Shifting, strengthening winds will help to counteract future sea-level rise in Antarctica—and by doing so, they may help stabilize ice sheets on some parts of the southern continent.
“That’s really significant,” says Aimee Slangen, who studies sea-level variation at the Australian Commonwealth Scientific and Industrial Research Organisation in Hobart and who was not involved with the study.The band of westerly winds that encircles Antarctica has been speeding up and creeping southward since the 1950s. The trend, largely driven by the Antarctic ozone hole, is expected to continue thanks to climate change—and could alone cause a drop in sea level of up to 40 cm over 70 years, according to research led by Leela Frankcombe, a geophysicist at the Centre of Excellence for Climate System Science in Sydney, Australia.
Along Antarctic shores, the effect of the winds could counteract almost half of the average global sea-level rise expected by the end of the century, which the latest report of the Intergovernmental Panel on Climate Change estimates to be up to 97 cm. That report notes, however, that tens of centimeters of additional rise might be caused by the unpredictable collapse of the western Antarctic ice sheet.
Winds of change
The reason for the sea-level drop is the Coriolis force, a consequence of Earth's rotation. As the Antarctic Circumpolar Current flows clockwise around Antarctica, this causes sea water to slope higher to the north and lower to the south. As the westerly winds strengthen by up to 15 percent over the next 70 years, both the current and this slope are expected to increase.
The work, published online this month in Geophysical Research Letters, also finds that the southern shift of the wind by up to 4 degrees latitude over the next 70 years could significantly decrease the transfer of heat from the air to the sea. Because the Southern Ocean plays a key role in global ocean circulation, this would lower the heat content of the entire ocean.In the absence of global warming, this chain of events could cause global sea levels to drop about 5 cm. “Our model really does show how fundamental the Southern ocean is,” says Frankcombe. “It can affect global sea levels.”Frankcombe’s results are not entirely surprisIing: other climate models have similarly predicted this effect. But her model has a higher resolution, and so is able to include the effects of eddies in the Southern Ocean, which most full climate models are not able to incorporate.
Shifting winds are not the only reason why local sea levels will be kept in check. Slangen has shown that as the massive ice sheets in Antarctica and Greenland lose weight by melting, the poles lose some of their gravitational pull. As a result, water slopes away from the poles and towards the equator. This means the net sea-level rise around the Antarctic Peninsula is expected to be about zero by 2100, she says.
Wet and warm
The big question, however, is whether these effects will have a stabilizing effect on the ice sheets.
In theory, sea-level rise would help to cause the collapse of the western Antarctic ice sheet. The base of this ice sheet is underwater; the weight of the continent’s ice presses it down onto bedrock, preventing it from floating like an iceberg. Thinning of the ice-sheet edges or rising sea levels would lever more of the ice up off bedrock, pushing the ‘grounding line’ ever-further inland. Slowing sea-level rise should hinder this, says Slangen. On the other hand, a drop in sea level might cause some ice to flow more steeply and quickly into the sea. “There’s a lot of complicated processes happening there,” she says.
Sridhar Anandakrishnan, a glaciologist at Pennsylvania State University, notes that the biggest factor affecting the stability of the western ice sheet is not sea level but water temperature. Warm waters can melt floating ice shelves from the bottom up at rates of metres per year, allowing continental ice to flow outwards to replace it. “Relative sea level rise is not what makes the ice shelves go away in the 200 year timescale,” Anandakrishnan says. Although a decline in heat exchange in the Southern Ocean might help, he adds, it depends on exactly where those cooler waters hit the ice. “I’m not convinced that’s going to help us a lot,” he says.
This article is reproduced with permission from the magazine Nature. The article was first published on November 15, 2013.