NOTE: there are some animated GIF’s in this post that may take time to fully load. Patience please.
By Steve Goddard
Monday’s NSIDC Arctic ice extent graph took a turn downwards, and is now showing 2010 a little more than 500,000 km2 higher than 2007. The animation below shows the change from May 1 to May 2.
By contrast, NORSEX shows something very different for May 2. They have no downwards turn, their ice extent measurement is right at the 1979-2006 average, and they show 2010 extent more than 1,000,000 km2 above 2007.
In order to look at this closer up, I superimposed the NORSEX 2010 data (red) on the NSIDC 2010 data (blue) at the same scale, and normalised to 2010, and saw some interesting things. The first problem is that they started to diverge right around the first of April, and as of May 2 they disagree by nearly 500,000 km2.
The next image shows that the X-Y scaling is identical (but normalised) in the two graphs. The grid is from NORSEX. Other colors (besides red) have been removed through chroma keying.
The second discrepancy is that the two sources show a large difference in growth since 2007. The image below normalizes the 2007 data – with identical horizontal and vertical scales. Using this view, NORSEX shows twice as much ice growth as NSIDC since 2007.
The animation below begins normalised to 2010 and finishes normalized to 2007. This technique does not show that either source is in error or has changed their data, rather the animation is done by me to enhance visualization.What it does show is the significant differences between the two records.
I believe that both groups use SSMI so it is difficult to understand what the problem is. Last year we saw something similar. NORSEX has a history of making adjustments in mid-season, so my sense is that NSIDC is probably more accurate. Any ideas from readers?