1. Adjunct Professor, Dept. of Economics, Universidade Federal de Pernambuco, Recife, 50740-590 Brazil (corresponding author).
2. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana–Champaign, IL 61822.
3. Senior Research Fellow, Environment and Production Technology Institute, International Food Policy Research Institute, Washington, DC 20006.
4. Fellow, Dept. of Economics, Universidade Federal de Pernambuco, Recife, 50740-590 Brazil.