Why is the summer sea ice melting away so rapidly?
The reason for the apparent huge error in the model projections is multiple positive feedbacks.
Before the large 2007 sea ice decline, most scientists had estimated the sea ice would remain intact year round until near the end of this century. Scientists now agree on 2030, if not long before. The science consensus now is that the main reason is the loss of albedo cooling and the resulting heat absorption effect. Loss of sea ice causes more loss by more warming — a positive feedback loop.
The latest 2011 research by C. Kinnard suggests it is more complicated. "Arctic sea ice extent is now more than two million square kilometres less than it was in the late twentieth century, with important consequences for the climate, the ocean.... Observations show a more or less continuous decline for the past four or five decades.... Both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years.... Enhanced advection of warm Atlantic water to the Arctic seems to be the main factor driving the decline of sea ice extent on multidecadal timescales, and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability."
Rapid Arctic surface warming, loss of Arctic albedo, melting of Arctic land ice and change in Arctic ocean currents have set up a complex positive feedback process causing the Arctic to lose sea ice from above and below at an increasing rate.
Perennial (year round) sea ice decline
Information published by the NSIDC suggests that low sea ice extent in the summertime may be having a direct effect on sea ice growth in the winter months. Warmer air temperatures and a lower-than-average sea ice extent were recorded in some areas of the Arctic Ocean in November.
In recent years, low sea ice extent in the summer has been linked to unusually warm autumn air temperatures, resulting from the larger areas of open water (due to ever increasing sea ice loss) that absorb more heat during the summer. This heat must escape back to the atmosphere in the autumn, before the ocean can freeze over. This escaping heat contributes to warmer-than-average conditions, which have been most apparent in October but may now be extending into November.