2012年11月

More on the 2012 Melt Season: the New Normal

Although the record for sea ice extent was reached already on August 26 of this year (see August’s IceBlog), this year’s melt season continues to have surprises for scientists as the sun continues its march southward and more and more of the Arctic enters 24-hour darkness.

The most marked is the encompassing nature of this year’s sea ice melt.  The melting continued after breaking the 2007 record, ending up at 3.41 million sq kilometers (1.32 million square miles), easily beating the previous record of 4.2 million and more than the three standard deviations below the 1979-2000 average.  Melting also continued  – albeit slowly – well past sundown on September 21, not beginning its recovery with new freezing until nearly a week past the autumnal equinox.

Indeed, the new sea ice minimum probably once again sets a “new normal,” just as 2007 did before.  All the post-2007 sea ice minimums were more within that range than the previous years, as though the “average” suddenly dropped a level.  Given how little older and thicker ice now exists, it seems most likely that the most-2012 minimums will now lie more around that “new normal” – until the bar for “normal” drops again.  Given that much thinner ice today, some scientists are again beginning to speculate that an ice-free summer may occur sometime within this decade.

A second surprising development was Greenland.  As the below satellite composite shows, melting occurred this past summer over nearly its entire area, the red area telling a clear visual story of unusual warming.  Such melting is however not entirely unprecedented.  Ice core records show that it has taken place about once every 150 years or so, and the last such occurrence was about that long ago.  Therefore, whether this year’s Greenland-wide melt is still another sign of accelerated global warming in the cryosphere will need to wait until coming years for judgment.  But if we see Greenland-wide melt again already in this decade, we are clearly off that 150-year pattern – and once again in “new normal,” uncharted territory.

Cryospheric climate change in the past decade has increasingly seen the former “extremes” becoming the “means” – events earlier forecast as possible, yet highly unlikely to become reality.  James Hansen and Makiko Sato (2012, Climate Change: Inferences from Paleoclimate and Regional Aspects) have argued that a better measure of future climate change is not to extrapolate from current changes in a linear fashion, but from the rate of change.  At least in the Arctic, this way of looking at forecasting the future seems increasingly correct.