In the winter of 2015, New York City's Hudson River froze—a rare occurrence. Prior to the 2000s, the record shows that the Hudson froze in 1720, 1780, and 1821—a period that overlaps with the so-called Little Ice Age, when the Northern Hemisphere was cooler overall. But since the turn of the century, the lower Hudson has frozen not once, but twice: in 2015 and 2003. Meanwhile in the Midwest, the National Oceanic and Atmospheric Administration (NOAA) recorded the winter of 2013-2014 as one of the region's coldest on record. That is the year, according to Google Trends, that the terms "Polar freeze" and "Arctic freeze" entered the public lexicon.