Greenland: oversaturated glaciers lose storage capacity

The meltwater of the Greenland Ice Sheet flows faster than expected

The meltwater flow on the Greenland ice sheet. © Geocenter Denmark
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Storage capacity on the limit: The upper layers of the Greenland ice sheet can absorb less meltwater than assumed. As researchers have observed, the excess water accumulates on the surface instead and thereby flows faster into the oceans. The already unstable ice sheet threatens more and more mass loss, as the scientists report in the journal "Nature Climate Change".

Global warming is making the glaciers of Greenland increasingly unstable, causing the ice giants to melt. The ice sheet has lost ground in this way for years - and not too tight: every year it loses as much ice as would fit in five Lake Constances. The ice not only heats up from above, but also from below through subglacial lakes.

A team of researchers led by Danes Horst Machguth from the University of Zurich has now investigated how climate change affects the near-surface layers of the ice sheet. This so-called firn layer consists of snow, which gradually turns into glacial ice. In Greenland it is up to 80 meters thick.

Supersaturated sponge?

From previous studies, it is known that the firn acts like a sponge: It stores meltwater that forms on the surface and then seeps into the fir, into so-called ice lentils. "It's unclear how the firn reacted to the recent warm summer in Greenland, " explains Machguth. "With our research, we want to find out if the firn could store the large amounts of meltwater or if the sponge was oversaturated."

The researchers investigated the meltwater runoff in the snow ice. © Geocenter Denmark

Over a total of three expeditions, the team of scientists traveled hundreds of miles on the ice, analyzed the ground with the help of radar, and drilled 20 meters deep holes in the firs at regular intervals. In doing so, they also oriented themselves to locations where similar cores were drilled 15 to 20 years ago. display

Unexpectedly massive ice cap

The comparison of old and new kernels showed in several places that there are considerably more ice lollies in Greenland ice than in the past and that the firn has stored the melt water much like a sponge. But that is not everywhere. Cores drilled in deeper elevations show a different picture: namely that the exceptional amounts of meltwater form a surprisingly massive layer of ice just below the surface.

Presumably, the intensive and repeated entry of meltwater so many Eislinsen formed that they finally prevented further penetration of the water, the researchers suspect. The many small lenses seem to have grown together to a layer of ice several meters, which now lies like a cover over the still porous Firn.

Faster mass loss

However, this has consequences: The massive ice cap prevents newly emerging meltwater from seeping into the firn. It remains on the surface, as can be clearly seen on satellite imagery: The water forms meltwater tempels and rivers on the surface and then flows off to the edge of the ice sheet.

In a twofold way this can promote the ice loss of the Greenland ice sheet. On the one hand, the meltwater quickly flows into the sea before it can freeze again, for example. "Compared to the original storage in porous firn, this mechanism increases mass loss, " explains co-author Mike MacFerrin of the University of Boulder, Colorado. On the other hand, melting water lamps reduce the albedo of the ice and thus increase the thawing of the surface, as studies on sea ice showed.

By what amount this process increases the mass loss of the whole ice sheet, the researchers can not yet estimate. However, the phenomenon could be widespread Wissenschaftler a similar change of the firn scientists have already observed in the Canadian Arctic. (Nature Climate Change, 2016; doi: 10.1038 / nclimate2899)

(University of Zurich, 05.01.2016 - DAL)