by Dana Nuccitelli, via Skeptical Science
The record Arctic sea ice decline this year has predictably and deservedly received a fair amount of media attention. Jonathan Leake of the Sunday Times recently penned an article on the impending sea ice record. The bulk of the article was quite good, but at the end succumbed to the standard mainstream media practice of seeking “balance,” thus including some comments by John Christy. Christy has become very reliable for arguing that anything and everything related to climate change probably just boils down to natural variability, as he recently told US Congress was the case with regards to the frequency of extreme weather events, contrary to the body of peer-reviewed scientific literature.
As we will see in this post, Christy once again misrepresented the body of scientific literature with regards to Arctic sea ice extent in his efforts to paint the Arctic sea ice death spiral as nothing out of the ordinary.
2012 vs. 1940
In Leake’s article, Christy was paraphrased as saying that there is
“…anecdotal and other evidence suggesting similar melts from 1938-43 and on other occasions.”
Christy’s comments to Leake via email slightly differed from Leake’s paraphrasing, as Christy claimed that evidence suggests summer melts during 1938-43 were “very low extent.” This is a rather vague and subjective statement – very low relative to what? Given the context, Leake understandably appears to have assumed that Christy meant very low relative to recent years, and perhaps he did, but it is also possible that he meant ‘very low’ relative to the early 20th Century, for example.
This begs the obvious question – in the scientific literature, how does Arctic sea ice extent during the period 1938-43 compare to the rest of the 20th Century and current levels? One of the most widely used long-term estimates of Arctic sea ice extent comes from Walsh and Chapman (2001), whose data are available from the University of Illinois (updated through 2008). A description of the vast array of data used by Walsh and Chapman is available via tamino here, and the data are plotted in Figure 1.
Figure 1: Average July through September Arctic sea ice extent 1870-2008 from the University of Illinois (Walsh & Chapman 2001 updated to 2008) and observational data from NSIDC for 2009-2011 (blue), with a fourth order polynomial fit (black soiid line). Black vertical dashed lines indicate the years 1938-43.
Clearly the extent of Arctic sea ice during 1938-43 was nowhere near as low as current levels, based on these data. According to this reconstruction, the minimum extent during that timeframe (9.8 million square kilometers in 1940) was higher than it has been at any time since 1979. In other words, Arctic sea ice extent has been lower than it was in 1938-43 during the entire satellite record, and the current average summer extent is approximately 4.3 million square kilometers lower than the 1940 minimum.