New study pinpoints regional growth and decline of Antarctic sea ice
By Bob Berwyn
FRISCO — After compiling more than 5 million individual daily ice motion measurements of sea ice motion around Antarctica, scientists from the U.S. and U.K. say they’re sure that the recent increases in Antarctic sea ice are linked to changing wind patterns in the region.
Essentially, the circumpolar winds are strengthening around Antarctica, said Dr. Ron Kwok, of NASA’s Jet Propulsion Laboratory in California. Why those winds are intensifying, and whether it’s linked with a warming atmosphere remains as a huge question, Kwok said. View a mult-year animation of Antarctic sea ice changes here.
“We are basically finding evidence of change over a long time scale … That’s why it’s inportant to quantify the mechanisms,” he said. “It’s probably associated with a changing climate. The Antarctic sea ice interacts with the global climate system very differently than that of the Arctic, and these results highlight the sensitivity of the Antarctic ice coverage to changes in the strength of the winds around the continent.”
The changes in the wind could be part of a natural cycle of variability in the region; it could be driven at least in part by the Antarctic ozone hole, or it may be linked with as-yet unknown factors, said Dr. Paul Holland, A British Antarctic Survey scientist who was the lead author of the recent study.
The question of Antarctic sea ice expansion was in the spotlight the past couple of months. Even as Arctic sea ice dwindled to a new record low extent in late September, Antarctic sea expanded to a record high extent, but not nearly by the same magnitude as Arctic sea ice loss, leading scientists to search for the reason in Antarctic sea ice growth.
“Until now these changes in ice drift were only speculated upon, using computer models of Antarctic winds,” said Paul Holland, a British Antarctic Survey researcher who was the lead author of the new study that ties the growth to changing wind patterns.
“This study of direct satellite observations shows the complexity of climate change. The total Antarctic sea-ice cover is increasing slowly, but individual regions are actually experiencing much larger gains and losses that are almost offsetting each other overall,” Holland said.
“We now know that these regional changes are caused by changes in the winds, which in turn affect the ice cover through changes in both ice drift and air temperature. The changes in ice drift also suggest large changes in the ocean surrounding Antarctica, which is very sensitive to the cold and salty water produced by sea-ice growth.”
The study is based in images of Antarctic sea ice captured over a period of 19 years by four US Defense Meteorological satellites.
“Sea ice is constantly on the move; around Antarctica the ice is blown away from the continent by strong northward winds. Since 1992 this ice drift has changed. In some areas the export of ice away from Antarctica has doubled, while in others it has decreased significantly,” Holland said.
“The interesting question to me is, why hasn’t there been a large decrease in Antarctica … the regional processes going on that maybe are more subtle than in the Arctic, but in the long-term those changes could be very important,” he said, adding that the observed changes vary across the vast Antarctic region.
“Regionally the trends are huge. In some areas, sea ice is declining. In the Bellingshausen Sea, it’s decreasing as fast as in the Arctic,” he said.
Global climate driver
Sea ice plays a key role in the global environment by reflecting heat from the sun and providing a habitat for marine life. At both poles, sea ice cover is at its minimum during late summer. However, during the winter freeze in Antarctica the ice cover expands to an area roughly twice the size of Europe. Ranging in thickness from less than a meter to several meters, the ice insulates the warm ocean from the frigid atmosphere above.
The ecoystems associated with sea ice cover are very sensitive to temperature changes, persisting in a narrow range of temperatures — just a couple of degrees Celsius, Holland said.
“Very small changes could have a huge impact on the biology,” he added.
Arctic v Antarctic
The new research also helps explain why observed changes in the amount of sea-ice cover are so different in the two polar regions. The Arctic has experienced dramatic ice losses in recent decades while the overall ice extent in the Antarctic has increased slightly.
The small Antarctic increase is actually the result of much larger regional increases and decreases. In places, increased northward winds have caused the sea-ice cover to expand outwards from Antarctica. The Arctic Ocean is surrounded by land, so changed winds cannot cause Arctic ice to expand in the same way.
Scientists have observed contrasting climate changes around Antarctica in recent decades. The Antarctic Peninsula has warmed as much as anywhere in the Southern Hemisphere, while East Antarctica has shown little change or even a small cooling around the coast.
Kwok and Holland said it’s important to distinguish between the Antarctic Ice Sheet — glacial ice — which is losing volume, and Antarctic sea ice — frozen seawater — which is expanding.
The research was funded by the Natural Environment Research Council and National Aeronautics and Space Administration and was published this week in the journal Nature Geoscience.