Why Is Antarctica’s Sea Ice Growing As Temperatures Rise?

The North Pole is losing about 30,000 square miles of sea ice per year. Over the past century, average global temperatures have climbed by 1.5 degrees Fahrenheit. And yet, over the past few years, the sea ice that surround the South Pole has steadily been growing.

This past September, at the end of the Southern Hemisphere’s winter, the extent of Antarctica’s sea ice reached 19.51 million square kilometers, breaking a 35-year record that dated back to the start of data being collected in 1978. (In comparison, from 1981 to 2010, the average extent on the same date was just 18.5 million square miles.)

Why are the Arctic and Antarctic such polar opposites? Climate change deniers have pounced upon the unexpected divergence to argue that the planet’s temperature isn’t actually rising. But new research suggests that a different mechanism—unrelated to climate change—is responsible for the ice growth. The real answer, says University of Washington oceanographer Jinlun Zhang, can be found blowing in the wind.

Specifically, according to a study he and colleagues published in the Journal of Climate, the vortex of winds that swirl around the South Pole has both strengthened and converged, a trend that can explain about 80 percent of the growth in ice extent that has been detected in recent years.

Atmospheric scientists had previously observed that these swirling winds had gradually strengthened since the 1970s. Using a computer model, Zhang’s team found that this mechanism drives ice growth—even in the face of rising temperatures—by pushing floating layers of sea ice together, compressing them into thick ridges that are slower to melt.

“Ice ridging increases the amount of open water and areas with thin ice, which are then exposed to cold air in winter, leading to enhanced ice growth,” Zhang says. “Meanwhile, the ridges, driven together by the wind, shrink less during the summer, because thicker ice tends to survive longer.” Based on this mechanism, the model accurately predicted ice growth in the same areas—the Weddell, Bellingshausen, Amundsen and Ross seas—that it’s been most distinctly observed.

Of course, the explanation brings to mind another question: Why is this vortex of swirling winds growing more powerful in the first place? Scientists are still unsure, but a few hypotheses have been put forth.

One possible culprit is the hole in the ozone layer, caused by lingering CFCs that were emitted before their use was phased out by the Montreal Protocol. Because ozone absorbs ultraviolet light from the Sun, missing ozone affects the local balance and transfer of energy, potentially leading to stronger winds. Another possibility is that the strengthened winds can simply be chalked up to natural variability.

Whatever the cause, the observed effect—a growth in Antarctic ice—has been relatively small, especially in comparison to the rapidly melting ice in the Arctic. For now, the winds are causing ice growth, but going forward, that trend is likely to be overwhelmed by a far more potent one: the continued rise in greenhouse gas emissions and the climate change they’re rapidly driving. “If the warming continues, at some point the trend will reverse,” Zhang says.

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Joseph Stromberg was previously a digital reporter for Smithsonian.