Climate Science

At a Loss

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Science  30 May 2008:
Vol. 320, Issue 5880, pp. 1134
DOI: 10.1126/science.320.5880.1134d

Sea-ice coverage in the Arctic plummeted in the summer of 2007 to levels never before observed, surprising even experts who had witnessed the decades-long decline and predicted that the ice pack would continue to shrink at an increasingly rapid rate. Why did so much ice disappear? Zhang et al. conducted a retrospective modeling study of the evolution of Arctic sea-ice coverage and found that preconditioning, anomalous winds, and ice-albedo feedback were responsible for most of the retreat. Years of warming climate there preconditioned the ice for disappearance by thinning it significantly, pushing it ever closer to the point of complete melting, while stronger than normal winds pushed unusually large amounts out of the Arctic basin. The ice thinning and exposure of open water that these processes caused left the remaining ice even more susceptible than normal to loss due to heating of the upper ocean, increasing the intensity of the positive ice-albedo feedback and accelerating the rate of ice loss. Once summer had passed and temperatures had dropped low enough for ice to begin to regrow, 10% more ice than usual had vanished, 70% of it due to melting and 30% due to ice advection. The large ice loss, coupled with prevailing climate trends, suggests that Arctic sea ice has become particularly vulnerable to anomalous atmospheric forcing. — HJS

Geophys. Res. Lett. 10.1029/2008GL034005 (2008).

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