It’s a nagging thorn in the side of climatologists: Even though the world is warming, the average area of the sea ice around Antarctica is increasing. Climate models haven’t explained this seeming contradiction to anyone’s satisfaction—and climate change deniers tout that failure early and often. But a new paper suggests a possible explanation: Variability in the heights of ocean waves pounding into the sea ice may help control its advance and retreat.
The sea ice growth around Antarctica isn’t particularly large—about 1.2% to 1.8% on average per decade between 1979 and 2012, according to the 2013 Intergovernmental Panel on Climate Change’s Fifth Assessment Report. It’s also not uniform. The increases are concentrated primarily in the Ross Sea in western Antarctica; meanwhile, in the nearby Bellingshausen and Amundsen seas, sea ice has significantly decreased just as it has done in the Arctic.
What role waves play in all this has been barely examined, from a climate modeling standpoint. Ocean waves undeniably pound into the ice, of course—polar explorer Ernest Shackleton noted “swells coming in and breaking the ice up” in his book South, says Alison Kohout, of the National Institute of Water and Atmospheric Research in Christchurch, New Zealand. But how does a single wave interact with an ice floe? And what is the cumulative effect of pounding waves on hundreds of kilometers of ice?
To begin to answer these questions, Kohout and her colleagues focused on the marginal ice zone (MIZ), the transitional region at the edge of the sea ice pack where the swell of ocean waves can still significantly affect the shape and size of the floes. The MIZ, which is packed with smallish floes—perhaps 100 meters across at most—receives the brunt of the pounding from large waves churned up by powerful storms in the open ocean. As the ocean waves scatter their energy into the floes, the floes may collide and bend or break, forming smaller floes or a slurry of ice on the water. Modeling how a single wave affects this region is tricky: In some ways, the sea ice behaves as individual floes, while in other ways it acts as a very thick fluid that “dampens” the wave’s energy.
To measure how far into the ice the waves still pack a punch, the researchers used wave height, energy, and frequency data gathered from five autonomous wave sensors positioned on Antarctic sea ice along a 250-kilometer line. They noticed an interesting thing. As predicted, small waves—less than 3 meters tall—lost energy rapidly as they propagated through the sea ice, as they would through a thick fluid. But larger waves didn’t lose their energy nearly as quickly. As a result, “when the waves are bigger, the ice is going to get munched up a lot quicker,” Kohout says. Storms kicking up large waves, therefore, would have a disproportionately powerful effect on sea ice breakup, they report online today in Nature. And conversely, if wave heights are decreasing in a particular region, that could “allow” the sea ice to expand, she says.
Next, the team compared satellite sea ice observations from 1997 to 2009 with modeled wave heights during that time. The correlation was strong: When waves got shorter in a given area—such as the Ross Sea—sea ice grew. When waves got taller, the ice retreated. “It was really quite exciting,” Kohout says. “This … really shows that it’s quite possible [wave heights] are playing an important role.”
Indeed, the lines “agree beautifully”, says climate modeler Paul Holland of the British Antarctic Survey in Cambridge, U.K., who was not involved in the study. But that doesn’t prove that wave heights control the expansion and retreat of the sea ice, as the paper suggests, Holland says. Maybe changes in ice cover affect the size of the waves. Or maybe some third effect causes changes in both wave height and ice cover. “I would suggest that the change in ice cover is due to changes in the winds,” Holland says. He co-authored a 2012 paper in Nature Geoscience that suggested that more northward-blowing winds around Antarctica are pushing the ice northward, increasing sea ice cover and also damping down waves. “That explanation is also entirely consistent” with the paper’s correlation between sea ice extent and wave heights. Still, “it’s not a closed case,” he adds.
Incorporating a hyper-regional effect such as how wave heights influence sea ice into global climate models is going to be tricky, says Sam Dean, also of New Zealand’s National Institute of Water and Atmospheric Research, in Wellington, and a co-author on the new study. “If it was easy, they would have done it. [But] this paper suggests that it might be worthwhile,” he says. Kohout says that’s the message she hopes to deliver: That wave heights should at least be considered in future climate models, “to show that it is or isn’t important, one way or the other.”