Sea ice down under: Antarctic ice and climate

The sea ice cover surrounding the continent of Antarctica is on average thinner than Arctic sea ice, and more susceptible to winds. This photo, from a 2003 research cruise, shows the trail of open water left by a research ship. In the background, an iceberg towers above the thinner sea ice cover. Credit: Ted Scambos, NSIDC

The Arctic Ocean is not the only place with sea ice. The ocean surrounding the continent of Antarctica also freezes over each winter. But we don’t hear much about sea ice on the bottom of the planet. What’s happening to Antarctic sea ice and why does it matter?

One reason that we hear less about Antarctic sea ice than Arctic sea ice is that it varies more from year to year and season to season than its northern counterpart. And while Arctic ice has declined precipitously over the past thirty years of the satellite record, average Antarctic sea ice extent has stayed the same or even grown slightly.

Yet researchers say that Antarctic sea ice plays an important role in climate, helping to protect the Antarctic Ice Sheet from waves, warmer surface water, and warmer air that can destabilize Antarctica’s ice shelves and help speed the flow of continental ice into the ocean. And in some regions, Antarctic sea ice is not as stable as it used to be.

This map shows the trends for sea ice concentration around Antarctica in April 2011. The blue shading indicates the region to the west of the Antarctic Peninsula where sea ice concentration has declined significantly in the last 30 years. Red areas around the continent show regions where sea ice concentration was greater than normal. Overall, Antarctic sea ice shows a slight increasing trend, but in certain regions it is declining rapidly. Credit: NSIDC

A different world

“The two polar regions are essentially geographic opposites,” said Sharon Stammerjohn, a sea ice expert at the University of Colorado Institute for Arctic and Alpine Research (INSTAAR). “Sea ice in the Arctic Ocean is land-locked, while sea ice in the Southern Ocean is surrounded by open ocean.” That means that while Arctic sea ice is confined in a given space, Antarctic sea ice can spread out across the ocean, pushed by winds and waves. That also means that ice extent varies much more in the Southern Hemisphere than it does in the North.

Overall, Antarctic sea ice has grown slightly over the past thirty years of the satellite record, but the trends are very small, and the ice extent varies a lot from year to year. In Southern Hemisphere winter months, ice extent has increased by around one percent per decade.  In the summer, ice has increased by two to three percent per decade, but the variation is larger than the trend.

Although Antarctic sea ice is increasing overall, certain regions around Antarctica are losing ice at a rapid pace. In the Amundsen and Bellingshausen seas, west of the Antarctic Peninsula, the sea ice cover has declined dramatically in the last thirty years, with the winter sea ice cover lasting three months less in 2010 than it did in 1979. The main areas where ice extent is growing are the Ross Sea (north of the largest U.S. base, McMurdo) and the eastern Weddell Sea (south of Africa), although there is a lot of variability.

Changing winds

Some research suggests that the changes in Antarctic sea ice—both where it is increasing and where it is decreasing—are caused in part by a strengthening of the westerly winds that flow unhindered in a circle above the Southern Ocean. “Antarctic sea ice is governed more by wind than by temperature,” said NSIDC lead scientist Ted Scambos. “The effects of climate change play out differently in the southern hemisphere than the northern hemisphere.”

Scientists say that this westerly wind pattern has grown stronger in recent years as a result of climate change. However, because Antarctic sea ice is so vulnerable to the changes in both the atmosphere and ocean, researchers are also looking at other climate patterns, such as the high latitude response to El Niño-Southern Oscillation, as well as the effects of changing ocean temperature and circulation, to understand how Antarctic sea ice is changing.

Stammerjohn and other scientists say that declining sea ice around the Antarctic Peninsula probably helps destabilize continental ice in that area by allowing the air above the ocean in that region to warm more than before. These continental ice areas are shrinking and therefore contributing to sea level rise. Stammerjohn said, “Though it is true that, on average, Antarctic sea ice is not changing, or even slightly increasing, the overall average hides a very large regional decrease that could have global consequences.”

References

Stammerjohn, S. E., D. G. Martinson, R. C. Smith, X. Yuan, and D. Rind. 2008. Trends in Antarctica annual sea ice retreat and advance and their relation to El Nino-Southern Oscillation and Southern Annular Mode Variability. Journal of Geophysical Research. Vol 113. Doi:10.1029/2007JC004269.

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