A good article here about the loss of Arctic ice. The uncertainties in the modelling are noted:
Computer models that simulate how the ice will respond to a warming climate project that the Arctic will be seasonally ‘ice free’ (definitions of this vary) some time between 2040 and the end of the century. But the observed downward trend in sea-ice cover suggests that summer sea ice could disappear completely as early as 2030, something that none of the models used for the next report by the Intergovernmental Panel on Climate Change comes close to forecasting1.
“There’s a tremendous spread between observations and model projections,” says Serreze. “It might be that natural variability is larger than assumed, or perhaps models don’t get the change in ice thickness right.” Uncertainty also remains over the strength of various natural ‘feedbacks’. For example, an exposed ocean is darker than an ice-covered surface and so absorbs more solar heat, causing yet more warming and melting.
A lack of fine detail about circulation patterns in the Arctic Ocean could also be throwing off the models. For example, a survey carried out in 2008 revealed 20 formerly unobserved eddies, each some 15 to 20 kilometres in diameter, in waters north of Canada. “Whether these are new features, and what role they might play for ocean-mixing processes, we don’t know yet,” says Yves Gratton, an oceanographer and Arctic researcher at the National Institute of Scientific Research in Montreal, Canada.
Ice loss could also accelerate if the ice pack’s underlying waters warm up. Unlike in most of the world’s oceans, the coldest water in the Arctic, at −1 °C to −2 °C, is at the surface; below a depth of 200–300 metres, saltier and warmer water of about 1 °C flows in from the Atlantic. The cold surface layer — called the halocline — isolates the sea ice from the warmer water below.The article also notes that it may well mean a lot of snow this winter in the US or Europe.
But the halocline is vulnerable to warming from above, says Henning Bauch, a marine geologist at the GEOMAR research centre in Kiel, Germany. A thinning halocline — something that has not yet been observed — would not only jeopardize the sea ice but could also melt the carbon-rich permafrost beneath shallow coastal waters2, releasing greenhouse gases into the atmosphere.