The Greenland ice sheet has lost 20 percent more ice than scientists previously thought, posing potential problems for ocean circulation patterns and sea level rise, according to a new study.
The additional loss comes from an area previously unaccounted for in estimates: ice lost at a glacier’s edges, where it meets the water. Before this study, estimates primarily considered mass changes in the interior of the ice sheet, which are driven by melting on the surface and glaciers thinning from their base on the ice sheet.
The study, released Wednesday in Nature, provides improved measurements of ice loss and meltwater discharge in the ocean, which can advance sea level and ocean models.
Loss from the edges of glaciers won’t directly affect sea level rise because they usually sit within deep fjords below sea level, but the freshwater melt could affect ocean circulation patterns in the Atlantic Ocean.
“We can take a look at the glaciers we have now and see how they’re behaving,” said Michael Wood, a study co-author and glaciologist. “That will give us a sense of what the future might hold for future ice loss from Greenland.”
The researchers tracked changes in 207 glaciers in Greenland (constituting 90 percent of the ice sheet’s mass) each month from 1985 to 2022. Analyzing more than 236,000 satellite images, they manually marked differences along the edges of glaciers and eventually trained algorithms to do the same. From the area measurements, the team could calculate the volume and mass of the changes in ice.
Glaciers can lose ice in many ways. One change can happen when large ice chunks break off at the edge, known as calving. They can also lose ice when it melts faster than it can form, causing the end of a glacier to retreat and move to higher elevations.
Scientists found that a total of 1,034 gigatons of ice was lost across all glaciers because of this retreat and calving on their peripheries. The loss accelerated since January 2000, with the glaciers losing a total of 42 gigatons each year. It has shown no signs of slowing down.
Most striking, nearly every glacier was shrinking — and in every corner of the ice sheet.
“This is a signal that’s touching every part of Greenland,” said Chad Greene, the study’s lead author and a scientist at NASA’s Jet Propulsion Laboratory. “There’s basically no part of Greenland that’s safe from climate change.”
Polar scientist Twila Moon, who was not involved in the research, said scientists have known that the ice sheet is experiencing long-term retreat on the coasts, but she called the study “the most complete quantification of that change published to date.”
The research “emphasizes that we have to remain attentive to the extensive peripheral changes happening across Greenland,” said Moon, a researcher at the National Snow and Ice Data Center.
The largest losses came from glaciers that experienced the biggest changes from season to season. Glaciers in Greenland accumulate mass and grow throughout the winter, then experience ice loss throughout the summer.
“The larger that range is … is a very strong indicator of how much ice that glacier has lost over the past two decades,” said Wood, a researcher at Moss Landing Marine Laboratories at San José State University.
There are a number of factors, Wood said, that play into the seasonal variations, including how much ocean water the glacier is in contact with, whether it has a steep or shallow bedrock slope, and how much meltwater it is receiving upstream.
Unfortunately, some of the glaciers with wild swings from season to season also happen to be very large, posing significant threats for global sea levels.
Losing the most at its front, the Zachariae Isstrom glacier in northeast Greenland shed 160 gigatons over the past four decades. The glacier holds enough water to add more than 18 inches to the global sea level if it were to melt completely.
West Greenland’s Jakobshavn Glacier, said to have released the iceberg that is believed to have sunk the Titanic, lost 88 gigatons over the time period. One of the ice sheet’s fastest-moving glaciers, it was responsible for nearly 4 percent of sea level rise during the 20th century.
The Humboldt Glacier in northern Greenland, the ice sheet’s widest glacier that ends in the sea, lost 87 gigatons. If it melted completely, it could raise global sea level by seven inches.
Ice loss at these edges wouldn’t directly affect sea levels because the ice is already sitting in the ocean and won’t change the volume of water. But it would allow ice on the land to flow into the ocean more quickly and accelerate sea level rise — it’s like pulling the plug out of the drain, Greene said.
The freshwater from the glacial ice could have a more direct effect on ocean circulation and the distribution of heat around the globe, the authors say. Changes in our ocean circulation could have notable effects on human societies, including bringing extreme weather events and disrupting rain patterns.
“If you have a bunch of freshwater sitting on top of the ocean, then when it freezes in the winter, it doesn’t expel any salt,” Greene said. “We count on that salt to drive ocean circulation.”
Greene and his colleagues warn that the added freshwater could weaken the Atlantic Meridional Overturning Circulation, known as the AMOC, which circulates warm, salty water from the tropics toward Europe and sends colder water south along the ocean floor. It also shuttles nutrients that help sustain ocean life.
But other researchers say any effects on ocean circulation need to be further monitored and investigated.
“The rapid loss from the Greenland ice sheet started in the 1990s and is ongoing,” said physical oceanographer Fiamma Straneo, who was not involved in the study. But “there is no evidence that it has affected the ocean circulation so far.”
That’s not too unexpected at the moment. The amount of freshwater from the edge calving is modest (42 gigatons per year) compared with total flow (about 221 gigatons per year), said Feng He, a polar scientist at researcher at the University of Wisconsin at Madison who was not involved in the study. Computer models agree that adding relatively small amounts of freshwater, even if this amount is doubled, will only have a small effect on Atlantic Ocean circulation in the near future.
But continued ice loss and meltwater from Greenland will become a problem in the long term, said Straneo, a professor of oceans and climate at Scripps Institution of Oceanography. One model showed that continued ice loss from Greenland will reduce the AMOC by 15 percent over 50 years, with substantial weakening by 2100.
“It’s a possibility that small and steady increase of the Greenland melting might push the AMOC into a collapsed state,” He said.