Guest post by Steven Goddard
In my May 1st piece, Dr. Walt Meier at NSIDC hypothesized that differences in algorithm between NSIDC and NANSEN (NORSEX) were causing the gap between the NSIDC interpretation of normal and the NANSEN interpretation of normal, as seen below. They use different baseline periods which introduces some difference – but the discrepancy should go the other way due to the fact that the NANSEN base period (1979-2007) includes more low ice extent years from the current decade.
So I tried an experiment to test this out, where I overlaid NANSEN on top of NSIDC for the entire winter – and found that they are nearly identical. This would tend to discount the theory that differences in the algorithm are to blame. It appears from this more likely that one or the other has an error in their historical database which is affecting the interpretation of “normal.” Dr. Meier has stated that he is confident about the accuracy of the NSIDC database.
Using a third reference point, I tried another experiment comparing NSIDC (blue) vs. AMSR-E (red) and did see something interesting. Starting in late March, NSIDC (in blue) began to show more ice than AMSR-E (in red) – which uses a different satellite.
The degradation caused the ice extent to appear lower than AMSR-E, but now the problem seems to be going the other way – with SSM/I showing more ice than AMSR-E. What does it all mean? Given that NSIDC and NANSEN seem identical this year, I don’t think this explains the discrepancy in their baseline. It does appear that there is still an error in the SSM/I data however.
UIUC has quit posting their SSM/I images because the quality has become so bad.
February 25, 2009 – The SSMI images for many days in 2009 were bad enough that we removed them from this comparison display (see note below and the NSIDC website). There is enough interest in these side-by-side comparison images that we will try to replace them with corresponding images from the AMSR-E sensor in the coming weeks.