External Forcing, Internal Climate Variability and the Arctic’s Rapidly Shrinking
Sea Ice Cover
Mark C. Serreze [Serreze@nsidc.org] and Julienne Stroeve, National Snow and Ice Data
Center / Cooperative Institute for Research in Environmental Sciences (NSIDC/CIRES),
University of Colorado- Boulder
As assessed over the available satellite record, 1979-present, Arctic sea ice extent in
September is shrinking at a rate of approximately 12% per decade. The observed trend is
larger than depicted in hindcasts from most global climate models using observed climate
forcings and furthermore appears to have steepened over the past decade, hastening the
transition towards a seasonally open Arctic Ocean. This apparent strong sensitivity of the sea
ice cover to external forcing appears to reflect several mutually supporting processes.
Because of the extensive open water in recent Septembers, ice cover in the following spring
is increasingly dominated by thin, first-year ice (ice formed during the previous autumn and
winter) that is vulnerable to melting out in summer. Thinner ice in spring in turn fosters a
stronger summer ice-albedo feedback through earlier formation of open water areas. A thin
ice cover is also more vulnerable to strong summer retreat under anomalous atmospheric
forcing on seasonal to decadal timescales scales. Finally, general warming of the Arctic in
all seasons has reduced the likelihood of cold years that could bring about temporary
recovery of the ice cover. Events leading to the September ice extent minima of recent years
exemplify these processes.