Antarctic: Deep water heats shelf

Overflowing hot water also endangers cool seas

Iceberg in the Antarctic Weddell Sea © Sunke Schmidtko
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In the Antarctic, the danger comes from below: warm deep water rises there higher and sloshes on the shallow shelf, as marine researchers have now found. As a result, the shelf seas warm up more and more. Especially in the relatively cool Wedellmeer, ice melting could accelerate in the future, as the scientists report in the journal "Science".

The ice and the glaciers of the Antarctic were long considered relatively stable. But that is changing gradually. Last year a study showed that even the supposedly extremely stable ice of the eastern Antarctic could be more susceptible to climate change than expected. In addition, there are indications that the defrosting of glaciers in the West Antarctic could already be irreversible.

Sunke Schmidtko from the GEOMAR Helmholtz Center for Ocean Research in Kiel and his colleagues have now found further evidence that temperatures on the West Antarctic shelf are rising significantly - and also figuring out why. For their study, they had evaluated all oceanographic data from the waters around the Antarctic between 1960 and 2014.

Data shows that warmer water sloshes onto the West Antarctic shelf and raises water temperatures there. © S. Schmidtko, edit. C. Kersten / GEOMAR

Warm deep water spills onto the shelf

The data shows that the temperatures in the West Antarctic Amundsensee and the Bellingshausensee since 1960 rise more than in the rest of the Antarctic shelf. "Based on the data, we could see that this process is being reinforced from the outside, " says Schmidtko. As it turned out, along the continental slope in front of the shallow shelf seas are masses of water at greater depths, which are very warm at Antarctic conditions at 0.5 to 1.5 degrees Celsius.

"These waters have warmed up in West Antarctica over the past 50 years, " said Schmidtko. "And they are not as deep as they were 50 years ago." Especially in the Amundsensee and the Bellingshausen-See, the warm deep water now spills strengthened on the shelf. This downward flow of heat may explain why long-term accelerated glacier melt has been observed in these regions. display

Danger for the previously stable Weddell Sea

This development is also questionable for the south-western Weddell Sea. For here, the seawater on the shelf was still relatively cold, the temperatures were minus 1.5 degrees. Therefore, there was no stronger defrosting of the ice shelf here. But if the increase in warm water continues, it could also lead to major changes in the future, with dramatic consequences for fil- Ice shelves come, as the researchers explain.

But that means that for the first time, glaciers outside of the West Antarctic will melt from below and slide faster into the sea. "The increased penetration of warmer water over the shelf edge is likely to enhance this process, " says co-author Karen Heywood of the University of East Anglia, "That would then affect the speed of the global sea-level rise."

Antarctic krill (Euphausia superba) Uwe Kils / CC-by-sa 3.0

Danger for the Krill offspring?

The rising, warm deep water could, however, also endanger the ecology of the Southern Ocean. The Antarctic shelf areas are, among other things, spawning grounds for the Antarctic krill, the basis for many food chains in this ocean. Studies show that the spawning cycles of this cancer change when heated. What that means in concrete terms for the Antarctic krill now needs to be examined more closely.

Why climate change heats and raises the deep waters of the Antarctic is still unclear. "We suspect that they are related to large-scale changes in the wind systems over the southern hemisphere, " says Schmidtko. "But which individual processes play a role in this must be considered in future studies." (Science, 2014; doi: 10.1126 / science.1256)

(GEOMAR / Science, 05.12.2014 - NPO)