The Greenland Ice Sheet has lost more than 1,000 gigatons (Gt) of ice to calving since 1985—increasing previous estimates of mass loss by 20%. This revised number comes from a recent study of the territory’s glaciers over almost 4 decades. The research also revealed marine-terminating glaciers that responded more strongly to seasonal temperature changes also lost more mass over time.
“Glaciers that are highly sensitive…to the warming that happens over the summer are also probably going to be the ones that are sensitive to what happens over longer timescales as well,” said Chad Greene, a glaciologist and remote sensing specialist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and lead scientist on the research.
The new estimate could mean Atlantic Ocean circulation is less stable than previously thought.
Everywhere, All at Once
Scientists previously calculated that Greenland had lost around 5,000 Gt of ice in the past several decades. This number came from three independent techniques: satellite altimetry, flux gates, and gravimetry. Each method assumes some steady amount of calving and can capture the bulk of changes occurring on an ice sheet but could miss changes near the ice sheet’s edges, beneath its surface, or within its many narrow fjords.
But for this ice sheet, those finer details matter. “Greenland is basically a big, giant bathtub of ice, and most of that ice ends up draining through narrow fjords on its way out to the ocean,” Greene said. The boundary between the edge of the ice and environment beyond is called the calving front or terminus.
For decades, glaciologists have recognized that calving front retreat is “a significant process through which Greenland contributes mass to contemporary sea level rise,” said Derrick Lampkin, a cryosphere scientist at George Mason University in Fairfax, Va., who was not involved with the study.
“The calving front is oftentimes what responds first to a changing environment,” Greene said, calling it “the frontlines of climate change.” But most remote sensing techniques can’t track a front’s solid ice discharge and meltwater runoff at the same time.
“The fact that retreat is happening in every single sector, that’s a huge thing.”
To bridge that gap, Greene and his team gathered more than 236,000 satellite observations of the ice sheet that span 1985–2022. Aided by machine learning, the team mapped the calving fronts of 203 marine-terminating glaciers and four land-terminating glaciers for every month over that time. Those glaciers cover 87% of the ice sheet’s total area and hold 90% of the ice sheet’s total mass.
The researchers found that the Greenland Ice Sheet has lost more than 5,000 square kilometers of area, or 1,034 Gt of ice, from calving front retreat that had not been accounted for before. That means that the ice sheet has lost about 20% more mass than previously calculated.
All but one glacier have retreated since 1985. “The fact that retreat is happening in every single sector, that’s a huge thing,” Greene said. “It’s striking to see it happening everywhere and all at once.” Zachariæ Isstrøm glacier in northeast Greenland lost the most area (980 square kilometers) and ice mass (160 Gt) of the glaciers studied.
The team published these results in Nature in January.
Seasonal Changes and Perennial Losses
The monthly observations of calving fronts also provided insight into how the glaciers responded to seasonal changes. Greenland experiences temperature swings of as much as 35°C from summer to winter. “Nature has provided this built-in science experiment,” Greene said.
As expected, most glaciers grew during colder winter months and retreated during warmer summer months. However, the team found that 88% of marine-terminating glaciers, but none of the land-terminating glaciers, exhibited this seasonal growth pattern. Moreover, glaciers that were more sensitive to seasonal temperature changes also lost more area and mass from 1985 to 2022. Jakobshavn Isbræ had the largest seasonal variability and ranked second in mass lost.
The connection between seasonal and long-term retreat of marine-terminating glaciers supports the theory that warming ocean temperatures are a key driver of ice change. Lampkin called that connection “significant” and said it “may reveal a diagnostic observation that can be used to characterize marine-terminating outlet glacial systems.”
However, he cautioned that scientists don’t understand ocean-land-ice interactions well enough yet to make long-term predictions of melting on the basis of seasonal changes or predictions for whole ice sheets on the basis of the behavior of small-scale glaciers.
Antarctica, for example, “has a different set of glaciological, climatological, and oceanic context that may not lend the approach detailed in this work to be directly applicable,” Lampkin said.
AMOC Could Run Amok
But it turns out that even though the Greenland Ice Sheet has lost more ice mass than previously thought, that loss hasn’t changed calculations of sea level rise. “The old numbers are still the correct ones,” Greene said. “We had already measured the response,” he said, referring to rising sea levels, “and now we’re zeroing in on one of the causes.”
More concerning is the potential impact of all that fresh water on ocean circulation. The Atlantic Meridional Overturning Circulation, or AMOC, brings warm water from the tropics up to the North Atlantic Ocean. Freezing temperatures separate seawater into freshwater sea ice and dense salt water, which sinks and is flushed away back to low latitudes, where the cycle begins again.
AMOC is “a big conveyor belt of energy,” Greene said. It drives weather patterns in the Northern Hemisphere and regulates sea ice cover. If the Greenland Ice Sheet is releasing far more fresh water than previously thought, AMOC could be closer to destabilization, or even collapse, than feared.
“It is still unclear the magnitude of freshwater delivery required to alter the strength of AMOC,” Lampkin said. However, “the increasing trend in mass loss has put us on track for certain alterations in contemporary AMOC behavior.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
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