The Russian ship MV Akademik Shokalskiy was trapped in thick Antarctic ice 1,500 nautical miles south of Hobart, Australia, on Dec. 27, 2013. (AP Photo/Australasian Antarctic Expedition/Footloose Fotography, Andrew Peacock)

This story has been updated.

On Monday we learned some troubling news about the continent at the bottom of the world — Antarctica. Most of Antarctica is covered with a vast, thick sheet of ice, an area larger than the continental United States and over two miles thick in some places. The smaller, western part of this ice sheet was already believed to have been destabilized — potentially triggering over 10 feet of sea level rise. But now it looks like one key sector of the far larger eastern region (known as the Totten Glacier) may be going through a similar ice loss.

That’s not good.

However, as climate skeptics are quick to note, there’s something odd and seemingly paradoxical about Antarctica’s ice. Even as oceanfront glaciers in key areas seem to be retreating, potentially awakening the vast ice sheets behind them, Antarctic sea ice — ice floating atop the oceans surrounding the continent — has actually been increasing. And this has often been cited as a supposed anomaly in the global warming story.

So, is a rise in Antarctic sea ice any reason to discount the latest news about east Antarctica, or climate concerns more broadly? The answer is no.

First, the facts: Antarctic sea ice has indeed been increasing. The trend has been noted for some time, but here’s the latest from the National Snow and Ice Data Center (NSIDC) in Boulder, Colo., which reports that Antarctic sea ice recently reached its “fourth highest summer minimum” ever. That means that in the southern hemisphere summer, when seasonal Antarctica sea ice tends to be the lowest, there was nonetheless more of it than in any other year except for 2003, 2008 and 2013. (At the other end of the seasonal cycle, Antarctic sea ice also hit a new record for its overall winter maximum extent last September.)

In calling this new fourth-place record, NSIDC noted “a remarkable recent uptick in [sea ice] extent year-round for Antarctica.” It also presented this figure of sea ice for Antarctica in February, showing an upward trend:


National Snow and Ice Data Center.

So, Antarctic sea ice is indeed rising. However, it’s important to keep this in perspective — it isn’t rising as much as sea ice in the Arctic (which gets a lot more attention) is declining. Such was the finding of a recent NASA study, which used satellites to look at sea ice in both areas, finding a global downward trend despite the relatively modest rise in the Antarctic region:


NASA’s Earth Observatory/Joshua Stevens and Jesse Allen

Nonetheless, NASA, like NSIDC, confirms the uptrend in Antarctic sea ice. But why is it happening? And what’s the significance of this trend in the climate system?

The answer seems to be that, while scientists aren’t entirely sure what’s causing increasing Antarctic sea ice, they have some plausible explanations for it. But no matter what the cause turns out to be, understanding it is unlikely to give us any reason to worry less about what’s happening to the land-based ice of Antarctica — which, unlike sea ice, presents the true concern due to its potential for driving very dramatic sea level rise.

In its latest update on Antarctic sea ice trends, the National Snow and Ice Data Center added the following:

The debate continues regarding the cause of the recent Antarctic trends, but the best explanation so far involves a combination of strengthening low pressure in the eastern Ross Sea (the Amundsen Sea Low) and the eastern Weddell Sea, and a persistently positive phase of the Southern Annular Mode. The freshening of surface seawater around Antarctica may also play a role.

A recent blog post at RealClimate by Eric Steig, a glaciologist at the University of Washington, reaches a similar conclusion. Steig explains that what’s crucial to Antarctic sea ice is wind patterns, since if winds blow sea ice away from the continent, more ice can form closer in as new areas of open water are created and then freeze over.

“We can explain sea ice trends in the Antarctic rather well if we take into account the full range of changes in winds that have occurred,” writes Steig. Critically, that includes stronger winds blowing from the west around the region (wonk term: “circumpolar westerlies“) that, Steig says, actually seem to be tied to more greenhouse gases in the atmosphere, and less ozone in the stratosphere — factors that humans are behind.

“Far from discounting climate change in the Southern Hemisphere, the apparent paradox of Antarctic sea ice is telling us that it is real and that we are contributing to it,” adds Guy Williams, an Antarctic expert at the University of Tasmania.

Moreover, though total ice extent may grow through this process, we shouldn’t misinterpret what that means. “Antarctic sea ice is unrestricted in extent, unlike Arctic sea ice,” explains climatologist Kevin Trenberth of the National Center for Atmospheric Research. ” As a result winds can blow it away from Antarctica and new ice forms in behind, but the ice then is very thin.  So increases in Antarctic sea ice do not equate with increased volume.”

Not everyone agrees about a human role, though. John Turner of the British Antarctic Survey, for instance, opines that “the reason why sea ice is increasing around Antarctica is still being debated, but it seems likely that this is just natural climate variability, which is large in the Antarctic region as shown by the paleoclimate records we get from ice cores and other records.”

So in sum: Scientists are working to explain recent Antarctic sea ice trends, and they’re finding that there may actually be a role for humans in all this. Moreover, in the long run, they also think that warming overall temperatures will eventually eat into Antarctic sea ice, causing it to also decline.

What does this have to do with our concerns about Antarctic glaciers and ice sheets? The answer is relatively little or nothing, explains Jamin Greenbaum of the University of Texas, Austin, lead author of the new study about Totten Glacier in East Antarctica.

“Sea ice formation is a function of sea surface conditions but the water that rapidly melts glaciers is often found below several hundred meters of water (as we described in our latest paper),” Greenbaum explained. “So, while I do find patterns of sea ice extent interesting, increased sea ice extent doesn’t necessarily have a direct effect on coastal glacier melting as the processes that are increasing sea ice extent may be independent of anything that would affect glacier melt.”

You could have growing Antarctic sea ice and still have warm water sneaking underneath coastal glaciers and speeding up their retreat. In fact, the latest scientific understanding suggests we have just that — meaning that when we look at Antarctica, and especially look below the surface, there’s still great cause for concern.

Read more:

The melting of Antarctica was already really bad. It just got worse.

As ice melts, polar bears migrate north

The U.S. has caused more global warming than any other country. Here’s how the Earth will get its revenge.