1. T. H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College of Science, Technology and Medicine, 48 Prince's Gardens, London SW7 2PE, U.K., Economics for the Environment Consultancy Limited, 16 Percy Street, London W1P9FD, U.K., The World Bank, 1818 H Street, Washington, D.C. 20433, and Centre for Social and Economic Research on the Global Environment, University College London, Gower Street, London WC1E6BT, U.K.