Amid global warming, why are we in a deep freeze?

During winter, more often than not, a large part of northern United States is pummelled by an Arctic blast, sometimes severe, sometimes less severe, that lasts for a week or two. But this winter's blast plunged not only the Midwest and the Northeast into a deep freeze with bone-chilling temperatures as low as negative 45 degrees Celsius, it also tested the mettle of millions