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OCEANOGRAPHY

Thinner and more fragile

Ice shelves in western Antarctica are losing volume, and one shelf may disappear by the end of the decade

One of the Larsen ice shelves in February 2000: faults and fissures due to high temperatures

Photo NASA/Landsat 7One of the Larsen ice shelves in February 2000: faults and fissures due to high temperaturesPhoto NASA/Landsat 7

In 1995, an ice shelf that spanned 2,500 square kilometers—an area equivalent to slightly more than one and a half times the size of the city of São Paulo—broke off from the continental ice mass that covers the Earth and disintegrated over the course of a few weeks into the Weddell Sea, the part of the Southern Ocean that borders on the Antarctic Peninsula and part of the frozen continent. It was the end of Larsen A, as the shelf was called. Seven years later, in 2002, Larsen B, a nearby shelf five times larger, lost nearly a quarter of its size in a month and a half. Enormous blocks of ice began to float around in the ocean before melting as a result of the rising temperatures in the region. Since then, the size of the Larsen B remnant has become the focus of constant monitoring.

A study of satellite data coordinated by the US space agency NASA and published in the June 2015 issue of the scientific journal Earth and Planetary Science Letters estimates that the remainder of Larsen B, some 1,600 square kilometers in size, will disappear in the next five years. The shelf, up to 500 meters thick at some points, is melting and collapsing into fragments at an accelerated pace. “The speed with which this is happening is surprising,” says Ala Khazendar, a geophysicist at NASA and principal author of the paper. Another recent paper, co-authored by Fernando Paolo, a Brazilian PhD candidate studying at the Scripps Institution of Oceanography at the University of California, San Diego, explains the mechanism that is  likely to be behind the thinning of another ice shelf in the Larsen group.

062-063_Antártica_232In an article published on May 13, 2015 in the scientific journal The Cryosphere, researchers from the British Antarctic Survey, from Scripps, including Paolo, and from other American universities showed that the thickness of the Larsen C shelf, five times larger than Larsen B, is losing volume and thinning as a result of two processes. “On the surface, there is likely melting and compacting of the surface layer of firn—denser snow that covers the ice shelf, probably due to a rise in atmospheric temperature. This causes loss of air in the firn on the surface of the shelf,” Paolo says. “Below, in the submerged part of the shelf, the ice is thinning due to melting caused by the inflow of warmer bottom waters. This also causes a change in ice flow, which stretches the shelf. Our measurements reveal considerable uncertainty, but we can only explain this degree of thinning of Larsen C if these two mechanisms are acting simultaneously.” The study was based on an analysis of data from satellites and from eight radar surveys conducted between 1998 and 2012.

Thinning and disappearance of ice shelves, which are extensions of glaciers and of the ice sheet that covers Antarctica, do not directly cause a rise in sea level. Shelf ice already sits on the ocean, and melting will not change the sea level. The same logic holds true for sea ice, which is much thinner and floats around Antarctica. Ice shelf thinning affects sea level indirectly. “The melting of the shelves enables the ice imprisoned in the ice sheet, which lies over the continent, to slide more easily towards the sea,” says glaciologist Jefferson Cardia Simões of the Federal University of Rio Grande do Sul (UFRGS), coordinator of the National Institute of Science and Technology of the Cryosphere. “There is consensus that global warming is causing the melting of the ice shelves on the Antarctic Peninsula,” says Ilana Wainer of the Oceanographic Institute at the University of São Paulo (IO-USP), who works on climate models of ocean-atmosphere interaction in the Antarctic region. “And it’s not just natural climate variation.” In the past 50 years, the average atmospheric temperature over the Antarctic Peninsula has risen 2.5ºC.

18% thinner
The ice shelves are more fragile on the peninsula, which contains the least-cold parts of the continent, and in West Antarctica. A paper published in Science on March 26, 2015, of which Paolo was principal author, indicates that the shelves in the western portion will lose half their volume in 200 years if their current rate of thinning continues. Between 1994 and 2012, according to altimetry data obtained by European Space Agency satellites, some shelves have thinned by as much as 18%. “The situation is more critical in the western part of the continent, but there are also signs of changes in the east,” Paolo says. When the researchers took into account the period analyzed in the study—the eighteen years as a whole—they noted a slight increase in the thickness of the shelves in East Antarctica. But this increase was concentrated in the first 10 years monitored. When they looked only at the data from more recent years, they detected stabilization or loss of mass in the eastern shelves. It is a sign that thinning also appears to be affecting the eastern shelves.

Scientific articles
PAOLO, F. S. et al. Volume loss from Antarctic ice shelves is accelerating. Science. V. 348, No. 6232, p. 327-31. April 17, 2015.
HOLLAND, P.R. et al. Oceanic and atmospheric forcing of Larsen C Ice-Shelf thinning. The Cryosphere. V. 9, No. 3, p. 1005-24. 2015.

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