Antarctic sea ice: an update
Antarctic sea ice extent continues to make headlines because it has grown even as much of the globe, and Antarctica itself, is warming. Arctic sea ice, in contrast, is showing a marked decline. Warmer air and ocean waters are bathing both poles, so why does the Antarctic sea ice trend resist decline?
Despite what some might think, high ice extent in Antarctica does not balance out low ice extent in the Arctic. Antarctica is showing strong warming in other areas, and is experiencing the consequences of this warming, such as the dramatic breakups of ice shelves on the Antarctic Peninsula. Yet Antarctic sea ice extent has increased.
The very different geography of each pole is a large factor. The Arctic is a large body of water surrounded by land, while Antarctica is an icy landmass surrounded by ocean, meaning that the ways in which ocean waters, currents, and winds interact with sea ice are very different as well. But why is the Antarctic story less clear to scientists than the Arctic one?
Less land ice, more sea ice?
While scientists expected Arctic sea ice to respond strongly to warming, and knew that the poles might have different patterns of warming, they are still surprised that Antarctic sea ice has generally expanded. This has been especially perplexing to watch in 2012, 2013, and 2014, during which the Antarctic sea ice winter maximum extent set consecutive records.
For a time, researchers thought melt water might be a factor. Warming ocean waters are causing Antarctica’s ice sheets to flow more quickly and churn more ice into the ocean. This ice then melts into the surface water layer that Antarctic sea ice sits in. But this additional melt water is not large enough to explain the changes.
“The scale of the new freshwater input is small compared with snowfall or even rainfall in the Southern Ocean,” said Ted Scambos, senior research scientist at NSIDC. The Antarctic system has always experienced about 2,000 gigatons of melt annually, and the recent increases have only added about 150 gigatons. On top of that, the Southern Ocean annually receives about 20,000 to 40,000 gigatons of precipitation. “Yes, it is warmer, and more of the land ice is melting . . . but it’s not a big effect compared to the area and mass of the sea ice,” Scambos said.
So if more land ice melting is not a major influence on sea ice, what are scientists looking at now?
Gust in the wind
In a recent study, scientists focused on the Antarctic ozone hole. When ozone is present high in the atmosphere, it absorbs sunlight and warms the atmosphere. A large hole in the ozone layer over Antarctica chills the air high above the continent, and sets up a stronger westward circulation. This can drive stronger westerly winds even near the surface. Scientists Paul Holland and Ron Kwok found that under the right conditions, these winds tend to spread sea ice out, away from Antarctica, creating a larger overall extent.
Satellite evidence of the ozone hole dates to 1976, and this timeline matches changes in sea ice, according to Scambos. “There are several lines of evidence suggesting that sea ice was ‘different’ prior to the mid 1970s, and generally a lot more variable,” he said. Although research and models support the connection between ozone loss and sea ice growth in Antarctica, at least over the short term, scientists would like a longer time series. Older satellite data, from some of the first weather imagers ever launched, are being pressed into service again to investigate sea ice conditions in the early 1970s and even the 1960s.
Scientists are also investigating other climatic influences. “Individual areas of Antarctic sea ice are governed by different climate patterns, and each area has its own degree of variability,” Scambos said. “Looking at the different climate patterns and how they have played out over the past several years will be a key step to explaining what is going on.”
For instance, much of West Antarctica’s climate is influenced by the Amundsen Sea Low, a perennial weather system located over the adjacent Bellingshausen and Amundsen Seas. Researcher Marilyn Raphael and her colleagues published a study showing how the system affects wind and sea ice patterns across West Antarctica’s coasts. When the Amundsen Sea Low is strongest, sea ice extent along much of West Antarctica decreases while extent in the nearby Ross Sea increases; a weak phase results in the opposite effect in each sea. Other scientists are investigating how the ozone hole and El Niño patterns interact with the Amundsen Sea Low to induce changes in sea ice. Similarly complex variations influence sea ice extent in other regions around the continent.
Antarctica’s stubborn paradox has proven puzzling. As scientists learn more, they hope to understand what shapes Southern Ocean sea ice. “Five to ten years from now, I think we’ll still be seeing regional variability dominate any global climate change signal,” Scambos said. “We’ll know more about the drivers of regional trends, and also the history of sea ice in Antarctica,” he said. “But I don’t think we’ll see a climate-driven decline until late in the 21st century.”
Holland, P. R., and R. Kwok. 2012. Wind-driven trends in Antarctic sea-ice drift. Nature Geoscience, doi:10.1038/NGEO1627.
Raphael, M. N., G. J. Marshall, J. Turner, R. Fogt, D. Schneider, D. A. Dixon, J. S. Hosking, J. M. Jones, and W. R. Hobbs. 2015. The Amundsen Sea Low: Variability, change and impact on Antarctic climate. Bulletin of the American Meteorological Society, doi:10.1175/BAMS-D-14-00018.1.
“Sea ice.” NASA Earth Observatory. Accessed 11 January 2016.