1. University of California, Berkeley, Energy and Resources Group, 310 Barrows Hall # 3050, Berkeley, CA 94703 (e-mail: )
2. Universidad Nacional Autónoma de México, Circuito Exterior S/N, Mexico DF, 04510, and Institute for Environmental Studies, Vrije Universiteit Amsterdam (e-mail: )
3. Department of Economics, University of Sussex, Falmer, BN1 9SL, UK, Vrije Universiteit Amsterdam, Tinbergen Institute, and CESifo (e-mail: )