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Monday, September 3, 2012

Indispensable Arctic sea ice graph

A lot of ingenuity has gone into graphics displaying various ways of measuring the Arctic sea ice death spiral.  There are graphs depicting sea ice extent (total area within the outer boundaries of the ice pack), area (extent minus the gaps of water), and volume.  One can find web pages that specialize in mapping past and current measurements of one of these parameters against different trend lines, in an attempt to get a feel for when the ice will disappear entirely, or almost entirely, during the summer.  There is even one that shows the death spiral as a ... spiral ... using a polar coordinate graphing scheme.

I personally find the most useful graph, though, to be this simple bar chart of sea ice volume.  Bar charts are very familiar, so it's easy to understand at a glance, and when it comes to sea ice, volume seems to me like the most important measurement.  Sea ice extent and area "recovered" rapidly last year to "near normal levels," but the all-time record low in both of those parameters this year makes it clear that much of the ice growth last winter was a very thin coating--which is where volume comes in.

Thanks to Lawrence Hamilton of the University of New Hampshire for permission to reproduce it.

What does this graph tell us?

Currently, volume stands at 3,600 cubic kilometers (cu-km), about half what it was as recently as 2008 (7,100 cu-km) and just over one-fifth what it was in 1979 (16.9 cu-km).  Records begin in 1979 because this set of data is from satellite monitoring, which began in that year.

Also, the trend line is obviously sharply down. If you imagine a line connecting the 1979 number and this years, it looks as though it will reach zero sometime within the next decade, perhaps 2018.

A quick review of some basics concerning Arctic sea ice:

- Its melting will not affect sea level rise, because the ice is floating and displacing the same mass of water as is created when it melts.

- Its melting is, however, likely to continue because of the positive feedback loop created when ice, which reflects sunlight, melts to expose open water (less reflective), leading to warmer temperatures.

- Its melting may also be leading to changes in the jet stream and in the weather the jet stream affects further south.

You can learn much, much more on the topic from the Arctic Sea Ice blog, which I cannot recommend too highly as a resource.