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Tuesday, July 22, 2008

Tamino's Arctic Ice Update

This entire post was lifted from Tamino's Open Mind blog -- you can go directly to his blog at this link (be sure to check out the interesting comments):

Arctic Ice Update

July 17, 2008 · 54 Comments

Last year the North Pole experienced a truly astounding reduction in sea ice. As a result, much of the older, thicker sea ice melted. The ice present now is mostly 1st-year ice, thinner than was present before; as a result, the new ice is more vulnerable to melting during the summer heat. This raises the possibility of similar dramatically low sea ice values this summer. In fact, scientists have made numerous predictions covering a wide range of possibilities, but my reading of the prognostications is that there’s about a 50-50 chance that this year’s minimum sea ice extent will be lower than last year’s. Members of the Polaris Project, for example, are split 4-to-2 against this year’s record breaking last year’s.

Monthly average sea ice extent for the northern hemisphere, as measured by satellite observations since late 1978, are available from the NSIDC (National Snow and Ice Data Center):

There are large ups and downs due to the annual cycle of melting and re-freezing caused by the seasons. But there’s also a distinct decline superimposed on that annual cycle. We can see this plainly by computing sea ice anomaly, the difference between a given month’s extent and the average for that month during some reference period. In this graph, the reference period is the entire time span of data, so the numbers aren’t the same as other sources which use a different reference period (usually 1979 to 2000) but the changes are the same (Note: I can’t seem to get this graph to display on the page — but if you click on it you’ll see the graph):

Another way to isolate the trend from the annual cycle is to compute the “mean” value using a wavelet transform:

This graph tells basically the same story as the previous one, which is evident if we plot them together (but using different axes; the left axis is for anomaly, the right axis for mean extent):

The decline is evident, and is unprecedented for the time we’ve been tracking sea ice; prior data (not from satellites) confirm that the rapid decrease of northern hemisphere sea ice is confined to the last few decades.

Not only has Arctic sea ice been in decline for decades, the decline is accelerating. The anomalies don’t just show a decline, they show a statistically significant greater rate of decline now than for the first two decades of satellite observations. This result isn’t just due to the dramatic 2007 dip; even if we eliminate data after 2007, using only the data up to the end of 2006, the recent increase in the decline rate is still statistically significant.

Will this year’s minimum be even lower than last year’s? The issue isn’t yet decided; the annual minimum in sea ice extent for the northern hemisphere tends to occur around early September, although last year the minimum didn’t happen until Sept. 24th — so we won’t know for nearly another two months how low the extent will go this year. Some maintain a continual watch on the ice cap, reporting daily whether it’s up or down compared to last year; to me that seems rather like watching the grass grow because you’re wondering whether or not the lawn will be as full and green this year as last.

The unusual wind patterns seen last year haven’t occurred this year (yet), so ice reduction isn’t as great so far this year as last year. But there're still nearly two months of the melt season yet to go, and the ice is indeed thinner than last year, so it’s possible we may yet exceed last year’s record.

I managed to locate daily data for sea ice extent from mid-2002 to the present. These data are processed by a different algorithm than those from NSIDC. So, the numbers are different; nonetheless, the changes indicated are extremely similar. Here’s the daily data:

We can directly compare the year 2007 to 2008 (so far):

This is essentially the same as the graph updated regularly on NSIDC.

While it remains to be seen whether this year’s minimum extent will be even smaller than last year’s, we already observe that Arctic sea ice is in decline and that the decline has gotten faster recently. In my opinion, whether or not we break the record again this year is a less important scientific question than whether or not the trend will continue. But clearly, in the press and in the public eye breaking the record has more influence than continuation of a trend. Therefore, a new record low might have significant impact on the “propaganda war” over global warming. As for the actual result … we’ll just have to wait and see. We won’t have to wait very long.

Categories: Global Warming

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