Time series of sea ice thickness observed by moored sonars in the Transpolar Drift in Fram Strait show significant thinning during the 1990–2011 period. The thickness of old level ice is reduced by 25 %, while the fraction of (ridged) ice thicker than 5 m is reduced by 50 %. The combined effect on the mean ice thickness is a reduction from an annual average of 3.0 m during the 1990s to 2.2 m at the end of the record. Due to the steady advection of ice from many sites across the Arctic, the ice observed in Fram Strait carries an integrated signal of Arctic change. The thickness of old level ice is approaching values more typical for seasonal ice. While such low thicknesses prevail, larger areas of sea ice can melt away during the melt season. Hence conditions favorable to melt (or favorable winds) will result in new summers of very low sea ice extent. It is an important implication that the sea ice extent then depends more on the actual weather each melt season than previously. A relatively warm summer may remove much larger areas of ice than a slightly colder summer. This is exemplified through the relatively cold summer of 2013, which featured a sea ice extent nearly 50 % higher than the preceeding warm summer of 2012. This, again, implies that one may expect larger year to year variability in summer sea ice extent in the coming years, with the ramifications this has on human activity in the Arctic. In addition to this general reflection of the state of the Arctic sea ice cover, the Fram Strait ice thickness observations are of particular relevance to development of offshore petroleum fields on the North East Greenland shelf just downstream of the monitoring site. Most relevant design parameters may be deduced from this data set, such as level ice thickness and frequency and depth of pressure ridges.

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