There’s no doubt that growing Antarctic sea ice is a mystery in the climate system — and an anomalous, seemingly contrary indicator. However, if a controversial and much-discussed new paper from famed former NASA scientist James Hansen and 16 colleagues is correct, then actually it could be a troubling climate warning sign. (Indeed, other scientists have reached similar conclusions.)
According to Hansen’s thinking, expanding Antarctic sea ice is precisely what you would expect to see if the Antarctic continent itself is losing a lot of ice mass from its vast ice sheet, adding to sea level rise.
The thinking goes like this: As ice shelves melt, and more inland ice slides towards the sea, a gigantic volume of cold, fresh water enters the ocean. This freshwater pulse, the researchers continue, promotes ocean “stratification,” in which a cold surface layer lies atop a subsurface warmer layer. The cold surface layer promotes more sea ice growth atop open water, while the warm lower layer sneaks beneath that ice and continues to melt submerged ice shelves, which plunge deep into the water at the fringes of the continent.
The fundamental physical reason for the expansion of sea ice in this scenario is that cold, fresh water is less dense than warmer, salty water. Or as the National Snow and Ice Data Center explains:
As deep ocean temperatures around Antarctic rise, they increase ice shelf melt, according to a study led by Richard Bintanja
. This meltwater is creating a cool layer near the surface of the ocean that promotes sea ice production. In addition, the meltwater is fresh, or much less salty and dense than surrounding saline ocean layers. So fresher meltwater floats upward, mixing with the cold surface layer, lowering its density. As this fresh layer expands, it forms a stable puddle on top of the ocean that makes it easier to produce and retain sea ice.
In this sense, expanding Antarctic sea ice might be anything but good news.
Indeed, in the troubling scenario outlined by Hansen and his colleagues, it’s part of a series of feedbacks that lead to rapid sea level rise. “Amplifying feedbacks, including slowdown of [the Southern ocean’s overturning circulation] and cooling of the near-Antarctic ocean surface with increasing sea ice, may spur nonlinear growth of Antarctic ice sheet mass loss,” write Hansen and his colleagues.
“Effects of freshwater injection and resulting ocean stratification are occurring sooner in the real world than in our model,” they add.
Reached by phone, Hansen added that one reason he thinks the model he is using is is right — and other models are not — is that it captures Antarctic sea ice expansion. “All the other models have sea ice disappearing as the planet get warmer,” he says. He also thinks the Antarctic ice expansion trend will continue, along with ice sheet melt.
“It will be clearer, give us a few more years,” he says.
Granted, the new Hansen study is simultaneously advancing a gigantic new synthesis of existing research and also pushing the envelope — it will need to be scientifically digested for some time, and has already drawn some critical comments from experts. However, the Hansen paper also just received its first official peer review by one of several reviewers designated by the journal, Atmospheric Chemistry and Physics Discussions – the University of Chicago geoscientist David Archer. And it is a strong review – Archer says that the paper is a “masterwork of scholarly synthesis, modeling virtuosity, and insight, with profound implications.”
It’s also worth noting that other explanations for the expansion of Antarctic sea ice have been offered that do not turn on meltwater and ocean freshening. Overall, it’s not clear that scientists yet fully understand what’s going on in this extremely remote part of the climate system.
Nonetheless, this serves to point out the danger of seizing on a mysterious anomaly in the climate system — like expanding Antarctic sea ice — and interpreting it to score political points. For what may seem like good news for the climate — and bad news for climate “alarmists,” as skeptics put it — could be the utter opposite.
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