Alex Morales at Bloomberg says, "A glitch in satellite sensors caused scientists to underestimate the extent of Arctic sea ice by 500,000 square kilometers (193,000 square miles), a California- size area, the U.S. National Snow and Ice Data Center said.
"The error, due to a problem called “sensor drift,” began in early January and caused a slowly growing underestimation of sea ice extent until mid-February. That’s when “puzzled readers” alerted the NSIDC about data showing ice-covered areas as stretches of open ocean, the Boulder, Colorado-based group said on its Web site.
"“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,” the center said. “Although we believe that data prior to early January are reliable, we will conduct a full quality check.’’"
A 'glitch' the size of California is a pretty healthy glitch. That might even threaten the validity of your conclusions. Of course. if you had locked in your conclusions before collecting data, you might be impervious to the data.
Hat Tip/Stop the ACLU