A frightening report from Climate Central details the increase in wildfires in 11 western states of the U.S.
Arctic sea ice has been declining so fast that most people in the know expect to see ice-free conditions in the Arctic ocean at its annual minimum before very long. The natural question is, how long?
We recently looked at trends of sea ice extent (and even forecast next year’s value) using not just the extent itself, but the latitude of the sea ice edge. To compute that, I used a 5th-degree polyonomial approximation from the original paper by Ian Eisenman (who identified the importance of geometry on Arctic sea ice).
It was pointed out that the figures computed by a different method — by actually averaging the latitude of the ice-ocean boundary based on gridded sea ice concentration — differed from the values computed from the polynomial approximation. This is to be expected, since the approximation is the value for ice which is re-arranged to be as far north as possible, whereas for actual sea ice the detailed distribution can strongly impact the average ice-ocean boundary latitude. But the difference seemed larger than expected.
So, I emailed Ian Eisenman and asked whether he could shed light on the matter. He was kind enough to respond in detail, saying this:
In the GRL paper, I focused on a measure of the ice edge latitude that was based on the average location of the contour line representing the transition from sea ice to ice-free ocean. This was computed from the full gridded daily ice concentration fields. In the Auxiliary Material, I briefly described an alternative way to account for continents based on a slightly different (and more subtle) physical justification that involves “rearranging” the ice cover. The alternative measure is perhaps more crude physically, but its advantage is that it can be computed directly from the ice extent, and I gave an approximate polynomial expression to allow others to easily compute this measure of the ice edge latitude from a time series of ice extent. The two measures – the more onerous one used in the paper and the quick rough one described at the end of the Auxiliary Material – produce different quantitative values, but both appear to do a sufficient job of accounting for the influence of coastline geography that the trend in ice edge latitude for each month (trend for 30 Januaries, etc) doesn’t have substantial seasonal structure for either measure (in contrast to ice extent).
It makes sense to me. It also confirms what I had already found, that the main result — indeed the main purpose of investigating ice edge latitude — was unchanged, namely: that most of the seasonal differences in Arctic sea ice trends are a geometric effect. When viewed in terms of ice edge latitude (whether computed from detailed gridded ice fields of the approximation formula), the trends in sea ice loss from one month to another are much more similar.
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.
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.
Since the turn of the millenium, we’ve been struck by some truly notable and very damaging heat waves. Probably foremost in the memory of those who follow such things are the 2003 European heat wave, 2010 in the Moscow region, and just this year in the U.S.
Remember the Alamo
I watched about 8 hours of the 24 Hours of Reality: Dirty Weather Report from Al Gore’s “Climate Reality” project. All in all, I’d say it was excellent. It also made me realize how much more we need to involve the general public, and especially the young adults and near-adults who will bear the brunt of coming climate changes, in our efforts to reverse the lethargy that grips our nation, and to a lesser degree the world, in dealing with the crisis that is global warming.