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.
I would let our friends at Heartland Institute know about this, but apparently they’re still trying to figure out that California isn’t New Mexico. On January 29 I let them know of their error; as of February 19 it remains on their site.