Ice shelf edge near Halley Station, Antarctica. (c) Ralph Timmermann, Alfred Wegener Institute
Read The Antarctic will contribute more to sea-level rise in the coming decades than previously forecast. A large ice shelf southwest of the German research station Neumayer III could become unstable towards the end of the 21st century. This floating ice tongue, the Filchner-Ronne Ice Shelf on the southern edge of the Weddell Sea, will be submerged in warm water and massively thinned in a few decades, researchers at Hartmut Hellmer from the Alfred Wegener Institute for Polar and Marine Research (AWI) in Bremerhaven, Germany report Journal Nature. The researchers used climate and ice models to study how global warming affects the Filchner-Ronne Ice Shelf. According to their results, higher air temperatures trigger a fateful chain of events in just 25 years. The heat initially causes the hitherto dense sea ice cover in the Weddell Sea to shrink. This changes the ocean currents. Currently sinking, salty water in front of the ice shelf diverts warm currents. But when the sea ice disappears, comparatively warm water with temperatures around freezing can cross a threshold in front of the ice shelf edge and flow into the deep basin under the ice.

In the model of the researchers there are first advances from 2037. By 2081, the entire eastern part of the basin was already bathed in a lukewarm bath, and in 2095 also the western section. As a result, according to the researchers, the annual melting rate at the bottom of the ice shelf increases from the current 20 centimeters to an average of four meters. In some places, the ice thickness can even decrease by 50 meters per year, write Hellmer and his colleagues. The ice shelf is more than a thousand feet thick where it communicates with the land. At the ice shelf edge, where the open sea begins, it still has a width of 200 meters.

Although the loss of a ice shelf does not contribute directly to the rise in sea level, as the ice is already floating. The Filchner-Ronne Ice Shelf, which is larger than Germany, but fills a huge bay and thus acts as a barrier for ice streams from both the East and West Antarctic. "If a ice shelf thins, can the ice on the mainland behind it set in motion, " says Hellmer. If more ice flows from the continent into the sea, however, the sea level rises. Bremerhaven researchers are still investigating how the ice streams on the mainland will react to the thinning.

However, a second study from the journal Nature Geoscience shows that even the inland ice in the region does not seem to rest as securely on the Antarctic mainland as previously assumed. Two large streams of ice that feed the Filchner-Ronne Ice Shelf have their base below sea level, notes Neil Ross of the University of Edinburgh and colleagues on geophysical surveys from the aircraft. display

The ice rests in a basin the size of Hesse, which is up to a thousand feet lower than the sea level. In addition, the subsurface of the so-called Aufsetzlinie? the connecting line between ice shelf and land? relatively steep towards the continent. This geometry makes the ice streams susceptible to change, the researchers report. "The ice in the region is still on the ground, but it only needs to be thinner to swim up, " says co-author Martin Siegert, also from Edinburgh.

The researchers are seeing signs that this has already happened in the past. At the bottom of today's ice-filled basin are apparently marine sediments. The land connection of the ice shelf was therefore earlier 200 kilometers further south than today.

So far, climatologists have not included the Weddell Sea region in their projections for future sea-level rise. "But the results are robust, and they show us what we need to cope with based on current knowledge, " says Angelika Humbert of AWI, author of a commentary in Nature Geoscience. So far, the polar explorers' attention has focused mainly on the west coast of the continent on the Amundsen Sea. Some ice shelters are already retreating there, and the landing lines are moving inland. "The processes that we currently see on the Amundsen Sea will be repeated on the Weddell Sea, " says Hartmut Hellmer. The risk for a collapse is great, says Martin Siegert: "The region still needs a push, so that significant changes, but not particularly strong."

Hartmut Hellmer (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven) et al .: Nature, Vol. 485, p. 225, 10.1038 / nature11064 Neil Ross (University of Edinburgh) et al .: Nature Geoscience, online pre-publication, doi: 10.1038 / ngeo1468 © science.de - Ute Kehse

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