Chapter 13: Variability of Sea Ice Extent Over Decadal and Longer Timescales
Recent syntheses of sea ice and related proxy information have provided an improved picture of Arctic sea ice variability over decadal to century timescales. A spectrum of variability is superimposed on a recent decrease of Arctic sea ice. An outstanding feature is the correspondence with the Atlantic Multidecadal Oscillation, which has timescales of 50–120 years. The linkage appears to arise through the inflow of Atlantic Water to the Arctic Ocean. Less robust, and by all indications non-stationary, associations with atmospheric modes such as the North Atlantic Oscillation have also been documented, primarily in recent decades. One possible reason for the nonstationarity of such associations is that the centers of action of major atmospheric modes may change over the timescale of centuries or even less. While the recent decrease of summer ice in the Arctic appears to be unique in the past 1,450 years, paleo reconstructions also suggest a minimum in Arctic ice coverage during the early Holocene. Unlike the Arctic, Antarctic sea ice shows essentially no trend over the past 30 years. The absence of a trend has been attributed to wind forcing and possibly ocean interactions. Observational information on Antarctic sea ice variability is virtually nonexistent beyond the past 100–150 years, so proxy information provides the only clues to longer-term Antarctic sea ice variability. Such information obtained from ice cores suggests that wintertime ice extent in the East Antarctic sector has decreased by about 20% since 1950, and that multicentury variations also characterize Antarctic ice extent.