As the planet warms, winter has shrunk by a month in most northern latitudes. In the last 50 years, the Northern Hemisphere lost a million square miles of spring snowpack and in the U.S. alone, snow cover has been reduced by as much as 30 percent. Here Porter Fox travels along the snow line to track the scope of this drastic change, clarifying how it will affect everything—from rapid sea level rise, to fresh water scarcity for two billion people, to massive greenhouse gas emissions from thawing permafrost.