Chukchi-Beaufort ice extent comparison – why feature only the last 7 years?
The most recent issue of Arctic Sea Ice News provided by the National Snow and Ice Data Center (NSIDC) – the official US keeper of sea ice data – (July 17, 2013) included an interesting graph of sea ice extent in the Chukchi and Beaufort Seas at July 12.
They present the data for 2007 to 2013, compared to the new 30 year average1, and note that the Beaufort Sea had “the most extensive ice cover seen there in the last seven summers.” It is also clear from their graph that the 2013 extent was virtually identical to the average in both regions (Fig. 1).
Figure 1. Graph of sea ice extent at July 12 each year from 2007 to 2013 from the National Snow and Ice Data Center, NSIDC. “Climatology” (last set of bars) is the 30 year average (1981-2010) extent at this date.1 It is clear from the graph that the 2013 extent was virtually identical to the 30-year average. Map from Wikipedia.
What puzzled me was why they featured only the last 7 years when satellite data go back to at least 1979. Is there something in that data they don’t want us to see?
There is no similar data in graph form available that I could find but there is the wonderful comparative sea ice mapping tool provided by Cryosphere Today, operated by the University of Illinois.
So, in the absence of numerical data to compare to the Fig. 1 graph, I chose visual data to ask the question: what could there be about the long-term record of Chukchi/Beaufort sea ice data that the NSIDC might not want us to know?
The ice coverage at mid-summer (July 12) provides a snapshot of what sea ice conditions are like for polar bears before the summer melt season gets into full swing, so this historical perspective is quite revealing. [See previous posts here, here, and here for more on Chukchi/Beaufort polar bears.]
I used Cryosphere Today to compare the sea ice extent and concentration values for 2013 on July 12 (which NSIDC tells us was virtually identical to the 30 year average) to selected years from 1981 (the earliest available) to 2012 (Fig. 2, click to enlarge, necessary to see the details).
Viewed together, it is apparent that compared to recent historical levels, only 2007, 2011 and 2012 had really anomalous ice coverage at July 12 in the Chukchi and Beaufort Seas. While 1987, 1993 and 1995 had somewhat lower than average sea ice in the Chukchi/Beaufort region, it was nowhere near the low extent at 2007; 2006 had at least average and perhaps higher-than-average ice extent.
Figure 2. Sea ice concentration maps from Cryosphere Today to compare the sea ice extent and concentration values for 2013 on July 12 (which NSIDC tells us was virtually identical to the 30 year average) to selected years from 1981 (the earliest available) to 2012. These maps are bundled into: 1980s (odd years); 1990s (odd years); early-to-mid 2000s (odd years plus 2006); 2007-2012 (2009 data is not available). Yellow arrows indicate the approximate location of the Chukchi Sea, while the blue arrows indicate the Beaufort Sea. Click to enlarge.
So the answer to my question appears to be that NSIDC would prefer that you and I not realize that at the middle of the summer in the Chukchi/Beaufort region, sea ice coverage was only anomalously low in 2007 and a few years following; that there has not been a long and continuous decline over the last 30-odd years in mid-summer ice extent; and that ice coverage in 2006 was about as close to ‘average’ in the Chukchi as 2013 (and probably above average in the Beaufort) — which means that aside from 2007 and a few years after, there has been lots of ice for Chukchi/Beaufort polar bears until at least mid-summer.
In other words, the ‘low ice’ years of 2007-2011 in the Chukchi/Beaufort were a minor blip in the record that probably means very little. Which begs the question: why did NSIDC feature the graph in the first place? That’s science for you – one question leads to another.
Footnote 1. Note that NSIDC finally, on July 1, 2013, changed the baseline (or average) against which all yearly values are compared from the former 22 year period (1979-2000, applied to all sea ice indices from 2002 to June 2013) to a climatologically valid 30 year period (1981-2010). “Data prior to July 2013 have been reprocessed to this new base period.” See their note here.