Shifting winds in southern hemisphere push warmer water beneath ice shelves
By Bob Berwyn
SUMMIT COUNTY — Warm ocean currents, driven by shifting hemispheric winds, are likely the main cause of recent ice loss in Antarctica, an international team of researchers reported this week. The new information is another step toward being able to forecast changes in global sea level, according to scientists with the British Antarctic Survey.
The scientists reached their conclusions after studying NASA satellite data that measures the height of the ice shelves and offers a glimpse at how ice shelf height is melting.
“We looked all the way around Antarctica,” said Lead author, D,r Hamish Pritchard of the British Antarctic Survey. “We’re looking specifically at the ice shelves … they act as brakes to the glaciers flowing off the ice sheet. It’s the interface between the ice sheet and the ocean, so they have a particular significance … There’s a strong pattern of change in West Antarctica,” Pritchard said.
The scientists especially wanted to pinpoint any patterns in the thinning of the shelves, which would help link the observations to possible causes. In particular, ice shelves in West Antarctica have been thinning, with measurable acceleration of ice flowing into the sea.
In some cases, especially along the Antarctic Peninsula, huge sections of ice shelves are disintegrating, though the researchers attributed the loss of ice shelves along the east coast of the Antarctic Peninsula to warmer winds rather than the influence of ocean currents.
“The motivation is to explain the pattern of the ice shelf melting. Certain glaciers along West Antarctia have accelerated dramatically, the most well-known being the Pine Island Glacier,” Pritchard said.
The team used 4.5 million measurements made by a laser instrument mounted on NASA’s ICESat satellite to map the changing thickness of almost all the floating ice shelves around Antarctica. Of the 54 ice shelves mapped, 20 are being melted by warm ocean currents, most of which are in West Antarctica.
In every case, the inland glaciers that flow down to the coast and feed into these thinning ice shelves have accelerated, draining more ice into the sea and contributing to sea level rise.
“In most places in Antarctica, we can’t explain the ice-shelf thinning through melting of snow at the surface, so it has to be driven by warm ocean currents melting them from below.
“We’ve looked all around the Antarctic coast and we see a clear pattern: in all the cases where ice shelves are being melted by the ocean, the inland glaciers are speeding up. It’s this glacier acceleration that’s responsible for most of the increase in ice loss from the continent and this is contributing to sea-level rise.”
To link the thinning with ocean currents, the researchers first looked at — and eliminated — other possible factors, including changes or reduction of snowfall, or whether warmer temperatures were leading to more compaction of the snowpack.
“Wwhat we were left with was the basal melting signal, from underneath,” Pritchard said, adding that weather observations have shown a marked shift in southern hemisphere winds, with an increase in Westerlies.
That’s changing the position of the relatively warm circumpolar deep water current, driving the warmer water underneath the ice shelves. Partial confirmation came from observations made by a remotely operated submarine which detected the warmer water slipping beneath the Pine Island ice shelf, Pritchard said.
“What’s really interesting is just how sensitive these glaciers seem to be. Some ice shelves are thinning by a few meters a year and, in response, the glaciers drain billions of tons of ice into the sea.
“This supports the idea that ice shelves are important in slowing down the glaciers that feed them, controlling the loss of ice from the Antarctic ice sheet. It means that we can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt – the oceans can do all the work from below.”
A different picture is seen on the eastern Antarctic Peninsula (the long stretch of land pointing towards South America). Here, the ice-shelf thinning found by this study can be explained by warm summer winds directly melting the snow on the ice-shelf surfaces. Both patterns, of widespread ocean-driven melting and this summer melting on the Antarctic Peninsula, can therefore be attributed to Antarctica’s changing wind patterns.
“This study shows very clearly why the Antarctic ice sheet is currently losing ice, which is a major advance,” said Professor David Vaughan is the leader of the EU ice2sea research project.
“But the real significance is that it also shows the key to predicting how the ice sheet will change in the future is in understanding the oceans. Perhaps we should not only be looking to the skies above Antarctica, but also into the surrounding oceans,” Vaughan said.
The study was carried out by an international team from British Antarctic Survey, Utrecht University, Scripps Institution of Oceanography and Earth & Space Research in Corvallis, Oregon. NASA’s ICESat – Ice, Cloud and Land Elevation Satellite – measurements were collected during the period 2003 – 2008 to detect changes in ice-shelf thickness through time.