Opinion OK, so the floating Arctic ice cap appears to be shrinking. Catastrophe if it goes on, right? As white ice reflects heat into space, past a certain point more and more heat will not be reflected, more and more ice will melt. Past such a "tipping point", the ice cap would never recover - it would vanish completely, taking with it the ice cover of Greenland which would cause huge rises in sea levels and Biblical flooding worldwide.
Not so much, according to the latest research by German climate scientists. It seems that even in the case of a completely ice-free summer with the sun shining down onto an unprotected Arctic Ocean 24 hours a day (as it does in summer time up there), the heat absorbed by the sea would not be enough to permanently remove the ice cap. It would recover, in fact, within two years: there is no tipping point.
According to Steffen Tietsche, a polar ice expert at the Max Planck Institute for Meteorology in Hamburg, this is because removal of ice works two ways. It lets the sun's rays warm the ocean beneath more strongly, but it also lets heat escape from the sea more easily. Thus, following an ice-free summer the Arctic will shed the extra heat fast due to the lack of its usual igloo-like ice blanket. Soon it will be so cold that the ice will reappear.
Tietsche and his colleagues write:
We examine the recovery of Arctic sea ice from prescribed ice-free summer conditions in simulations of 21st century climate in an atmosphere–ocean general circulation model. We find that ice extent recovers typically within two years.
The excess oceanic heat that had built up during the ice-free summer is rapidly returned to the atmosphere during the following autumn and winter, and then leaves the Arctic partly through increased longwave emission at the top of the atmosphere and partly through reduced atmospheric heat advection from lower latitudes. Oceanic heat transport does not contribute significantly to the loss of the excess heat.
Our results suggest that anomalous loss of Arctic sea ice during a single summer is reversible, as the ice–albedo feedback is alleviated by large-scale recovery mechanisms. Hence, hysteretic threshold behavior (or a “tipping point”) is unlikely to occur during the decline of Arctic summer sea-ice cover in the 21st century.
Details of the full paper in Geophysical Research Letters can be read here.
So the gradual decline in ice extent seen in recent decades may continue, but even if a very hot summer seriously eats away at the sea ice - even so much as to completely melt it all - it will recover; there will be no sudden disaster this century.
Meanwhile down at the south pole the sea ice around Antarctica is actually increasing. The ice there has been covering another 100,000 square km more sea each decade for the last 30 years, despite the well-publicised losses of ice shelf in the Western Antarctic. ®