More than 98 inches of snow has fallen in Boston this season, while workers have spent about 170,000 hours plowing the streets and distributed more than 76,000 tons of salt on roadways. At the same time, much of the American West, Rocky Mountains, and Northern and Central Plains have experienced warmer-than-average temperatures. California, in the grip of an epic drought, had its fourth-driest January ever recorded with just 15 percent of average precipitation. So what is going on with this extreme weather, and what does it have to do with global climate change?