It was big news in September when scientists announced that Arctic sea ice had melted back to its lowest extent on record, thanks largely to the heat-trapping greenhouse gases generated by human activity. It was smaller news a few weeks later when they noted that the sea ice at the opposite end of the globe had swelled to its — largely because the swelling wasn't all that pronounced, because September marks the end of winter in Antarctica, when you’d expect there to be lots of ice, and because it’s widely believed that winds, not temperatures, are the major factor behind why Antarctica’s sea ice swells and shrinks.
Trends in Antarctic sea ice motion over a 19-year study period are shown by the arrows, in meters per second per year. The colors show the change in northward ice speed, with reds being fastest and blues slowest.
Click image to enlarge.
Credit: NASA/JPL-Caltech/British Antarctic Survey
Still, “widely believed” isn’t a strong scientific statement, which is why a paper published on November 11 was important, if not especially surprising. Writing in Nature Geoscience, Paul Holland of the British Antarctic Survey and Ronald Kwok of NASA’s Jet Propulsion Laboratory have found the case’s smoking gun. “Our main conclusion is that the drift of ice has changed over the past two decades,” Holland said in an interview. “And we’ve linked these changes to changes in winds over that same period.”
That evidence comes from both satellites and computer models. The satellites have been looking down for several decades, noting not just the seasonal swelling and shrinking of sea ice, but also the way the ice moves. They also measure the atmosphere's temperature at different altitudes and locations, and since temperature differences are the ultimate driver of winds, those readings — along with ground-based readings, which tend to be few and far between in Antarctica — are fed into models (in this case, from the European Center for Medium-Range Weather Forecasting) that reconstruct wind speed and direction.
If the winds are truly responsible for sea ice movement, Kwok said, “they should match each other. And they do, so we can be confident that in most places, at least, the expansion of ice seems to be wind-driven.”
The winds affect sea ice in two ways, Holland explained. The most obvious is that it pushes sea ice around. If it blows north, for example, away from the Antarctic continent, it spreads the ice over a wider area. But those winds also carry cold air from the continent, which aids the formation of new sea ice. If the winds are blowing toward the south, by contrast, they push ice up against the continent, while also bringing relatively warm air that inhibits ice formation.
Holland said both things are happening “in some places, such as the Ross Sea, sea ice has been increasing over the years. But in the Bellingshausen Sea, it’s decreasing.” The increases slightly outbalance the decreases, with the result that Antarctic, winter sea ice has gone up by a small amount: about 1 percent per decade over the past 30 years.
This raises the question of why have the winds changed? “There are several possibilities,” Holland said. The first is natural variability. Global and regional climate have been changing in all sorts of ways since long before humans even existed, let alone started burning fossil fuels in earnest, and while human-driven climate change is now dominating those natural changes, they’re still going on.
Another possibility is that the Antarctic ozone hole, created by the artificial chemicals known as chlorofluorocarbons, have altered the atmosphere’s energy balance around Antarctica, triggering changes in wind patterns. The third, Holland said: “Growing concentrations of greenhouse gases could have affected the winds, but I haven’t seen any direct evidence of that.” They’re all plausible, he said, “but I regard none of them as being proven.”
They have the smoking gun, in other words, but they don’t yet know who fired it.