Denver Post: As the global climate warms in the next two decades, the snowpack on Aspen Mountain will retreat about 650 feet up the slope and ski season will be trimmed by almost a week, a new study says. One way to fight that will be with snow guns, according to the study on the impact of and adaptation to climate change at two iconic ski resorts — Aspen and Park City, Utah. Aspen, the study calculates, may have to triple its snowmaking in the coming decades. “This is just a way …