Thinning of Arctic sea ice 1990-2010 as observed by upward looking sonars - or why the Arctic Ocean could be ice free in summer in less than ten years

Abstract

Time series of sea ice thickness observed in Fram Strait by moored sonars show a 35 percent reduction in multiyear (MY) modal ice thickness since 2005. The MY mode reflects the thickness of level ice which has survived at least one melt season, and is hence a reflection of the thermodynamic equilibrium. During the 1990s the late winter MY modal thickness was 3.4+-0.4 m. Following excessive export of MY ice during the winter of 2005, late winter MY modal ice thickness dropped to 2.2 +-0.1 m, which persisted until the end of our record in 2010. The reduced MY modal ice thickness is a result of the Arctic sea ice cover entering a new state, where dynamic and thermodynamic effects appear to have combined to shift the equilibrium towards thinner ice.

This new state includes a dramatic reduction in the fraction of ridged sea ice, compared to the 1990s. The vast fields of ridged ice thicker than 5 m, constituting 28 percent of the winter Arctic sea ice cover during the 1990s, is nearly gone. At the end of winter in 2010, ice thicker than 5 m constituted only 6 percent of the total ice mass observed. The combined effect on late winter mean ice thickness is a reduction from 4.3+-0.4 m during the 1990s to a record low value of 2.0 m in late winter 2010. We speculate that increased ocean heat flux plays an important role in the thinning of the thick ice. With the thickest ice nearly gone and the MY level ice thicknesses close to thicknesses typical for first year sea ice, we are approaching a state where favorable conditions could melt most of the Arctic sea ice cover during one summer.