23 June 2009

Connolley-Grumbine sea ice bet

William Connolley and I (both people who have worked professionally on sea ice) are arranging our bet regarding this year's minimum sea ice cover. I've already mentioned a bit about my prediction, which is for this year's average sea ice extent for September, in the Arctic, as computed by the NSIDC, to be about 4.92 million km^2. To the extent that my working model is good, the standard error of that estimate is about 0.5 million km^2.

In making that prediction, I'm taking the approach that the last two years' dramatic low covers were part of a continuing process of decline in the ice pack, though a fair portion of it still being peculiar circumstances of Arctic weather in summer/fall 2007 and 2008. This is not the usual view in the field, where the most common take is that 2007 represents a step change in the system -- one sort of thing going on before 2007, massive change in 2007, and things should continue more or less similarly for a number of years. (Until the next step change.)

William is taking the view that 2007 and 2008 were just bizarre years, not a fundamental change as most are taking it. The reasonable prediction in his view is to take the years 1979-2006, run a straight line through them, and use that as your basis. I've finally done the math for it (not at all difficult in the spreadsheet but I've been running around), and his prediction for 2009 is 5.84 million km^2 (also with about 0.5 million km^2 standard error -- but I'm rounding down for his figure, and rounding up for mine. That is, my scheme makes a better fit than his.).

So William, how about even quatloos over/under 5.38 million km^2? If it's under, you pay me 50, if over, I pay you 50? If your approach is right, 84% of the time you'll win this. If mine is, it's 84% of the time that I'll win. So, as you advised, it's a wager we each think is biased in our favor.


Alastair said...

From what you say it seems that I am taking a more conventional view in assuming that the rate of decline is increasing.

It is difficult to decide what the rate of decline is at present but it seems to me that the change for 2006-7 should give a good approximation. Using 1st September values from IJIS, then that was was a decline 1,382,500 sq km per yr. Subtracting that from last year's extent of 4,957,656 sq km gives 3,575,156 sq km. That should let me win my bet that the average during September will be less than 4,000,000 sq km. :-)

Cheers, Alastair.

Robert Grumbine said...

Don't worry, you're still apart from the field. Not least, you ignored that 2007-8 showed an increase in figuring your 'per year'.

I'm not sure what the thinking is about the size of the next step, but don't think it's expected to be as large as 2006-7. What I'm reasonably confident about is that nobody is thinking that the next step will occur this year. We'll see on Friday, when the ARCUS estimates are released.

Alastair said...

I have only now realised what my method is!

It is based on the idea that there is a biennial oscillation in the ice melt. In other words a big melt is followed by a wee melt, followed by a big melt and so on. The big melts are becoming larger and the wee melts are becoming larger.

The last big melt was 07 and the last wee melt was 08. So I predict O9 will be another big melt. The last big melt increased over the penultimate wee melt by 1.4 M sq km so I am predicting that this year the next big melt will increase by at least 1.4 M sq km on the last wee melt.

There are other ways you could play with the figures using the big melt wee melt concept but since it is not going to give an accurate result, e.g. because of the weather, then I have just used the simplest method and got a ball park figure.

Cheers, Alastair.

William M. Connolley said...

I need to think about this and draw some pix. In principle, yes.

EliRabett said...

What bothers me about STs position is how unphysical it is

Robert Grumbine said...

Belette: 'long as you're back to me before, say 1 August. :-)

Eli: I'm afraid the ST reference flew by me. ST = Space Telescope?

EliRabett said...

Do idle SToats
of Penguins Dream
As Arctic ice
Through summers channels stream


C W Magee said...

If you want to put a curve into my pool, the current state of play is here:

William M. Connolley said...

Hi Bob. OK, I accept. I still haven't found time to work it out myself but I'm sure I can trust your numbers.