As with the NASA Russian temperature debacle last year and the forced recalculation of US surface temperatures for the last century in 2007, the latest problem was discovered after NSIDC received emails from puzzled readers, asking why obviously sea-ice-covered regions were showing up as ice free open ocean. A statement on the NSIDC web site, published February 18, 2009, explains the current faux pas this way:
As some of our readers have already noticed, there was a significant problem with the daily sea ice data images on February 16. The problem arose from a malfunction of the satellite sensor we use for our daily sea ice products. Upon further investigation, we discovered that starting around early January, an error known as sensor drift caused a slowly growing underestimation of Arctic sea ice extent. The underestimation reached 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 we account for during quality control measures prior to archiving the data.