SOMEWHAT SCARY GRAPH
MOSTLY NONSCARY GRAPH
Scary plot from treehugger
Less scary plot from NSIDC
Non-scary plot from Meteorology News
All show the SAME DATA.
A hysteresis is like driving around Madison Wisconsin: the way from A to B has no resemblance to the way from B to A. If Greenland melts due to a little warming, it will take a LOT of cooling to grow the ice sheet back. Or as Lech Walesa once said, it is easier to make a fish soup from an aquarium than to make an aquarium out of a fish soup. (By which he was referring, of course, to Poland's emergence from communism, which has an unfavorable hysteresis.)
Does something like this apply to sea ice? The argument is that a little more CO2 will melt enough ice that the Arctic albedo will DECREASE (was "increase", corrected. See comments.) enough to melt more ice, such that once one crosses a certain threshhold, the Arctic would suddenly warm all of its own, and it would take a huge cooling push to get sea ice back.
Eisenman has a simple energy balance model which has convinced him that 1) yes, it's possible and 2) no it's a long way into the future, above 4xCO2. He finds that most of the Winter sea ice needs to be gone before a small increase in forcing flips the Arctic Ocean really into a warm state where it becomes impossible to restart the ice cycle without a very large reversal of the forcing.
He made two very interesting points related to the sea ice trends.
The charts we see show much more decline in summer sea ice than in winter sea ice. But it happens that the latitudes of winter-only sea ice are mostly covered in land. If you plot, instead of ice area, the latitudinal extent of sea ice, you actually get very similar declines for all seasons.
Secondly, the 2007 anomaly that everybody got all excited about wasn't as spectacular as it looks. Five or six comparable anomalies have occurred on the latitude metric, just in other seasons. It's simply the fact that the 2007 anomaly occurred in the season of the sea ice minimum that it jumps out visually. It's a display of quantitative information question. Plots of annual ice minimum looked really scary in in late '07, but other ways of looking at the data much less so.
None of this is to say the ice isn't in decline, nor that we won't have ice free summers soon. It is to say that the scare of 2007 was a bit overblown and that though there may well be a tipping point in the Arctic, it's fairly far off at present, at least in Eisenman's opinion.
The guts of the talk are here at PNAS if you can access that. He made many other interesting points in an engaging and enlightening talk. A first rate presentation and in my opinion a first-rate hire for the department that gets him.