m monthly extent of 4.26 million square kilometers (1.64 million square miles), 23 percent lower than the previous minimum in 2005. Is this drastic reduction the result of natural variability superimposed on a general declining trend, or is Arctic sea ice cover shifting into a different climatic state characterized by completely ice-free summers? To help answer this, Haas et al. study the Arctic's Transpolar Drift, a current that carries ice from Siberia across the North Pole to the east coast of Greenland. Through helicopter-borne electromagnetic measurements, the authors calculate sea ice thicknesses over the Transpolar Drift during the late summers of 2001, 2004, and 2007, adding to a ground-based data set that extends to 1991. They find that average ice thickness has reduced by 44 percent since 2001. A model of ice ages shows that the area of older, thicker ice has decreased due to changed drift patterns. The authors suggest that the shift to younger and thinner ice could soon result in an ice-free North Pole during summer.
Reduced ice thickness in Arctic Transpolar Drift favors rapid ice retreat.
Christian Haas: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany; now at Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada;
Andreas Pfaffling: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany; now at Norwegian Geotechnical Institute, Oslo, Norway;
Stefan Hendricks and Lasse Rabenstein: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany;
Jean-Louis Etienne: Septieme Continent, Paris, France;
Ignatius Rigor: Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, U.S.A.
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