The Arctic Ocean is entering its spring melt season, after reaching a winter maximum that was the sixth lowest in the satellite record and that appears to have left the North Pole covered, not by thick multi-year ice but by thin first-year ice. Meanwhile, instead of contracting, sea ice around the continent of Antarctica continues to expand in extent ever so slightly.
As we’ve explained previously, there is an enormous difference between the degree to which Antarctic sea ice is expanding (to a slight but statistically significant degree) and Arctic sea ice is declining (catastrophically). Furthermore, there are fundamental differences between the very nature of sea ice at both polar regions – notably the fact that Antarctica as a continent is surrounded by ice that largely drifts away and melts each summer, and doesn’t generally have the kind of multiyear ice that is disappearing from the Arctic. So the comparison isn’t apples-to-apples.
Even so, explaining the mechanism responsible for the relative stability of Antarctic sea ice – even as some Southern Ocean temperatures increase – is something that has been occupying a number of researchers. A British Antarctic Survey study led by Paul Holland late last year concluded that changes in local wind patterns were spreading the ice out across a greater area. An earlier study by the Georgia Institute of Technology proposed that warmer Antarctic air temperatures are creating greater precipitation, which falls as snow insulates the region’s sea ice cover from warming from below.
This theory addresses an important distinction between the thermodynamics of Arctic and Antarctic sea ice; in the words of Georgia Tech’s Judith Curry, “In the case of the Arctic most of the melting is driven from the warmer atmosphere above. In the Antarctic most of the melting has been driven from the ocean below.” This warming sub-surface water has been implicated in the demise of some West Antarctic glaciers and has also been observed to be impacting glaciers in Greenland.
Now a new theory expands on the impact of that deeper, warm water. According to Richard Bintanja and colleagues at the Royal Netherlands Meteorological Institute, glacial melting is causing the spread of plumes of cold surface water, which – similar to the effect hypothesized by Curry and colleagues at Georgia Tech – is insulating the ice from the warmer waters below and, being cooler, is freezing more easily in fall and winter, thus ensuring a continuing supply of fresh ice.
Of course, the climate is a complex system and it doesn’t necessarily follow that these assorted theories are mutually exclusive (although this latest study suggests that colder surface waters would result in less water evaporating and falling as snow). Holland, the British Antarctic Survey researcher, responded to the new study by saying that, ”The possibility remains that the real increase is the sum of wind driven and melt-water driven effects, of course. That would be my best guess, with the melt water effect being the smaller of the two.”
Photograph of an icebreaker in early-season sea ice in Antarctica’s Ross Sea, by Kieran Mulvaney.