Thinning ice threat to sea level

Scientists are finding parts of Antarctica are melting much more rapidly than previously thought. Photo / AP

Researchers find warm water eroding ice shelves in east and west Antarctica.

A hundred years from now, humans may remember 2014 as the year we first learned that we may have irreversibly destabilised the great ice sheet of West Antarctica, and set in motion more than 3m of sea level rise.

And 2015 could be the year of the double whammy - when we learned the same about one gigantic glacier of East Antarctica, which could set in motion roughly the same amount all over again.

The findings about East Antarctica emerge from a new paper just out in Nature Geoscience by a team of scientists from the United States, Britain, France and Australia.

They flew research flights over the Totten Glacier of east Antarctica - the fastest-thinning sector of the world's largest ice sheet - and took measurements to try to figure out the reasons behind its retreat. And the news wasn't good: It appears that Totten, too, is losing ice because warm ocean water is getting underneath it.

"The idea of warm ocean water eroding the ice in west Antarctica, what we're finding is that may well be applicable in east Antarctica as well," says Martin Siegert, a co-author of the study.

The floating ice shelf of the Totten Glacier covers an area of 145km by 35km.The glacier holds back a vast catchment of ice that, were its vulnerable parts to flow into the ocean, could produce a sea level rise of more than 3m - which is comparable to the impact from a loss of the West Antarctica ice sheet. And that's "a conservative lower limit", says lead study author Jamin Greenbaum, at the University of Texas at Austin.

In its alignment with the land and the sea, the Totten Glacier is similar to the west Antarctic glaciers, which also feature ice shelves that slope out from the vast sheet of ice on land and extend into the water. These ice shelves are a key source of instability, because if ocean waters beneath them warm, they can lose ice rapidly, allowing the ice sheet behind them to flow more quickly into the sea.

The researchers used gravitational measurements, radar and laser altimetry to get a glimpse of what might be happening beneath the massive glacier, whose ice shelves are more than 490m thick in places. Using radar, they could measure the ice's thickness. By measuring the pull of the Earth's gravity on the plane, the scientists were able to determine how far below that ice the seafloor was.

The result was the discovery of two undersea troughs beneath the ice shelf, allowing a greater depth of water beneath the floating ice. These subsea valleys, the researchers suggest, may explain the glacier's retreat, allowing warmer deep waters to get underneath the ice shelf, accelerating its melting.

One of these canyons is 4.8km wide, in a region that was previously believed to simply hold ice lying atop solid earth. On the contrary, the new study suggests the ice is afloat.

The availability of warm water, and the observed melting, notes the study, "support the idea that the behaviour of Totten Glacier is an east Antarctic analogue to ocean-driven retreat under way in the west Antarctic ice sheet".

If Antarctica loses volumes of ice that would translate into major contributions to sea level rise, that rise would not be distributed evenly around the globe. Antarctica is so massive that it pulls the ocean towards it, but if it loses ice, its gravitational pull will relax, and the ocean will slosh back towards the Northern Hemisphere - which will experience additional sea level rise.