Antarctic Sea Ice Growth Could Reverse

Antarctic sea ice's apparent immunity to global warming is just an illusion, say researchers who may have uncovered the secret of why the southern ice has been growing.

The new research also strongly suggests that the Antarctic sea ice could be in for a big reversal in coming decades.

For years scientists have puzzled over how the sea surface temperature around Antarctica has risen, but sea ice there has been increasing at the same time.

"We just want to understand this paradox," said Jiping Liu of Georgia Institute of Technology. "For the past 30 years, the Arctic sea ice has been decreasing while Antarctic sea ice has been increasing. We've been trying to explain this."

To do that, Liu and veteran climate modeler Judith Curry analyzed Southern Ocean temperature records and the best simulations of sea surface temperature.

They found that higher sea surface temperatures during the last half of the 20th century probably revved up the hydrological cycle above the Southern Ocean, creating a situation in which more sea ice can grow. In other words, higher sea surface temperatures increased evaporation in more temperate zones, which ramped up precipitation closer to Antarctica.

That additional precipitation has yet another effect that helps increase sea ice: It lowers the salinity of the surface water, which slows the melting of sea ice, Liu explained. The result is that the growth of sea ice has outpaced melting.

But the ice building and preserving effects are only temporary, Liu told Discovery News, who with Curry published their findings in the latest issue of the Proceedings of the National Academy of Sciences.

If the sea and air warm even more in the 21st century, as projected, much of that extra snowfall could turn into rainfall, which would rapidly melt ice all around the southernmost continent.

While other researchers certainly respect the work of Liu and Curry, they are cautious about the details.

For one thing, it's not easy to leap from a model to the actual detailed workings of the ocean, said Doug Martinson of Lamont-Doherty Earth Observatory of Columbia University. Martinson has spent the better part of two decades studying the behavior of sea ice firsthand.

"Judy (Curry) is very good at this and I think they are using the models responsibly," said Martinson. "But it's a little scary to run them to look at fine details. There are so many feedbacks. It's hard to say exactly what's going to happen."

"The overall story is correct," agreed polar researcher Bruno Tremblay of McGill University. "But there are different ways you can achieve the increase in ice."

One thing that is clear: The Arctic and the Antarctic are very hard to compare, Martinson said, which is why work like that Liu and Curry are undertaking is important.

"They are apples and oranges," Martinson said. "They are dramatically different systems."

In one case, there is an icy ocean surrounded by land. In the other, there is an icy continent surrounded by icy water.

Another thing Martinson agreed with, unequivocally, he said, is the final line in Liu and Curry's paper: "(I)mproved representation in models of atmosphere–sea ice–ocean interactions will be critical for forecasting Antarctic sea ice changes as climate warms."

"That's a hands down yes," said Martinson.