In the wake of last week’s announcement that Arctic summer sea ice extent had reached its lowest level in the satellite record – and possibly the lowest for the last several thousand years – a few ‘skeptic’ blogs were keen to trumpet what they proclaimed to be a different sea ice record.

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“The world’s media is extremely excited at the thirty-year record low extent of sea ice at the North Pole which occurred just days ago: but almost nobody is reporting on the fact that something almost equally unusual is going on down around the coasts of Antarctica,” wrote Lewis Page in The Register.

That echoed a post on Forbes a few days earlier from James Taylor, who observed that “Antarctic sea ice set another record this past week, with the most amount of ice ever recorded on day 256 of the calendar year (September 12 of this leap year)… Sea ice around one pole is shrinking while sea ice around another pole is growing.”

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So what does that mean? Is everything in fact OK? Are the two somehow balancing each other out? And why is one increasing while one declines?

The first thing to remember is that these are two entirely different ecosystems. There is no global sea ice budget that needs to be balanced. Secondly, we’re talking two different measurements: Winter extent in the Antarctic versus summer extent in the Arctic. Theoretically, a better apples-to-apples comparison might be summer extent in the two, but that won’t work, and the reason why it won’t work gets to the heart of why scientists are much more interested in and concerned by what’s happening to sea ice in the Arctic.

The Antarctic is a continent surrounded by ocean. As a consequence, in the summer, sea ice is free to break up and drift northward unimpeded, where the great majority of it melts every summer, climate change or no climate change.

The Arctic is an ocean surrounded by land: There are very few escape routes for the sea ice that forms each winter, and so although some melts each year, some stays, surviving the summer

and thickening each winter.

The Antarctic has very little of the multiyear sea ice traditionally found in the Arctic; it is the disappearance of that multiyear ice, and the overall thinning – and thus a dramatic decrease in volume, as well as extent – of the ice cap that is so significant when discussing summer sea ice loss in the northern hemisphere.

It is worth pointing out as an aside that, as we can see from the graphs at left, any increase in Antarctic winter sea ice is hardly on the same level as the summer decrease at the other end of the world. (I recognize that by comparing the two, I too am guilty of the apples-to-oranges misdemeanor I mentioned earlier, but I bring it up only to address the implication of equivalency in the blogs quoted at the beginning of this post.)

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But even so, while it may be relatively insignificant, there is indeed a slight upward trend in Antarctic sea ice extent in the winter. So, why? Does it somehow ‘disprove’ global warming? Hardly. Implicit in that notion is that the waters of the Southern Ocean must be somehow cooling. But they aren’t. In fact, the Southern Ocean is warming faster than the global trend. This ocean warming is a driving force behind concerns over the stability of some west Antarctic glaciers.

As Judith Curry of Georgia Tech University explained to the Houston Chronicle’s Eric Berger, “In the case of the Arctic most of the melting is driven from the warmer atmosphere above. In the Antarctic most of the melting has been driven from the ocean below.” But, research by Curry – who, as Berger points out, is a noted critic of the Intergovernmental Panel on Climate Change – and colleagues suggests that Southern Ocean warming is leading to greater precipitation, which falls as snow. This increases the albedo of sea ice, reflecting heat away from its surface and protecting it from atmospheric warming; and also freshens the surface layer of the ocean, stabilizing it and holding back the warmer water from below.

Under that scenario, the Antarctic sea ice situation may remain relatively stable for a few decades yet; ultimately, however, even the fresher upper ocean will become warm enough to melt the ice from below, while precipitation will begin to fall as rain, melting the ice from above. When that happens, winter sea ice in the Antarctic will also begin to decline in extent.

As John Cook of Skeptical Science observes, “Antarctic sea ice is a complex and unique phenomenon. The simplistic interpretation that it must be cooling around Antarctica is decidedly not the case. Warming is happening – how it affects specific regions is complicated.”

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A view of the sunset over a tabular iceberg in Antarctica’s Weddell Sea. (Michel Setboun/Getty Images)

Graphs from the National Snow and Ice Data Center.