Global warming likely to reverse trend in coming decades
By Summit Voice
FRISCO — Climate scientists have long suspected that increasing winds around Antarctica have been the main cause of growing sea ice extent in the southern hemisphere, and new research from the University of Washington shows how and why that might be happening — even as overall global temperatures warm.
Global warming deniers have tried to use the growth of Antarctic sea as a weapon in their battle against science, but climate researchers point out that the loss of Arctic sea ice far outweighs the small increase in the southern hemisphere. And the new research suggests that, as global temperatures continue to increase, Antarctic sea ice is all but certain to start shrinking.
Overall, Antarctic sea ice has been increasing by about 1 percent annually, which has led to record sea ice extent in the region the past few years. As of September 16, Antarctic sea ice extent reached about 7.51 million square miles, a record for the date and about 3.9 percent above the 30 year average. By contrast, this year’s Arctic summer minimum ice extent is approximately 30 percent below the 30-year average.
The new University of Washington study, to be published in the Journal of Climate, shows that stronger westerly winds swirling around the South Pole can explain 80 percent of the increase in Antarctic sea ice volume in the past three decades. The research was funded by the National Science Foundation
“People have been talking about the possible link between winds and Antarctic sea ice expansion before, but I think this is the first study that confirms this link through a model experiment,” commented Axel Schweiger, a polar scientist at the UW Applied Physics Lab. “This is another process by which dynamic changes in the atmosphere can make changes in sea ice that are not necessarily expected.”
UW oceanographer Jinlun Zhang said there is overwhelming evidence that the the Southern Ocean is warming, which led to a conundrum for researchers.
“Why would sea ice be increasing? Although the rate of increase is small, it is a puzzle to scientists,” Zhang said in a press release from the university.
Satellite data show that the polar vortex swirling around the South Pole has strengthened, and that it’s also driving the sea ice together, causing ridging and making the ice thicker overall. The thicker ice lasts longer into the summer, and exposes more sea water to cold temps, which, in turn, leads to further ice formation.
Zhang and colleagues used computer models to try and pin down the relationship between wind and Antarctic sea ice, showing that thick ice has increased by about 1 percent per year from 1979 to 2010, while the amount of thin ice stayed fairly constant. The end result is a thicker, slightly larger ice pack that lasts longer into the summer.
“You’ve got more thick ice, more ridged ice, and at the same time you will get more ice extent because the ice just survives longer,” Zhang said.
When the model held the polar winds at a constant level, the sea ice increased only 20 percent as much. A previous study by Zhang showed that changes in water density could explain the remaining increase.
The results don’t explain why the circumpolar winds are strengthening, but scientists have theorized that it could be related to global warming, ozone loss or even just natural variability.
.To explain the differences in sea ice trends at the two poles, scientists have also pointed out geographic and climatic differences: According to Zhang, surface air warming in the Arctic appears to be greater and more uniform, Zhang said.
Another difference is that northern water is in a fairly protected basin, while the Antarctic sea ice floats in open oceans where it expands freely in winter and melts almost completely in summer.
The sea ice uptick in Antarctica is small compared with the amount being lost in the Arctic, meaning there is an overall decrease in sea ice worldwide.
Many of the global climate models have been unable to explain the observed increase in Antarctic sea ice. Researchers have been working to improve models to better reproduce the observed increase in sea ice there and predict what the future may bring.