1. Department of Economics, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4, and Resources for the Future (e-mail: )
2. Environmental Defense Fund, 257 Park Avenue South, New York, NY 10010 (e-mail: )
3. Department of Economics, Duke University, 213 Social Sciences Building, 419 Chapel Drive, Box 90097, Durham, NC 27708 (e-mail: )