The disorderly story of Antarctic sea ice

An icy North America and a ship’s besetment in Antarctic ice offer no comfort on global warming. [14 January 2014 | Peter Boyer]

A brisk offshore katabatic at Cape Denison, Commonwealth Bay, before a massive grounded iceberg prevented the escape of sea ice from 2010.

The two biggest seasonal events on Earth are winter snow on northern lands and ice cover on the Southern Ocean. Both featured in the news last week.

In the Northern Hemisphere, huge snowstorms followed by bitterly cold conditions across North America were loudly claimed to be living proof that there’s no such thing as global warming. But apart from the obvious rebuttal that a cold snap in North America does not equate to a cold planet, there’s evidence that this one results at least partly from long-term atmospheric warming.

The evidence has to do with the northern polar vortex, a flow of very cold air circling the Arctic. The polar vortex depends for its strength on Arctic air being much colder than air to the south. When it weakens, the vortex wanders southward, bringing the cold air with it. With the Arctic Ocean and the air above it warming at over twice the global average, the temperature difference between Arctic and more southerly air is much less than it was. A weakened vortex went all the way to the Gulf of Mexico last week, and Americans suffered the consequences.

Anti-warming claims were also made about the pile-up of Antarctic sea ice in and around Commonwealth Bay, Antarctica, which for two weeks blocked the passage of a chartered Russian ice-strengthened ship, Akademik Shokalskiy, on a university-organised climate research voyage. US climate blogger Anthony Watts and Australian newspaper columnist Andrew Bolt, both of whom are hostile to science supporting human-induced global warming, were among many who took delight in the notion of a shipload of “warmists” stuck in ice. Bolt castigated news media which, after publicising the expedition’s climate research program, seemed to him to go silent on this when the ship became locked in. Watts posted a picture showing open water in Commonwealth Bay when Douglas Mawson landed there 101 years ago.

The message from both Watts and Bolt was that heavy ice conditions today in a place that was clear of ice 100 years ago signalled cooling, not warming, and should embarrass both the voyage’s organisers and all “warmist” scientists. It’s a neat story, but it’s plain wrong. Commonwealth Bay was indeed mostly clear of ice when Mawson and his men set up camp at Cape Denison in 2012, as it has been for most summers since, including the summer of 2008-09 when I spent a month there, but this has nothing to do with air temperature.

Mawson didn’t name his base camp “the home of the blizzard” for nothing. Heavy, cold air above the base’s icy hinterland is drawn by gravity down to the coast, where it can reach hurricane force. This is known as a katabatic. Katabatic winds are strong enough to blow sea ice away from the shore, leaving large areas of open water for some distance offshore in which the freezing of seawater is constantly thwarted by the wind. Such open-water areas are especially prevalent along this part of the Antarctic coast.

But Antarctica is full of surprises. Some 150 km east of Mawson’s base is Mertz Glacier. The glacier’s long tongue of ice projecting into the ocean was an additional factor in keeping Commonwealth Bay free of pack ice drifting westward in the prevailing eddies. In 2010 a massive drifting iceberg, 100 km long, hit the Mertz Glacier Tongue, causing a large piece to break off and float away. The iceberg drifted a short distance further to the west before grounding itself in Commonwealth Bay, where it’s remained for over three years.

The loss of much of the ice tongue and creation of a new ice-island has dramatically transformed the whole region. Ice previously blown out to sea is now trapped behind and around the iceberg, and is being pushed together so that much it is now over three metres thick. So the ice conditions that stopped Akademik Shokalskiy in its tracks during its visit to the coast weren’t the result of any long-term cooling trend, as Watts and Bolt would have it, but the chance arrival of an iceberg in 2010 plus a sudden, unexpected break-out of accumulated sea ice.

The bigger picture is not so clear. Over the past 20 years Arctic multi-year sea ice has dramatically diminished. The more exposed waters of the Antarctic means it’s never had much multi-year ice, but its seasonal ice cover seems over the same period to have actually increased slightly. If we want to know why, in a generally warmer world, Southern Hemisphere sea ice seems to go against the trend, we need to begin with the big picture. While the Arctic is land surrounding an ocean, the Antarctic is its complete opposite: an ocean surrounding a continent.

Current research identifies several likely factors in increasing Antarctic sea ice. One is strengthening Southern Ocean westerlies combining with the effect of Earth’s rotation, forcing ice northward faster than it can melt and thus expanding the sea ice zone. Another is increased melting of land ice around Antarctica’s coast, producing colder, fresher water which floats on the ocean surface, making more favourable conditions for formation of sea ice.

There’s more than one trend at work here. A position paper soon to be published by Hobart’s Antarctic Climate and Ecosystems Cooperative Research Centre reports that increasing sea ice in the Ross Sea region south of New Zealand has masked a big decrease in a region to the west of the Antarctic Peninsula.

Bear in mind that the area of Antarctic sea ice does not affect sea level, in contrast to changes to Antarctica’s land-based ice sheet, by far the world’s biggest body of ice. That’s currently losing about 200 million tonnes of ice a day and rising, with big implications for future sea level.

As with all things to do with climate, working through the detail of what’s happening to Antarctic sea ice calls for perseverance and openness to what the evidence is revealing. I doubt that the polemical world occupied by Bolt and Watts has much room for such things.

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