April 1, 2013
Warming Oceans May Be Driving Sea Ice Expansion In Antarctica
Lawrence LeBlond for redOrbit.com - Your Universe Online
A growing number of studies have pegged global warming and climate change as a cause of sea-ice decline in recent decades. However, a newly published study in the journal Nature Geoscience is showing a vastly different scenario. According to researchers at the Royal Netherlands Meteorological Institute (KNMI), ocean warming may actually be driving sea-ice expansion in the Antarctic.
While sea ice at the North Pole has shrunk significantly over the past thirty years, scientists have been scratching their heads as to why sea ice near the South Pole has grown in extent over the same period. The new study suggests Antarctica´s melting sheet has created a surface layer of fresher water that insulates nearby sea ice, allowing for ice expansion during the winter.
The Antarctic meltwater has a relatively low density, so it accumulates in the top layer of the ocean. These fresher waters are then able to re-freeze more easily during autumn and winter.
While the fresher meltwater is able to freeze more readily, the land ice sheet continues to melt because its massive glaciers descend hundreds of feet underneath the surface and are exposed to warming ocean waters, according to the scientists, led by Richard Bintanja and Geert Jan van Oldenborgh.
Bintanja explained that it is “paradoxical and counter-intuitive” that global warming can cause loss in the northern hemisphere and lead to growth in the southern hemisphere. But that´s exactly what they have found in their study.
"This was one of the big apparent contradictions we had which (skeptical) people were always very happy to point out," senior researcher Oldenborgh told The Australian. "Previously, people first thought it was due to the ozone hole, but the problem there is the ozone hole forms in spring and the big increase in sea ice is in autumn and winter.”
"Then a few years ago, a group of people formulated a theory that this was due to the changing winds in Antarctica, and . . . we can see how that influences the sea ice, but we don't see a connection at all with the temperature of the Southern Ocean,” noted Oldenborgh.
"Our mechanism explains both the temperature and the sea ice. . . . We are very happy that we think we've solved part of the problem," he explained.
Satellite gravity measurements of Antarctica had revealed that the continent is losing mass, rather than gaining it, Oldenborgh explained, adding that this falls in line with predictions of the Intergovernmental Panel on Climate Change (IPCC).
Oldenborgh said it is too early to make any concrete predictions, but “we do know things are changing.”
The Antarctic ice sheet is losing mass at a rate of more than 275 gigatons annually, but may be the driving force of the small, but statistically significant expansion in surface sea ice, added Bintanja.
For their study, the authors analyzed satellite data, along with buoy observations of both ocean temperature and salinity for the period 1985 to 2010. They compared the observed changes in data with the output of a global climate model that simulated how losing 275 gigatons of meltwater from Antarctica´s ice sheet each year would affect ocean conditions.
Their model showed meltwater forming a cool freshwater cap that facilitated the expansion of sea ice, leading them to make a pretty sound guess that this was the most likely cause of the trend.
The team´s model paints a pretty good picture of what may be occurring in the Southern Ocean, but it is not the only plausible explanation for Antarctic sea-ice expansion.
“The mechanism could be completely true, but this study does not demonstrate that increased melting has made a significant contribution to the increase in sea-ice cover,” says Paul Holland, an ocean modeler with the British Antarctic Survey in Cambridge, UK.
Holland last year co-authored a study (Nature Geoscience) that showed sea-ice expansion was caused by regional patterns in the wind.
Holland explained that winds change the extent of sea ice by physically moving the ice and also by warming or cooling the sea surface. Holland and his colleague Ron Kwok, a climate researcher at NASA´s Jet Propulsion Laboratory (JPL) in Pasadena, California, used satellite data for sea-ice motion from 1992 to 2010.
Their model showed that in certain regions of Antarctica, such as the Weddell Sea, sea ice changes almost entirely due to the physical force of wind. In other areas, such as the King Hakon Sea, the changes result from the combined effects of wind force and temperature.
Bintanja explained that wind effects are important locally but meltwater influences sea-ice expansion regionally.
Holland, however, argued that ice melt is not uniform around the Antarctic coastline — as assumed by Bintanja and Oldenborgh — but is more concentrated in certain areas. He does believe that both wind and meltwater may be expanding the sea ice near the South Pole, but more research is needed to make solid estimates.
Bintanja and Oldenborgh assert that their study also shows another puzzling scenario. The cool melt water layer may limit the amount of water sucked from the oceans that falls as snow on Antarctica. This is because cold air typically holds less moisture than warm air.