SCIENCE WEDNESDAY -- May 1, 2013 at 5:34 PM ET
The Antarctic's Ice Paradox
Scientists have been trying to create a clearer picture of how the Antarctic responds to climate change. Photo by Nerilie Abram.
It's no secret that the ice sheet is melting in Greenland. Last year, the Arctic ice cap shrunk to a record low, with only 24 percent of the Arctic Ocean covered by ice, a 50 percent drop from its 1979-2000 summer average. At the height of the 2012 summer, Greenland had experienced melting across 90 percent of its surface. For a journalist, it's an easy story to tell: temperatures climb, the ice shrinks.
But at the opposite end of the world, in Antarctica, the picture isn't quite as clear. Satellite images from 2012 showed that Antarctic sea ice reached its highest levels extent on record, evidence skeptics often point to as proof that climate change isn't happening. And for years, the East Antarctic ice sheet, which covers the majority of the continent, appeared to be stable or perhaps even gaining mass.
Sea ice around Antarctica, June 21, 2012. Although average sea ice extents around Antarctica have been increasing over the last several years, land-based ice sheets are melting at fast rates. Courtesy NOAA.
There are several reasons why changes in Antarctica are simply harder to explain than those in the Arctic, says Waleed Abdalati, professor of geography at the University of Colorado. The geography is different and data collection is challenging, he said. Plus, the climate and a large hole in the ozone layer have buffered large parts of the continent from warmer air.
"The fact that you have a large, tall, thick ice sheet situated at the South Pole creates a climate down there that tends to isolate it from the rest of the world," Abdalati said. "And it does that in large part by setting up a circulation pattern where winds blow around the perimeter of the continent and block warmer air from lower latitudes."
Unlike the Arctic, it has been almost impossible at times to collect data in Antarctica. Satellite data only dates back to 1979; research station data to the mid-1950s, in some areas. The ice sheet east of the Transantarctic mountains was less explored and less understood until recently; parts are still inaccessible. And throughout the continent, which is larger than the United States, the ice sheet varies dramatically, making the ice losses and gains more difficult to tease out, Abdalati said. Greenland, on the other hand is smaller, easier to access and has been therefore studied in greater detail.
So what is really happening in the Antarctic?
The short answer is that the ice around Antarctica is growing, but the ice on the continent is shrinking, and both are a result of climate change. Just how much it's changing and how fast depends on where you look.
In some parts of the continent, the Antarctic ice is melting just as rapidly as the ice in the Arctic Circle. In fact, the Antarctic peninsula, which juts into the Southern Ocean, was one of the earliest climate change "hot spots", where rapid warming caused enormous segments of glaciers to break off into the ocean. In a recent study in the journal Nature Geoscience, new ice core data from James Ross Island, located at the northern tip of the peninsula, shows that warming on the peninsula has reached its highest rate in 1,000 years.
Nerilie Abram works with ice core samples from Antarctica. Photo by Paul Rogers.
Nerilie Abram, a research fellow at Australian National University and lead author of the study said that the ice melt in peninsula has increased 10-fold over the last 600 years, with the largest increases occurring in the last half century.
"The Antarctic peninsula contributes to more than half of the ice melt in Antarctica, and even there it varies," Abram said. "On James Ross Island, almost 5 percent of the ice cap is melting and refreezing. But if you went down to the coastal areas that are at sea level around island, the amount of melt is even higher."
The peninsula is particularly vulnerable because of its location in the Southern Ocean, which has a warming effect, she added.
At the West Antarctic ice sheet, ice is melting faster than it ever has in that location, but at a less dramatic pace than on the peninsula, said Eric Steig of the University of Washington, whose study was also published recently in Nature Geoscience. There, warmer air temperatures combined with warmer ocean water circulating below the ice shelves both cause the ice to melt at the "upper bound of normal," he said.
At this rate of melt, the ice shelves in Antarctica won't last, he explains, and its not obviously linked to human activity the way it is on the peninsula. And of course, that rate could change.
He suspects that the melt on the West Antarctic ice sheet is similar to what occurred on the peninsula 20 years ago.
And while the ice loss has been fairly well documented in the west and the peninsula, data on the East Antarctic ice sheet paints a murkier picture. The most recent estimate published in the journal Science in 2012 states that the East Antarctic ice sheet is gaining mass by 14 gigatonnes per year. However, that data, as the study points out, has a 43 gigaton margin of error, Abdalati says, making it difficult to confirm the gain with any certainty.
Perhaps the most confusing detail - the growth of sea ice around Antarctica - is also a direct response to climate change, said Andrew Carleton, professor of physical geography at Penn State University. Despite the warming climate, sea ice surrounding the continent has increased about one percent every decade between 1979 and 2008, according to the National Snow and Ice Data Center.
It seems counter intuitive, but that growth is a result of the glacial melt happening on the rest of the continent, Carleton said. As fresh water melts into the ocean it decreases the salinity of the seawater, he explained. Water with less salt content freezes at a higher temperature, so even with warming air temperatures melting the glaciers, the Antarctic Ocean continues to gain sea ice.
"It seems paradoxical, but it makes sense," Carleton said.
The story of Antarctica is not a simple one, Abdalati says, and scientists are still working to make sense of how it fits in the bigger picture of climate change.
"It's not that it doesn't fit. It all fits," he said. "We have to ask 'what is the story telling us?'"