Thirty years ago Charles F. Baes, Jr., a chemist at the U.S. Department of Energy’s Oak Ridge National Laboratory, wrote that the earth was undergoing a great “uncontrolled experiment,” one that would soon reveal the global consequences of rising greenhouse gas concentrations. Today scientists know that deforestation, land use and the burning of fossil fuels are warming our planet. We are less certain, however, about how climate change will alter forests and grasslands, as well as the goods and services these ecosystems provide society.