The NSIDC press release for the 4th September charts the continuing decline of Arctic sea ice, showing the August mean extent historical series, with 2007 dropping like a stone towards the bottom of the graph. “Don’t care,” is the response of some, “look at the trend for the Antarctic; it’s going up. It’s just natural variations.”

There are a lot of reasons why this idea is wrong-headed, but it gave the old man a thought.

How much sea ice is there at both poles put together? Is there any observable trend in the dual-hemisphere sea ice extent?

Doing a full time series for every month since 1979 (the satellite era) would take ages, so is there a shortcut? September and March are the two months where the sea ice generally reaches its respective maximum and minimum mean monthly extent in both hemispheres (how’s that for alliteration?) .

So I have extracted the mean monthly sea ice extent for these two months, for 1979-2006, from the NSIDC’s readily available dataset. Adding the NH and SH means together gives us a ‘global’ mean monthly extent, at the periods when the metric reaches its extremes at both poles (nominally). Then I summed the two separate months. Note here that there is more ice around in September (24-27 million km2) than there is in March (17.9-21 Mkm2). Also note that the range is greater in the SH than the NH.

What happened?


(Sorry about the clumsy link; I can’t get the graphics insert sorted)

Oh. This was a bit of a surprise. I actually expected there to be little or no observable trend. But there is one. The trend is noticeably downwards.

All of this is an exercise in playing with the numbers, of course, but it should put one idea to bed: even though the sea ice measurements in the Antarctic show a small positive anomaly in the modern record, it is not enough to offset the large negative anomaly in the Arctic. Put simply; globally, there appears to be less ice than there used to be. Now why would that be?

More later.