There is no longer any doubt that many glaciers are melting in the “eternal” ice of the Antarctic and thus - as expected - contribute to the measured global rise in sea levels. The melting of the ice sheets is mostly attributed to two effects of climate change, namely to the rising temperatures of the atmosphere as well as of the sea water. A German-British research group has now been able to show that two of the fastest melting glaciers in the southern polar region lie on a heat source, the heat of which has nothing to do with climate change. Under the Thwaites and Pope glaciers in West Antarctica, a particularly hot earth crust ensures that their lowest ice layers melt and the glaciers flow very quickly into the sea as ice streams.
In contrast to the relatively stable ice of the huge East Antarctica, the ice masses on the much smaller West Antarctica facing the Pacific are constantly in flux.
The Thwaites and its neighbor, the Pope Glacier, have lost almost 5,000 billion tons of ice there in the past 40 years.
That corresponds to more than a third of the total ice loss of the southern continent during this period.
The meltwater contributed about five percent of the world's sea level rise.
Thin earth crust under the ice
The East and West Antarctica differ not only in the respective mass and drift rates of the ice. Geologically, the two parts of Antarctica are worlds apart. The huge eastern part is an old stable continent, a so-called craton like the Canadian Shield or Siberia. West Antarctica, on the other hand, is a relatively young, tectonically active area in which many earthquakes occur. There are also hidden active volcanoes under the ice masses. This is especially true for the interior along the Amundsen Sea. There, the kilometer-thick ice sits on a rift zone in which the earth's crust gradually breaks apart. Similar rift zones run through large parts of East Africa, for example, but are also located under the Upper Rhine Valley between Basel and Mainz or under the upper reaches of the Rio Grande in North America.
In such rift zones, the earth's crust is often barely 20 kilometers thick and therefore much thinner than under old continents such as East Antarctica.
In addition, the earth's crust there is brittle because the flanks of the rift zone are slowly moving away from each other.
Through this thin and brittle crust, much more geothermal heat can reach the surface from the earth's interior than in stable areas.
There are several power plants in the Upper Rhine Valley, in which the abundant geothermal energy is used to generate electricity.
Melt water as a lubricant
Ricarda Dziadek's research group from the Alfred Wegener Institute in Bremerhaven has now determined the geothermal flow in the West Antarctic Rift System. For their calculations, the researchers use, among other things, data from an expedition on the research ship “Polarstern” in spring 2017. At that time, the area was mapped in detail from a helicopter for a few weeks with aeromagnetic measuring devices. From the magnetic measurement values obtained, the researchers working with Dziadek first calculated temperatures in the earth's crust and then the heat flow on the earth's surface, which is covered with thick ice.
As Dziadek and her colleagues now
write
in the online journal
Communication Earth & Environment
, the geothermal heat flow in this area averages almost 90 milliwatts per square meter. This corresponds to the values measured in other rift zones around the world. However, the heat flow under the West Antarctic is by no means uniform. The research group was able to determine a very high value of up to 150 milliwatts per square meter under the Thwaites and Pope glaciers.
The geothermal energy now causes the lowest ice layers of these glaciers to melt. The resulting meltwater then acts as a lubricant, on which the ice above can flow particularly quickly towards the coast. With stable glaciers, the glacier tongues floating in the water, the so-called ice shelves in front of the ice masses, slow down this river. But since climate change is gnawing at the ice shelves of the West Antarctic glaciers, their braking effect is considerably reduced. Together with the increased geothermal energy, this leads to an enormously accelerated flow of the Thwaites and Pop glaciers into the sea, says AWI researcher Karsten Gohl, one of the co-authors of the study.