the Air Vent

Because the world needs another opinion

NSIDC Issues Corrections to Webpage

Posted by Jeff Id on December 24, 2008

Based on The Air Vent post carried by Watts Up With That, the National Snow Ice Data Center has issued several corrections to the documentation of their sea ice area time series.

Most will remember my earlier post which plotted global sea ice trends. After initially concluding that the global ice level wasn’t decreasing measurably Tamino pointed out a problem in my analysis. After issuing my corrections, thanks and apologies to Tamino and the um…..thousands of readers of Watts Up With That, I went back to work investigating what was really happening to the ice area time series.

It was actually quite lucky that Tamino mentioned the step in the data and criticized me for not reading carefully (something which was mentioned in several comments on the various threads). When I first learned of it, I found the criticism was based on an entirely different set of ice area data with different source documentation. Still, I checked closely and found the tiny step in the time series and was convinced that I had missed something. I had spent a huge amount of time learning the data before I made my post so it was frustrating to say the least. Understand, I used several resources to check my work; not the least of which was the National Snow Ice Data Center (NSIDC) anomaly graph which has the same shape as the one I generated.

The first graph below is from the NSIDC website, the second is my calc. Differences in the noise between the two are explained by the daily resolution used in my graph compared to what my eyes tell me must be monthly data for their plot. They also seem to have an additional year (2007) in their data plot which is not available in the bootstrap time series I used.

nsidc-anomaly

nh-sea-ice-area-anomaly-bootstrap-algorithm1

After reading everything with great care, this graph and a similar one for the SH were used to verify my results before my original post. This paragraph below used to be on the NSIDC website describing the data of these time series.

In computing the total ice-covered area and ice extent, pixels must have an ice concentration of 15 percent or greater to be included; thus, total ice-covered area is defined as the area of each pixel with at least 15 percent ice concentration multiplied by the ice fraction in the pixel (0.15-1.00). Total ice extent is computed by summing the number of pixels with at least 15 percent ice concentration multiplied by the area per pixel. Sea ice concentrations are assumed to be 100 percent around a circular sector centered over the Northern Hemisphere pole (known as the pole hole) which is never measured due to orbit inclination. The Southern Hemisphere also has a pole hole; however, it does not affect this sea ice data set; since only land is under this hole. For SMMR, the hole is 611 km in radius and is located poleward of 84.5 degrees north. For SSM/I, the hole is 310 km in radius and is located poleward of 87 degrees north.

After checking this for about the hundredth time along with the rest of the extensive documentation, I wrote to the NSIDC and asked them to confirm that the area anomaly for the NH wasn’t corrected another way. Several emails back and forth later they confirmed that the area anomaly wasn’t accounted for. I then pointed out that the graph above and the paragraph in the data description were in error. After a short time, the NSIDC replied that they had their sea ice team was reviewing the data and planning an immediate update to their site.

That same day the site was corrected to read:

In computing the total ice-covered area and ice extent with both the NASA Team and Bootstrap Algorithms, pixels must have an ice concentration of 15 percent or greater to be included. Total ice extent is computed by summing the number of pixels with at least 15 percent ice concentration multiplied by the area per pixel, thus the entire area of any pixel with at least 15 percent ice concentration is considered to contribute to the total ice extent. Total ice-covered area is defined as the area of each pixel with at least 15 percent ice concentration multiplied by the ice fraction in the pixel (0.15-1.00). There is a circular section over the Northern Hemisphere pole (known as the pole hole) which is never measured due to orbit inclination. For the purposes of ice extent, pixels under the pole hole are always considered to be at least 15 percent. For total ice-covered area, the pixels under the pole hole are not used. The Southern Hemisphere also has a pole hole. However, it does not affect this sea ice data set because there is only land under this hole. For SMMR, the hole is 611 km in radius and is located poleward of 84.5 degrees north. For SSM/I, the hole is 310 km in radius and is located poleward of 87 degrees north. Note: The difference in pole hole areas between SMMR and SSM/I results in a discontinuity in the Northern Hemisphere ice-covered area time series across the instrument transition.

Link HERE. They are still considering how to handle the area anomaly graph.

Since this changes how you interpret area data substantially, there is no easy method for updating the trend graph. Still, the step in the data is quite small as shown below.

ice-area-nh-bootsrtap-zoomed-in

It occurs right after 1987.5 which corresponds to the 87 July/August boundary which is different from tamino’s reference. If we assume worst case that the NH hole in the data was 100% filled with ice (it wasn’t), the calculation from before produces a slight downslope in comparison to the flat trendless line in my original post. The result is only a trend equaling a 4% reduction in global sea ice over a nearly 30 year period. Not exactly disastrous either way. I am going to continue my work on this by matching (regressing) the last two years from other sites on the end of the data. With the recent global cooling, it should be interesting to see where global sea ice is today.

I need to offer thanks to Anthony Watts for putting the original post on his blog. His professionalism was commendable in handling this matter quickly and transparently. IMO this openness to correction is lacking on several AGW blogs. I also need to thank the NSIDC (particularly, Dave, Molly and the Sea Ice Team) who really blew me away with their responsiveness and professional demeanor in making these corrections.



9 Responses to “NSIDC Issues Corrections to Webpage”

  1. Phillip Bratby said

    Well done Jeff. Saw this first at WUWT. Merry Xmas and Happy and Productive New Year.

    Phil

  2. Mongo said

    Jeff – as Phil above says – great job. I wish GISS would be so transparent.

    Merr Christmas!

  3. Layman Lurker said

    Jeff, in the final analysis, your oversight was totally understandable. You also demonstrated your depth of understanding of the data with the previous comments you made about complications running “corrected” NH anomaly graphs – now seemingly confirmed by NSIDC. The whole incident – from the original post, to the discovery of the oversight and your reaction, to the follow through with NSIDC – should help regular readers of this blog to trust your posts that much more.

    Merry Christmas!

  4. Jeff,

    Excellent work. Keep it going!

    Merry Christmas and Happy New Year!

  5. DeWitt Payne said

    So that means the NSIDC extent time series data requires no correction for the transition between SSM/R and SSM/I? It’s probably a pretty safe assumption that the pole hole ice concentration for the large and smaller pole hole is greater than 15%.

  6. Geoff Sherrington said

    Terminology problem when takling of a 15% filled pixel. I thought a pixel was by definition uniform.

    Are “pixels” being used interchangeably with “grid areas”?

    Or are we talking of some grey scale or colour scheme where the hue is 15% of maximum value, or similar?

    The pixel area is probably not constant given geodesy and polar geometry. I’m confused too. And at Christmas.

    But, Jeff Id, you are making remarkably fine contributions to the art and you are to be thanked. Even of you can’t spell “Geoff”.

    Best of the year for 2009 Geoff (Australia)

  7. Geoff Sherrington said

    Jeff Id – Another topic – do you want a ahort paper on Compact fluorescent Globes and some pros and cons?

  8. DeWitt Payne said

    Geoff Sherrington Says:
    December 26, 2008 at 10:29 am,

    A pixel can have a lot of different shades depending on the number of bits used. Typically, gray scale on a computer display has 8 bits of resolution or 256 shades from black to white. Video signals typically cut off the top and bottom of the range and only go from 215 to 235 or something like that. For the satellite images each pixel is measuring a brightness temperature at different microwave frequencies in the vicinity of 90 GHz. That brightness temperature can be converted to ice concentration so indeed an individual pixel can have an ice concentration varying from zero to 100%. Conventionally, a cut off level is used. 15% is usual but William Chapman at CT suggested that they use 10%. You are correct that there is a geometric factor, pixels at the edge of the image cover a larger surface area than pixels at the center, which is used in the calculation of actual extent and area. It’s apparently fairly complex though. IIRC, there was an article a while back on CA where Steve M. tried to convert the raw image data to area (or extent, I can’t remember which) and wasn’t entirely successful.

  9. Keep up the good work, Jeff. We read your site every day over at climategate.com
    All the best for the New Year!

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