Again, an upward correction: In north-eastern Greenland, ice loss could be six times greater than previously assumed by the end of the century and thus contribute significantly to sea-level rise. This comes from a combination of GPS and satellite data documenting widespread strengthening of ice flow and thinning in the northeastern portion of the island's ice sheet. The scientists fear that there may be similarly underestimated ice dynamics in other areas of Greenland.
With the many concerns about climate change, a particularly anxious gaze is directed at the largest island on earth: Greenland's ice sheet, which is sometimes more than three kilometers thick, threatens to turn more and more into water and thus drive up the global sea level. It is not only the thawing of the surfaces of the glaciers due to the rising temperatures that plays a role: the ice giants are also increasingly slipping and thus flowing faster into the sea. So far, however, the extent and further development of this effect remains unclear. In particular, information from the remote north-east of Greenland has been missing so far. A gigantic stream of ice flows there: it connects the interior of the ice sheet with two glaciers over a distance of 600 kilometers, which drain around twelve percent of the Greenland ice sheet into the sea.
Targeting the Northeast Greenland Ice Stream
In 2012, after decades of melting, the floating ice caps collapsed and the ice has retreated at an accelerating rate ever since. The loss is associated with a diminishing braking effect on the ice flow. However, it remained unclear how much ice is lost as a result and how far the effect extends into the ice sheet. Because this area of the ice sheet only moves at less than one meter per year and is therefore comparatively difficult to monitor, so that precise forecasts have only been possible to a limited extent. "The models are mainly based on observations at the front of the ice sheet, which is easily accessible and where there is a lot of activity," says lead author Shfaqat Abbas Khan from the Technical University of Denmark in Lyngby.
To provide more clarity, from 2016 to 2019 Khan and his colleagues collected data from GPS stations as far as 200 kilometers inland of the Northeast Greenland Ice Stream. They then combined this information with elevation data from the CryoSat-2 satellite mission. The results then formed the basis for model simulations that shed light on the processes in the ice sheet and project further developments. "We were able to detect subtle speed changes that, when coupled with a model of ice flow, ultimately give us insight into how the glacier slides on its bed," says co-author Mathieu Morlighem of Dartmouth College in Hanover.
Widely reinforced slides
As the researchers report, their results show that the extent of the ice loss has so far been greatly underestimated. "Our data shows that what we're seeing at the front extends well into the interior of the ice sheet," Khan said. "We can see that the entire basin is thinning and the surface velocity is accelerating. Each year, the glaciers we study have retreated further inland, and we predict this will continue for decades and centuries to come. Under the current climatic conditions, it is difficult to imagine how this retreat can be stopped,” says the scientist.
Model calculations based on the data indicated that the Northeast Greenland Ice Flow could contribute six times as much to sea level rise by 2100 as previously estimated. Specifically, this is an increase between 13.5 and 15.5 millimeters. That's equivalent to the contribution of the entire Greenland ice sheet over the past 50 years, the scientists say.
"It's possible that what we're seeing in northeast Greenland is happening in other areas of the ice sheet as well. This is because many glaciers have accelerated and thinned near the edge in recent decades,” says Morlighem. “Using GPS data, we could now also determine how far inland this acceleration spreads. If this is strong, the contribution of ice dynamics to Greenland's total mass loss will be greater than current models suggest. Co-author Eric Rignot from the University of California, Irvine concludes: "Data collected in the vast interior of ice sheets, such as those described here, can help us to better represent the physical processes in numerical models in the future, which in turn will lead to more realistic projections of global sea level rise.”
Source: Technical University of Denmark, professional article: Nature, doi: 10.1038/s41586-022-05301-z