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Satellite Sensor Malfunction Caused Underestimation Error in NSIDC Arctic Sea Ice Extent Analysis Beginning in January

The US National Snow and Ice Data Center (NSIDC) reported that a malfunction of the satellite sensor used for its daily sea ice products resulted in a slowly growing underestimation of Arctic sea ice extent beginning in January. The center has removed the most recent data and is investigating alternative data sources that will provide correct results.

Nsidcdrift
Daily total Arctic sea ice extent between 1 December 2008 and 12 February 2009 for Special Sensor Microwave/Imager SSM/I compared to the similar NASA Earth Observing System Advanced Microwave Scanning Radiometer (EOS AMSR-E) sensor. Credit: National Snow and Ice Data Center. Click to enlarge.

The error, known as sensor drift, resulted in an underestimation of approximately 500,000 square kilometers (193,000 square miles) by mid-February. Sensor drift, although infrequent, does occasionally occur and it is one of the things that NSIDC accounts for during quality control measures prior to archiving the data.

NSIDC gets sea ice information by applying algorithms to data from a series of Special Sensor Microwave/Imager (SSM/I) sensors on Defense Meteorological Satellite Program (DMSP) satellites. These satellites are operated by the US Department of Defense, and their primary mission is support of US military operations.

The daily updates in NSIDC’s Arctic Sea Ice News & Analysis rely on rapid acquisition and processing of the SSM/I data. Because the acquisition and processing are done in near-real time, NSIDC publishes the daily data essentially as is. The data are then archived and later subjected to quality control. NSIDC performs quality control measures in coordination with scientists at the NASA Goddard Space Flight Center, which can take up to a year. High-quality archives from SSM/I, combined with data from the earlier Scanning Multi-channel Microwave Radiometer (SMMR) data stream (1979–1987) provide a consistent record of sea ice conditions now spanning 30 years.

Sometimes errors are dramatic and obvious. Other errors, such as the recent sensor drift, may be subtler and not immediately apparent. We caution users of the near-real-time products that any conclusions from such data must be preliminary. We believe that the potential problems are outweighed by the scientific value of providing timely assessments of current Arctic sea ice conditions, as long as they are presented with appropriate caveats, which we try to do.

On February 16, 2009, as emails came in from puzzled readers, it became clear that there was a significant problem—sea-ice-covered regions were showing up as open ocean. The problem stemmed from a failure of the sea ice algorithm caused by degradation of one of the DMSP F15 sensor channels. Upon further investigation, we found that data quality had begun to degrade over the month preceding the catastrophic failure. As a result, our processes underestimated total sea ice extent for the affected period. Based on comparisons with sea ice extent derived from the NASA Earth Observing System Advanced Microwave Scanning Radiometer (EOS AMSR-E) sensor, this underestimation grew from a negligible amount in early January to about 500,000 square kilometers (193,000 square miles) by mid-February. While dramatic, the underestimated values were not outside of expected variability until Monday, February 16. Although we believe that data prior to early January are reliable, we will conduct a full quality check in the coming days.

Sensor drift is a perfect but unfortunate example of the problems encountered in near-real-time analysis. We stress, however, that this error in no way changes the scientific conclusions about the long-term decline of Arctic sea ice, which is based on the consistent, quality-controlled data archive discussed above.

—NSIDC

Comments

Jimr

I suspect this is one of many incorrect readings (only 193,000 sq.mi.) and accusations involved in global warming.

Reel$$

Much of the satellite data used to support AGW can be questioned. Especially sea level measurements as the altimeter radar has error bias (at times) in excess of the change it measures.

Thanks to the alert readers of the Watts Up With That website for flagging NSIDC.

aym

As usual the denalists overstate the point. As NSDIC points out, the data miscongruity only started around January. In other words, only for around 8 weeks has the sensor been wrong.

As stated in the site, other sources are compared to give an overall picture, especially before being archived for scientific endevours, which still shows as a drastic reduction of ice overall. The record breaking minimal ice extent of 2007 has been compared with other data sources, as NSDIC says total data workups take a year, and those numbers still stand. Last years almost breaking record, is still being scrutinized but those numbers are still valid as they occured before the sensor problems.

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