Emergent patterns in nature and society

Good news about climate change?: South winds can counteract sea level rise

Nature News recently reported that changing winds dampen Antarctic sea-level rise. In a nutshell, driven by the ozone hole and continued by climate change, southern circumpolar  winds might increase in force reducing somehow the exchange of heat between atmosphere and ocean. As a result this will counter act sea level rise by dampening feedbacks related with gravitational effects of the heavy ice sheets that would displace water towards the tropics. I quote:

The work, published online this month in Geophysical Research Letters1, 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 has 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 by about 5 centimetres. “Our model really does show how fundamental the Southern Ocean is,” says Frankcombe. “It can affect global sea levels.”

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 that the net sea-level rise around the Antarctic Peninsula is expected to be about zero by 2100, she says


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 West 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 farther inland. Slowing sea-level rise should hinder this, says Slangen. By contrast, 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 in Philadelphia, 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,” says Anandakrishnan. 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.

Although the note mention that wind circulation can counteract half of the expected sea level rise from Antarctica over the next 100 years, it is still uncertain if this new feedback will be enough to avoid the West Antarctica Ice Sheet collapse.

1. Frankcombe, L. M., Spence, P., Hogg, A. M. , England, M. H., & Griffies, S. M. Geophys. Res. Lett. http://dx.doi.org/10.1002/2013GL058104 (2013).


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