Sea ice is the central component and most sensitive indicator of the Arcticclimate system. Both the depletion and areal decline of the Arctic sea ice cover,observed since the 1970s, have accelerated since the millennium.While the relationship of global warming to sea ice reduction is evident and underpinnedstatistically, it is the connecting mechanisms that areexplored in detail in this review.Sea ice erodes both from the top and the bottom.Atmospheric, oceanic and sea ice processes interact in non-linear ways onvarious scales. Feedback mechanisms lead to an Arctic amplification of theglobal warming system: the amplification is both supported by the icedepletion and, at the same time, accelerates ice reduction. Knowledgeof the mechanisms of sea ice decline grew during the1990s and deepened when the acceleration became clear in the early2000s. Record minimum summer sea ice extents in 2002, 2005, 2007 and 2012provide additional information on the mechanisms.This article reviews recent progress in understanding the sea ice decline. Processes arerevisited from atmospheric, oceanic and sea ice perspectives. There isstrong evidence that decisive atmospheric changes are the major driver ofsea ice change. Feedbacks due to reduced ice concentration, surface albedo,and ice thickness allow for additional local atmospheric and oceanic influences andself-supporting feedbacks. Large-scale ocean influences on Arctic Oceanhydrology and circulation are highly evident. Northward heat fluxes in theocean are clearly impacting the ice margins, especially in the Atlanticsector of the Arctic. There is little indication of a direct and decisiveinfluence of the warming ocean on the overall sea ice cover, due to anisolating layer of cold and fresh water underneath the sea ice.
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