Wintertime Arctic sea ice growth slows long-term decline: NASA

Arctic sea ice [image credit: cbc.ca]

Yes, it does say ‘slows’. There’s some rather convoluted logic about the present and future of Arctic sea ice going on here. Good luck to readers who think they can unravel it. But NASA does have to concede there’s a winter negative feedback going on, while doing its best to downplay possible consequences so as to keep the usual warming obsessions afloat.

New NASA research has found that increases in the rate at which Arctic sea ice grows in the winter may have partially slowed down the decline of the Arctic sea ice cover.

As temperatures in the Arctic have warmed at double the pace of the rest of the planet, the expanse of frozen seawater that blankets the Arctic Ocean and neighboring seas has shrunk and thinned over the past three decades.

The end-of-summer Arctic sea ice extent has almost halved since the early 1980s. A recent NASA study found that since 1958, the Arctic sea ice cover has lost on average around two-thirds of its thickness and now 70 percent of the sea ice cap is made of seasonal ice, or ice that forms and melts within a single year.

But at the same time that sea ice is vanishing quicker than it has ever been observed in the satellite record, it is also thickening at a faster rate during winter. This increase in growth rate might last for decades, according to a new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

This does not mean that the ice cover is recovering, though. Just delaying its demise.

“This increase in the amount of sea ice growing in winter doesn’t overcome the large increase in melting we’ve observed in recent decades,” said Alek Petty, a sea ice scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and lead author of the study. “Overall, thickness is decreasing. Arctic sea ice is still very much in decline across all seasons and is projected to continue its decline over the coming decades. ”

Petty and his team used climate models and observations of sea ice thickness from the European Space Agency’s CryoSat-2 satellite to explore sea ice growth variability across the Arctic. The climate model results compared well both with CryoSat-2’s measurements and the results of another commonly used Arctic sea ice model, giving the authors confidence in the climate model’s ability to capture Arctic sea ice variability.

“The global climate model seems to do a good job of capturing the Arctic sea ice state and shows that most of the thickness change in the central Arctic is from thermodynamics, that is, ice formation and ice melt, although around the Arctic sea ice edge dynamics, which is ice transport, can play a bigger role,” Petty said.

These model simulations showed that in the 1980s, when Arctic sea ice was on average 6.6 feet (2 meters) thick in October, about 3.3 extra feet (1 meter) of ice would form over the winter.

That rate of growth has increased and may continue to do so for several more decades in some regions of the Arctic; in the coming decades, we could have an ice pack that would on average be only around 3.3 feet thick in October but could experience up to 5 feet (1.52 meters) of ice growth over the winter.

It seems counterintuitive: how does a weakening ice cover manage to grow at a faster rate during the winter than it did when the Arctic was colder, and the ice was thicker and stronger?

The answer has to do with the fact that sea ice thickens primarily from below, as the seawater under the ice freezes. If the sea ice layer floating over the ocean thins, the upper ocean is less insulated from the very cold Arctic winter atmosphere. That lowers ocean temperatures and builds more ice from below.

Continued here.