Antarctic sea ice extent reached a record high this year on 22 September, topping 20 million square kilometers for the first time since 1979, according to the National Snow and Ice Data Center. (Average maximum extent from 1979 to 2014 is in red.)

Antarctic sea ice extent reached a record high this year on 22 September, topping 20 million square kilometers for the first time since 1979, according to the National Snow and Ice Data Center. (Average maximum extent from 1979 to 2014 is in red.)

NASA's Scientific Visualization Studio/Cindy Starr

What's going on with Antarctic sea ice?

Q: What’s happening with Antarctic sea ice this year?

A: The National Snow and Ice Data Center (NSIDC) announced this week that the sea ice surrounding Antarctica reached its maximum extent—its widest halo around the continent—in 2014 on 22 September: more than 20 million square kilometers, which also set a record for the highest extent of sea ice around the continent since satellite measurements began in the late 1970s. (That area of “maximum extent” is actually the average extent from the previous 5 days.)

Q: How does that compare with the loss of Arctic sea ice?

A: Overall, this uptick in Antarctic sea ice is still only a fraction (about a third) of the rapid loss of sea ice in the Arctic Ocean. According to NASA, the Arctic has lost about 54,000 square kilometers of ice per year, while the Antarctic has a net gain of about 19,000 square kilometers. This year, sea ice extent in the Arctic was the sixth lowest on record, at 5 million square kilometers on 17 September.

Q: OK, but isn’t the planet warming up? Why is Antarctic sea ice growing at all?

A: This enigma has puzzled scientists, and it’s an active area of research; both the Intergovernmental Panel on Climate Change’s fourth (2007) and fifth (2013) assessment reports commented on it. The short answer is that while the overall climate is warming, it’s a complicated system: The warming climate is also changing weather patterns. Multiple studies have been looking into how these changes might affect sea ice extent: for example, changes in prevailing wind patterns or in the magnitude of ocean waves, both of which can herd the ice toward or away from the coast.

And as scientists have begun to look more closely at what’s happening around Antarctica, one point stands out: While there is a net gain, that gain is actually the sum of even stronger increases and decreases in sea ice extent at different locations around the continent. Some places, like the Ross Sea, have seen growth of sea ice—but others, like the Bellingshausen Sea west of the Antarctic Peninsula, have seen significant sea ice loss. That could mean that changing wind patterns are sweeping warm air over some parts of the continent and cold air over others—which may in turn be influenced by the ozone hole in the atmosphere.

Q: Why did Antarctic sea ice hit a record high this year?

A: NSIDC notes a couple of possible factors. One, again, is changing wind patterns; in July, NSIDC noted that a strong-low pressure pattern over the Amundsen Sea during June had brought lower-than-usual temperatures. Another possible contribution is actually related to melting—of the continental ice sheet. Deep ocean water, which is relatively warm, has been melting portions of the ice sheet at its base. When that cold, fresh water enters the ocean, it forms an extra-chilly layer on the ocean surface around the continent. That’s another condition that favors sea ice growth. And, as Walt Meier of NASA’s Goddard Space Flight Center noted here, Antarctic sea ice already covers a huge area—it doesn’t take much growth for it to set a new record.

"What we're learning is, we have more to learn," said Ted Scambos, lead scientist at NSIDC, in the agency’s press release this week.