A brief walk with Jeff Mitton and Scott Ferrenberg through the pine forests of Niwot Ridge, nearly a mile above the town of Boulder, Colorado, makes it easy to understand why people are worried about climate change. The world has been getting hotter, and so has Niwot Ridge.
Mitton, the model of a naturalist with his white beard and big smile, studied evolutionary biology after getting his Ph.D. in 1973. When I visit, he is toting a tripod-mounted camera. Ferrenberg, carrying a small ax, is tall and lanky with a salt- and-pepper goatee; he spent time in California, Pennsylvania and Arizona, working jobs from insect research to technical support for forest-fire control, before moving to Boulder and eventually becoming a graduate student in Mitton's lab. They were then part of a team studying the effect of higher temperatures on alpine trees by planting big heating lamps in the hills above Boulder and watching to see what happened.
One late spring day in the forest, the two men spotted something odd: Mountain pine beetles were moving about. These bugs had recently been in the news. By 2010, pine beetles had damaged 4 million acres of Rocky Mountain forest in northern Colorado and southern Wyoming, and beetle watchers were warning of worse to come. It wasn't unusual to see mountain pine beetles in the Colorado hills: They had been there for as long as anyone could remember. But everyone knew they couldn't live at the high altitudes of Niwot Ridge and that the beetles usually didn't come out until July.