the AMOC is one of the central tipping elements of the climate system.
Record melt in August 2023
According to the National Snow and Ice Data, mainland Greenland experienced its third highest melt season in 2023 since records began 45 years ago. The ice sheet thawed to a total of 32.9 million square kilometers.
While the melt progressed slowly in the first few months of the season, by mid-June it had increased to almost half the ice sheet's area. On August 22nd, a special weather situation caused a record-breaking melting event covering around 750,000 square kilometers. The mass loss on the mainland was also higher in 2023 than the long-term average.
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