A late-winter expansion of Arctic sea ice is a good example of ice-forming dynamics that could keep the Arctic from hitting a "tipping point" in the near future.

Some scientists have predicted that rising temperatures could create a runaway feedback loop in the Arctic. Sunlight-reflecting ice sheets would give way to sunlight-absorbing water, driving up temperatures and melting even more ice. The Arctic climate would change so dramatically that winter ice couldn't form again, producing planet-wide ripples in weather patterns.

But some research suggests that other, previously underappreciated forces may stabilize the melt before it's complete. The Arctic will soon be ice-free in summer, and winter ice will decline, but it won't suddenly become permanently ice-free.

"Everyone thought there would be a tipping point," said Dirk Notz, a Max Planck Institute climate scientist. "But that's too simple."

The most recent Arctic sea-ice spurt was caused by a cold snap over the Bering and Barents Seas that allowed ice to form until later than usual in March, nudging total ice cover towards averages seen between 1979 and 2000. But, Notz emphasized, this was just a single data point. Since 1979, the Arctic's maximum winter sea-ice cover, measured before the summer melt, has dropped by about 6 percent.

The spurt does, however, demonstrate the ability of thin ice, such as that at the edge of Arctic ice sheets, to grow very rapidly. That's a big reason why Arctic ice sheets should be able to re-form in winter. Indeed, as Notz described in a review of polar-sea-ice research in December in Proceedings of the National Academy of Sciences, there's no evidence of the Arctic hitting a tipping point in the last several million years, even though temperatures and sea-ice levels have fluctuated widely.

Over the last few years, Arctic sea-ice cover has reached modern historical lows, stoking the tipping-point fears. Though a tipping point isn't out of the question, it would likely happen at greenhouse gas levels beyond what's expected, said Notz.

The same can't be said for ice sheets in western Antarctica and possibly Greenland. The dynamics of that ice, much of which rests atop solid land rather than floating on water, are different. Melting ice could slide off continental shelves and into the ocean faster than it's replaced by fresh snowfall. This may have been what fueled two sudden, massive sea-level rises at the end of the last Ice Age.

Tipping-point evidence is stronger for western Antarctica than Greenland, said Notz. But even the absence of a tipping point wouldn't necessarily be reassuring. "It doesn't mean Greenland won't melt away," he said. "It just means it will happen gradually."

Image: Ice in the Bering Sea/NASA.

See Also:

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.