The sequence of extreme September sea-ice extent minima observed since 2002 suggests acceleration in the response of the Arctic sea-ice cover to external forcing, hastening the transition towards a seasonally open Arctic Ocean. This acceleration results from 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. Finally, general warming of the Arctic has reduced the likelihood of cold years that could bring about temporary recovery of the ice cover. This talk summarizes these linked processes through analysis of satellite-derived sea-ice extent, ice concentration and ice age, and atmospheric conditions from atmospheric reanalyses and satellite observations. Events leading to the recent extreme September ice-extent minima exemplify these processes.