Daily Archives: November 22, 2012

Check my arithmetic

In the last post, I forecast next year’s September sea ice extent using not only extent data, but latitude of the ice edge. To compute that from extent, I applied a formula given by Eisenman in the supplemental info of his publication.

A reader pointed out that there seemed to be a discrepancy between his numerical values and mine — and he’s right. I’ve double-checked the approximation formula and my program, but I can’t resolve the discrepancy.

Therefore, I’d be grateful if some of you would reproduce the calculation of latitude based on extent data, using Eisenman’s approximation formula. The formula as given is this:

The data I used for sea ice extent is here (it’s monthly averages from NSIDC):


If you could report the range of latitude values you get from the extent data in that file, using the formula as given by Eisenman, that would confirm or deny that I’ve implemented his formula as stated.


P.S. The figure I used for the radius of the earth is 6371 km.

Sea Ice Forecasts

For several years, I’ve forecast the Arctic sea ice extent to be observed during September (when it reaches its minimum). For the most part my forecasts have been successful, although I was farther off this year than previously because sea ice extent dipped well below the existing trend line — and that’s how I make my predictions, by extending the existing trend line for September sea ice extent, 1 year into the future.

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