Blame the Sea? Ocean may be melting ice shelf from below

Significant portions of a large Antarctic ice shelf just south of one that suddenly broke apart in February 2002 are rapidly thinning and may suffer a similar, catastrophic demise in less than a century, scientists say.

Satellite observations collected between 1992 and 2001 suggest that the upper surface of the Larsen C ice shelf dropped as much as 27 centimeters per year during the period. About a quarter of that shrinkage, or 7 cm, may have resulted from snow packing down into denser material called firn, says Andrew Shepherd, a glaciologist at the University of Cambridge in England. Uncertainties about such factors as the height of ocean tides and the temperature and the salinity of water beneath the ice shelf would account for no more than a small fraction of the remaining loss in height above water, says Shepherd.

Therefore, he notes, as much as 20 cm per year of the upper surface’s drop stems from melting. About nine-tenths of any mass of floating ice lies below the water’s surface, suggesting that Larsen C is thinning overall by as much as 2 meters each year. Shepherd and his colleagues report their analysis in the Oct. 31 Science.

The likely cause of the thinning, says Shepherd, is relatively warm water beneath the ice shelf. Although water at a depth of 300 m just offshore of the ice shelf’s edge is about –1.5C, that’s about 0.65C above the melting point for ice at that depth. Many factors, including salinity and pressure, affect ice’s melting temperature.

Even a small temperature increase in the water below an ice shelf can make a big difference in the overlying ice’s melting rate, says Eric J. Rignot, a glaciologist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. His research suggests that each 0.1C rise in water temperature can, in a year’s time, melt away about 1 m of shelf ice.

In February 2002, 3,250 square kilometers of the Larsen B ice shelf splintered into thousands of icebergs (SN: 3/30/02, p. 197: Available to subscribers at All Cracked Up from the Heat? Major hunk of an Antarctic ice shelf shatters and drifts away). Shepherd says his team’s data suggest that during the previous decade, Larsen B, which was about 200 m thick when it collapsed, was thinning at an average rate of about twice that of Larsen C.

Larsen C is stable and isn’t shedding more icebergs than normal, says Shepherd. However, at the ice shelf’s current rate of thinning, Larsen C could reach a 200-m thickness, and therefore be susceptible to disintegration, in 70 years or so. If the waters in the region continue to warm, the ice shelf’s demise could occur even sooner.

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