1. R. S. Bradley and M. Vuille are at the Climate System Research Center, Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA.
2. E-mail: (R.S.B.)
3. H. F. Diaz is at the Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80303, USA.
4. W. Vergara is in the Latin America Environment Department, World Bank, 1850 I Street, NW, Washington, DC 20433, USA.