Growth trend of Antarctic sea ice could reverse this century
Posted September 10, 2010
The Arctic and the Antarctic may both be cold, icy, foreboding places — but the Earth’s two polar regions are quite dissimilar.
One is an ocean surrounded by land, while the other is land surrounded by ocean. Polar bears rule in the Arctic, while penguins are iconic of the Antarctic.
Even sea ice behaves differently. In the Arctic, it’s historically thicker and more multi-year survives from year to year. However, over recent decades, sea ice extent in the summer has dropped significantly, faster than can be explained by climate models. In contrast, the trend for the extent of annual sea ice around Antarctica for the same time period has increased slightly.
New research out of the Georgia Institute of Technology offers an explanation of the apparent paradox of expanding sea ice in Antarctica — and suggests the opposing trends between the Arctic and the Antarctic may soon end.
Photo Credit: Robyn Waserman/Antarctic Photo Library
The sea ice edge during the summer in the Ross Sea.
“We wanted to understand this apparent paradox so that we can better understand what might happen to the Antarctic sea ice in the coming century with increased greenhouse warming,” said Jiping Liu , a research scientist in Georgia Tech’s School of Earth and Atmospheric Sciences , in a press release.
In a paper that appeared in the journal Proceedings of the National Academy of Science in August, Liu and co-author Judith A. Curry , also at Georgia Tech, describe how increased snowfall, associated with warming of the upper waters of the Southern Ocean around Antarctica, helped extend sea ice over the last three decades.
As the atmosphere warmed during the last half of the 20th century, precipitation in the Southern Ocean increased, mainly in the form of snow. Freshening of the upper waters of the ocean, which are also warming, insulates the sea ice from the heat below by reducing vertical mixing. This insulating effect reduced the amount of melting occurring below the sea ice.
In addition, snow reflects sunlight away from the sea ice, which reduces melting caused by the atmosphere. Sea ice also reduces the exchange of carbon dioxide between the ocean and atmosphere.
Winter sea ice extent in the Antarctic is about 18 million square kilometers, more than doubling the size of the continent, according to the National Snow and Ice Data Center (NSIDC) . Sea ice coverage drops down to about 3 million square kilometers in the summer.
Based on satellite data, Antarctic sea ice has increased about a quarter-of-a-million square kilometers for the past 30 years, Liu said later in an e-mail to The Antarctic Sun.
The trend won’t last, according to the authors, who cite climate models that predict warming in the Southern Ocean due to increased loading of greenhouse gases in the atmosphere that will eventually trump the natural variability.
“The increased heating from below (ocean) and above (atmosphere) and increased liquid precipitation associated with the enhanced hydrological cycle results in a projected decline of the Antarctic sea ice,” the authors say.
In other words, more rain and less snow will melt the sea ice. And as the sea ice shrinks to expose more ocean, its albedo effect (reflectivity), also diminishes, allowing more heat into the ocean. That increases the melting of sea ice, which further reduces the sea ice albedo in a positive feedback loop.
Along the western side of the Antarctic Peninsula, one of the fastest-warming regions on the planet, sea ice extent and duration have already decreased markedly. Liu said the Georgia Tech study focuses on the whole Southern Ocean, not any particular region.
“Our finding raises some interesting possibilities about what we might see in the future. We may see, on a time scale of decades, a switch in the Antarctic, where the sea ice extent begins to decrease,” said Curry, chair of the School of Earth and Atmospheric Sciences at Georgia Tech.
NSF-funded research in this story: Jiping Liu and Judith A. Curry, Georgia Institute of Technology, Award No. 0838920 .