Canaries in different coal mines

It’s currently autumn in the Southern Hemisphere — which means that Antarctic sea ice is once again marching north, responding to the cold, dark polar winter. It’s one of Earth’s greatest seasonal changes. Sea ice is the ephemeral lovechild born from an ocean coupled with a cold atmosphere. It is very sensitive to the complex interplay of thermodynamic (freezing and melting) and mechanical (compacting, ridging, rafting, breaking) processes driven by both parents.

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As such, sea ice is a canary in the coal mine for changes to polar climate. That much has certainly been obvious in the Arctic. The dramatic decline of Arctic sea ice since 1981 is now firmly in the public consciousness as proof that global warming is real, and that it is a serious and pressing issue. The Arctic canary is unwell, to say the least. However at the other end of the planet, the Antarctic canary seems to be singing away happily, as the total extent has grown, albeit weakly, over the same period. The past two years have each been record-breakers, and 2014 looks to be building the same way. You can track how it’s going here, which shows how much more sea ice there is relative to the 1981-2010 average.

Highs and lows

Perhaps the most important fact about the (slight) increase in total Antarctic sea ice extent is that it masks major and contrasting regional changes. For example, there has been a strong decrease in sea ice duration in the Bellingshausen Sea, while the duration has increased in the western Ross Sea. Such curiosities have led sea ice scientists to investigate several possible mechanisms, and explanations for these patterns are now starting to emerge.

The Amundsen Sea Low is a pattern of low atmospheric pressure in the Pacific part of the Southern Ocean, which drags warm air south and pushes cold air north. This southward flow of warm air meets Antarctica in the Bellingshausen Sea, explaining why ice in this area is now in decline. Meanwhile, the cold air is being pushed north from the western Ross Sea — where sea ice extent is increasing. So the Amundsen Sea Low can be used to explain at least two variations in Antarctic sea ice.

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The Southern Annular Mode (also called the Antarctic Oscillation) is a term that describes the north-south position of the westerly wind belt that encircles Antarctica in the Southern Ocean. These winds are known variously as the “roaring 40s”, “furious 50s”, and “screaming 60s” depending on their latitude, and when they meet sea ice they drive it northwards (away from Antarctica). Like many other climate patterns (such as El Niño/La Niña), SAM has “positive” and “negative” phases. A positive SAM pushes the winds south to higher latitudes, meaning they encounter more sea ice, pushing more of it northwards and increasing the total ice extent. The Amundsen Sea Low also strengthens with the Southern Annular Mode’s positive phase. The mode has been strongly positive over the past three decades, helping to explain the overall increase in Antarctic sea ice extent, as well as the regional variations.

But that isn’t global warming … or is it?

Here’s the kicker: The strengthening of SAM over recent decades has been directly linked to human activity. Since the 1940s, ozone depletion and increasing greenhouse gases have caused the westerly winds to intensify and migrate south toward Antarctica.