Ice arches and positive feedback
The shrinking extent of sea ice in the Arctic has been a cause of concern for some decades, and the record low extent measured with passive-microwave radiometers in September 2007 gathered a good deal of publicity. The September minimum was 7.11 million km2 on average during 1979-1998. In 2007 it was 4.30 million km2. The two minima since then have each been greater. 2008 saw the second lowest and 2009 the third lowest extent.
You can check out the state of Arctic sea ice at the National Snow and Ice Data Center in Boulder, Colorado. Thus far during the present winter, 2009-2010, the extent has been tracking pretty closely the course followed in 2007, so two successive years of increased minimum annual extent do not justify us in concluding that the ice pack might be recovering. Equally, there is no sign of an impending catastrophe at the top of the world, but we would still like to understand why 2007 was a record-breaker. There have been several interesting attempts to explain it.
A particularly interesting attempt by Ron Kwok and colleagues appeared last month. They focus on a detail of the bigger picture, the outflow of sea ice through Nares Strait, the narrow gap between northwest Greenland and Ellesmere Island. To put this study in context, we need to step up from thinking about sea-ice extent to thinking about sea-ice mass.
Very roughly, the ice is 3 m thick on average, for a total mass each average September of about 19 million gigatonnes (but only 12 million Gt in 2007). The mass of the ice pack is the result of a balance between freezing, melting and export. The exported bergs and floes eventually melt, but not within the Arctic Ocean.
Most of the export, about 2000 Gt/yr, is through Fram Strait, between Greenland and Svalbard. Of the other possible outlets, the channels between the islands of the Canadian arctic archipelago contribute little. Apart from being narrow, they are most often blocked at their northward ends by plugs or “arches” of immobile ice. The arches form during the winter and persist until the end of summer, so that for much of the year there is effectively no southward ice export.
Kwok and colleagues found that no arch formed in 2007 at Nares Strait, which was therefore an open passageway for the full 365 days. Between 1998 and 2006, the open-channel state prevailed for only 140 to 230 days per year. From 2004 to 2006, when ice thickness measurements are available from the ICESat laser altimeter, the mass export was about 80-85 Gt/yr, but in 2007 it was 230 Gt/yr.
Why worry about such tiny amounts? The export through Nares Strait in 2007 was only 10% of that through Fram Strait, and insignificant in comparison with the total mass of the pack. The answer is that the ice in the Lincoln Sea, just north of Nares Strait, is some of the thickest, at about 5 m on average, in the whole Arctic Ocean.
The decline of Arctic sea ice is usually discussed in terms of its extent, but that is mainly because we have lots of information about extent. Measurements of thickness are harder to come by, and therefore so are estimates of total mass (area times thickness, multiplied by 900 kg m-3, the density of ice). But one of our main concerns about Arctic sea ice is that apart from shrinking in extent it is also getting thinner.
The impact on ice extent of the free evacuation of thick Lincoln Sea ice in 2007 was small, but it depleted the thick end of the frequency distribution of ice thickness. An ice pack with relatively more thin ice is less likely to survive the summer melt season, so the non-formation of ice arches in Nares Strait constitutes a positive feedback, magnifying the vulnerability of the ice pack as a whole. And it may be a positive feedback in another sense. Presumably arching, that is, blockage, is more likely when the supply of thick chunks of ice is greater. If an episode of free outflow decreases that supply, future episodes of free outflow become more probable.
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