1. Center for Sustainability and the Global Environment (SAGE), University of Wisconsin, 1710 University Avenue, Madison, WI 53726, USA.
2. Department of Geography and Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, MD 20742, USA.
3. Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305, USA.
4. National Center for Atmospheric Research (NCAR), Post Office Box 3000, Boulder, CO 80307–3000, USA.
5. Center for Limnology, University of Wisconsin, 680 North Park Street, Madison, WI 53706, USA.