1. School of Geography and the Environment, University of Oxford, Oxford OX1 3QY, U.K.
2. Climate Policy Initiative, Washington, D.C. 20005-7043, United States
3. Department of Sanitation, Water and Solid Waste for Development, Swiss Federal Institute of Aquatic Science and Technology (Eawag), 8600 Dübendorf, Switzerland
4. Department of Engineering Science, University of Oxford, Oxford OX3 7DQ, U.K.