The rapid disappearance of the Arctic sea ice raised for many the prospect of a "tipping point" at which ice loss becomes irreversible and an ice-free North becomes a permanent condition.
Especially after the dramatic fall in 2007, efforts have been made to model the probable behavior of the rapidly waning sea ice. That led to an important paper with a simple method and an elegant result. Several scientists at the university of Washington did model runs in which CO2 forcing was gradually increased, looking for a tipping point ("threshold behavior"). They didn't find any. The sea ice disappeared gradually as conditions warmed. When they allowed conditions to cool, the sea ice came back. No tipping points in the model.
Well, brace yourselves for the shock, but "skeptics" missed the point of the paper, and shouted the results from every rooftop. Stupid scientists! "The Next Ice Age Now" crowed "More global warming propaganda debunked."
The actual result, rather than the result deniers fantasized had come about, was this: the dramatic fall in sea ice cover to date represents a steady response to the long-term climate warming. It is progressing fast because the climate is changing fast. The ice cover is still falling apart, not (so far) because of a tipping point, but because of rapid global warming. It could recover if we successfully halted or reversed global warming. Short of that, it will continue its death spiral.
Not really a pro-"skeptic" message, if you read past the headline. Fortunately for their peace of mind, they rarely do.
Work to understand the rapid changes to the Arctic continues. Chris R points us to this 2008 paper:
It is argued that deep atmospheric convection might occur during winter in ice-free high-latitude oceans, and that the surface radiative warming effects of the clouds and water vapor associated with this winter convection could keep high-latitude oceans ice-free through polar night. In such an ice-free high-latitude ocean the annual-mean SST would be much higher and the seasonal cycle would be dramatically reduced - making potential implications for equable climates manifest. The constraints that atmospheric heat transport, ocean heat transport, and CO2 concentration place on this mechanism are established. These ideas are investigated using the NCAR column model, which has state-of-the-art atmospheric physics parameterizations, high vertical resolution, a full seasonal cycle, a thermodynamic sea ice model, and a mixed layer ocean. Citation: Abbot, D. S., and E. Tziperman (2008), Sea ice, high-latitude convection, and equable climates, Geophys. Res. Lett., 35, L03702, doi:10.1029/ 2007GL032286.It's not bathtub reading, but basically what Abbot and Tziperman set out to do was to explain evidence from the paleoclimate record of a warmer Arctic (1) and a less pronounced seasonal cycle at the poles. Both the late Cretaceous and the early Paleogene climate had these features, but existing climate models do not reproduce them well. When they created a more sophisticated model of the Arctic, they found it could settle into a stable ice-free state secondary to changes in cloud cover and atmospheric circulation (they discuss the same issues in a slightly earlier paper published by the Royal Meteorological Society.) The former has been cited 23 times; the latter 16, according to Google Scholar. So while this is serious science, the theory has not exactly caught fire. That they continue to explore the idea in Feb 2009 and again in July 2009, without a lot of other climate scientists taking up the charge,deepens the suspicion that Abbot/Tziperman have not convinced their colleagues that this is a thing, despite a 2011 paper (h/t Artful Dodger) which Abbot wrote with the great Raymond Pierrehumbert, which cites one of the 2009 papers as part of a broader discussion of possible sea ice tipping points.
On the other hand, it's not as if we have a lot of great explanations for the paleoclimate record laying around:
The consensus among these proxies suggests that Arctic temperatures were ∼19 °C warmer during the Pliocene than at present, while atmospheric CO2 concentrations were ∼390 ppmv. These elevated Arctic Pliocene temperatures result in a greatly reduced and asymmetrical latitudinal temperature gradient that is probably the result of increased poleward heat transport and decreased albedo. These results indicate that Arctic temperatures may be exceedingly sensitive to anthropogenic CO2 emissions.The specter of an Arctic tipping point has not been laid to rest. If a similar amount of forcing to today's somehow got the Pilocene's Arctic 19C warming than today's, than some kind of hole in the floor seems a logical area of concern. (I wonder idly if the phlegmatic Dmitrenko ever modeled 19 degrees of warming over the East Siberian Arctic Shelf.)
UPDATE: Check out Dosbat, Chris R's insanely good blog, for even more Arctic/Climate change goodness. Added to the blogroll, as well.