Greenland ice sheet is thinning six times faster than expected

Greenland, the world’s largest island, is largely covered by a massive ice sheet that has existed for at least the last 18 million years. It has an average thickness of 1.5 km (0.9 mi) but reaches over 3 km (1.9 mi) at its thickest. This ice cap has been thinning slowly for decades, but the process has accelerated in recent years under the influence of global warming. The Northeast Greenland Ice Stream (NEGIS) is a prominent ice-flow feature that drains the large central basin via several fast-flowing glaciers that terminate in the sea, and is the only ice stream that extends into the interior of the ice sheet. 

After a long period of stability, several of these large glaciers draining the region began thinning in 2000, which has given cause for concern. This thinning could result in increasing mass loss from the deep central portion of the Greenland ice sheet which would, in turn, affect sea levels and coastlines worldwide. 

A recent study by researchers from Dartmouth College, the Technical University of Denmark (DTU), and the University of California, Irvine, reports that the loss of ice from Greenland’s largest basin is occurring much faster than previously thought, and could contribute up to six times more to global sea-level rise by 2100 than current climate models project. The researchers say that ice loss from the Northeast Greenland Ice Stream alone could add half an inch or more of water to sea levels by the end of this century, which is equivalent to the contribution from the entire Greenland ice sheet during the past 50 years.

The study, published in Nature, made use of satellite data and numerical modeling with GPS data, collected from the harsh interior of Greenland over the past decade. In 2012, the intrusion of warm ocean currents caused the floating extension of the NEGIS to collapse, which has accelerated ice flow along the glaciers and triggered a wave of rapid ice thinning that has spread upstream. The researchers found that this thinning stretches inland from the Greenland coast as far as 200–300 kilometers (124–186 miles), and that other glaciers on Greenland may be suffering the same fate.

“Many glaciers have been accelerating and thinning near the margin in recent decades – GPS data helped us detect how far inland these changes, happening near the coast, propagate,” said co-author Mathieu Morlighem, the Evans Family Distinguished Professor of Earth Sciences at Dartmouth. Morlighem led the development of the numerical models for the study that simulated the flow of ice from the interior of the ice sheet to the coast.

“The Greenland ice sheet is not necessarily more unstable than we thought, but it may be more sensitive to changes happening around the coast,” Morlighem said. “If this is correct, the contribution of ice dynamics to overall mass loss on Greenland will be larger than what current models suggest.”

The study hinges on data collected by a team led by first author Shfaqat Abbas Khan, a professor at DTU Space, using a network of GPS stations that stretch inland on the NEGIS to behind the Nioghalvfjerdsfjord Gletscher and Zachariae Isstrøm glaciers, one of Earth’s most hostile and remote terrains.

Morlighem and coauthor Youngmin Choi, a former graduate student in Morlighem’s research group at UC Irvine, who is now at NASA’s Jet Propulsion Laboratory, compared the GPS data collected from the ice sheet to numerical models they developed to capture the dynamic response of the NEGIS after the collapse of the Zachariae Isstrøm glacier’s floating extension in 2012. They tested their model against various friction laws until their results matched the field data from the ice sheet. They then ran the model into the future and found that the NEGIS could lose six times more ice than existing climate models estimate.

“We can see that the entire basin is thinning and the surface speed is accelerating,” Khan said. “Every year, the glaciers we’ve studied have retreated farther inland, and we predict that this will continue over the coming decades and centuries. Under present-day climate forcing, it is difficult to conceive how this retreat could stop.”

Though winter 2021 and summer 2022 were particularly cold, the NEGIS glaciers have kept retreating. This is partly because Northeastern Greenland is an Arctic desert where precipitation is as low as 25 mm (0.9 inches) per year in places, so low that the ice sheet does not regenerate enough to offset ice loss through melt. However, estimating how much ice is lost and how far into the ice sheet the process occurs is not easy. The ice sheet’s interior – which moves less than one meter (3 feet) per year – is difficult to monitor, which limits the ability to make accurate projections.

“Models are mainly tuned to observations at the front of the ice sheet, which is easily accessible, and where, visibly, a lot is happening,” Khan said. “Our data show us that what we see happening at the front reaches far back into the heart of the ice sheet.”

Study co-author Eric Rignot, professor of Earth system science at UC Irvine, said that as more precise observations of the change in ice flow velocity are included in models, it is likely that estimates of global sea-level rise, such as the 22–98 centimeters (8–38 inches) by 2100 projected by the UN Intergovernmental Panel on Climate Change, will need to be corrected upwards.

“We foresee profound changes in global sea levels, more than currently projected by existing models,” Rignot said. “Data collected in the vast interior of ice sheets, such as those described in our research, help us better represent the physical processes included in numerical models and in turn provide more realistic projections of global sea-level rise.”

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By Alison Bosman, Earth.com Staff Writer