Ice Loss in Greenland “Vastly Underestimated”
By TECHNICAL UNIVERSITY OF DENMARK
Ice is continuously streaming off Greenland’s melting glaciers at an accelerating rate, dramatically increasing global sea levels. New results published in the journal Nature on November 9 indicate that existing models have underestimated how much ice will be lost during the 21st century. Hence, its contribution to sea-level rise will be significantly higher.
By 2100, the Northeast Greenland Ice Stream will contribute six
times as much to the rising sea level as previous models suggested, adding
between 13,5 to 15,5 mm (0.53 to 0.61 inches), according to the new study. This
is equivalent to the entire Greenland ice sheet’s contribution in the past 50
years. Scientists from Denmark, the United States, France, and Germany carried
out the research.
“Our previous projections of ice loss in Greenland until 2100 are
vastly underestimated,” said first author Shfaqat Abbas Khan, Professor at the
National Space Institute at the Technical University of Denmark (DTU Space).
“Models are mainly tuned to observations at the front of the ice
sheet, which is easily accessible, and where, visibly, a lot is happening.”
Ice loss occurs more than 200 km inland
The study is partly based on data collected from a network of
precise GPS stations reaching as far as 200 km inland on the Northeast
Greenland Ice Stream—located behind the Nioghalvfjerdsfjord Gletscher and
Zachariae Isstrøm glaciers, one of Earth’s most hostile and remote terrains.
The GPS data were combined with surface-elevation data from the CryoSat-2
satellite mission and high-resolution numerical modeling.
“Our data show us that what we see happening at the front reaches
far back into the heart of the ice sheet,” said Khan.
“We can see that the entire basin is thinning, and the surface
speed is accelerating. Every year the glaciers we’ve studied have retreated
further 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.”
Significant contribution to rising sea levels
In 2012, after a decade of melting, the floating extensions of Zachariae Isstrøm collapsed, and the glacier has since retreated inland at an accelerating pace. And though winter 2021 and summer 2022 have been particularly cold, the glaciers keep retreating.
Since northeastern Greenland is a so-called
Arctic desert – precipitation is as low as 25 mm per year in places – the ice
sheet is not regenerating enough to mitigate the 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 at less than one meter per year, is
difficult to monitor, which limits the ability to make accurate projections.
“It is truly amazing that we are able to detect a subtle speed
change from high-precision GPS data, which ultimately, when combined with a
model of ice flow, inform us on how the glacier slides on its bed,” said
coauthor Mathieu Morlighem, a professor of earth sciences at Dartmouth College.
“It is possible that what we find in northeast Greenland may be
happening in other sectors of the ice sheet. Many glaciers have been
accelerating and thinning near the margin in recent decades. GPS data helps us
detect how far this acceleration propagates inland, potentially 200-300 km from
the coast. If this is correct, the contribution from ice dynamics to the
overall mass loss of Greenland will be larger than what current models
suggest.”
The Zachariae Isstrøm was stable until 2004, followed by steadily
retreat of the ice front until 2012, when a large portion of the floating
sections became disconnected. As more precise observations of change in ice
velocity are included in models, it is likely that IPCC’s estimates of 22-98 cm
global sea level rise will need to be corrected upwards.
“We foresee profound changes in global sea levels, more than
currently projected by existing models,” said coauthor Eric Rignot, professor
of Earth system science at the University of California, Irvine.
“Data collected in the vast interior of ice sheets, such as those
described herein, help us better represent the physical processes included in
numerical models and in turn provide more realistic projections of global
sea-level rise.”
Reference: “Extensive inland thinning and speed-up of Northeast
Greenland Ice Stream” by Shfaqat A. Khan, Youngmin Choi, Mathieu Morlighem,
Eric Rignot, Veit Helm, Angelika Humbert, Jérémie Mouginot, Romain Millan, Kurt
H. Kjær and Anders A. Bjørk, 9 November 2022, Nature.
DOI:
10.1038/s41586-022-05301-z
