Largest ice sheet melt­ing from be­low

The Denver Post - - NEWS - By Chris Mooney

Sci­en­tists at in­sti­tu­tions in the United States and Aus­tralia on Fri­day pub­lished a set of un­prece­dented ocean ob­ser­va­tions near the largest glacier of the largest ice sheet in the world: Tot­ten glacier, East Antarc­tica. And the re­sult was a trou­bling con­fir­ma­tion of what sci­en­tists al­ready feared — Tot­ten is melt­ing from be­low.

The mea­sure­ments, sam­pling ocean tem­per­a­tures in seas over a kilo­me­ter (0.62 miles) deep in some places right at the edge of Tot­ten glacier’s float­ing ice shelf, af­firmed that warm ocean water is flow­ing in to­ward the glacier at the rate of 220,000 cu­bic me­ters per sec­ond.

These waters, the pa­per as­serts, are caus­ing the ice shelf to lose be­tween 63 and 80 bil­lion tons of its mass to the ocean per year, and to lose about 10 me­ters (32 feet) of thick­ness an­nu­ally, a re­duc­tion that has been pre­vi­ously noted based on satel­lite mea­sure­ments.

This mat­ters be­cause more of East Antarc­tica flows out to­ward the sea through the Tot­ten glacier re­gion than for any other glacier in the en­tirety of the East Antarc­tic ice sheet. Its en­tire “catch­ment,” or the re­gion of ice that slowly flows out­ward through Tot­ten glacier and its ice shelf, is larger than California. If all of this ice were to end up in the ocean some­how, seas would raise by about 11.5 feet.

“This ice shelf is thin­ning, and it’s thin­ning be­cause the ocean is de­liv­er­ing warm water to the ice shelf, just like in West Antarc­tica,” said Don Blankenship, a glaciol­o­gist at the Univer­sity of Texas at Austin and one of the study’s co-authors. Blankenship was not on the re­search ves­sel, but he and his col­leagues helped the Aus­tralia-based re­searchers with un­der­stand­ing the con­tours of the seafloor so they could plan their field in­ves­ti­ga­tions into where warm and deep waters could pen­e­trate.

The lead au­thor of the re­search, pub­lished Fri­day in Sci­ence Ad­vances, was Stephen Rin­toul, a re­searcher with the Univer­sity of Tas­ma­nia in Ho­bart and Aus­tralia’s Commonwealth Sci­en­tific and In­dus­trial Re­search Or­ga­ni­za­tion, or CSIRO. Tot­ten glacier is, more or less, due south of Aus­tralia and rel­a­tively close to one of Aus­tralia’s bases of op­er­a­tions on the ice con­ti­nent, Casey Sta­tion.

Rin­toul and his col­leagues, on board the gov­ern­ment ves­sel Aurora Aus­tralis, were able to nav­i­gate ex­tremely close to the Tot­ten ice shelf edge in Jan­uary 2015, when an open­ing in the sea ice al­lowed the ship to get in closer than one ever has be­fore.

The re­searchers took ocean mea­sure­ments at 10 sep­a­rate points along the float­ing Tot­ten ice shelf. And at two of the sta­tions, they found that the ocean un­der­neath was ex­tremely deep. There was a six-milewide canyon at a depth of 600 me­ters (nearly 2000 feet) that then branched into two nar­rower canyons, each reach­ing greater depths. One of them was over 800 me­ters deep (more than 2,500 feet) the other was 1,097 me­ters deep (3,600 feet). Each was about one to two miles wide.

It was in these deep un­der­sea canyons, and a few shal­lower ar­eas as well, that warm ocean water, called mod­i­fied cir­cum­po­lar deep water, was flow­ing in­ward pow­er­fully to­wards Tot­ten glacier. And the pre­vi­ously mea­sured loss of ice from the ice shelf matched closely with the amount of heat that the ocean was de­liv­er­ing, the pa­per found.

Granted, call­ing the waters reach­ing Tot­ten at great depths “warm” is a bit of a mis­nomer — they are slightly be­low the freez­ing point. How­ever, at the ex­treme pres­sures and depths in­volved, the freez­ing point of ice it­self low­ers, mak­ing these waters more than warm enough to melt ice.

Mea­sur­ing the warm water reach­ing Tot­ten was, un­til now, a miss­ing puz­zle piece in deter­min­ing what’s hap­pen­ing with the glacier. Prior re­search, for in­stance, had shown the pres­ence of cav­i­ties that warm water could en­ter, and sci­en­tists be­lieved this was oc­cur­ring be­cause they had ob­served Tot­ten thin­ning and low­er­ing in the water. But as NASA glaciol­o­gist Eric Rig­not put it to The Washington Post at the time, “it is one thing to find po­ten­tial path­ways for warm water to in­trude the cav­ity, it is another to show that this is ac­tu­ally hap­pen­ing.”

Now, sci­en­tists are show­ing that it’s ac­tu­ally hap­pen­ing.

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