Arnulf Grübler is a senior research scholar in the Transitions to New Technologies Project at the International Institute for Applied Systems Analysis (IIASA). His books include The Rise and Fall of Infrastructures and Technology and Global Change. Nebojsa Nakicenovic is leader of IIASA's Transitions to New Technologies Project. He is coeditor with Arnulf Grübler of Diffusion of Technologies and Social Behavior; with Arnulf Grübler and Alan McDonald of Global Energy Perspectives; and convening lead author of the IPCC's Special Report on Emissions Scenarios. William D. Nordhaus is Sterling Professor of Economics at Yale University and a Director on the Resources for the Future Board. His books include Invention, Growth, and Welfare and Managing the Global Commons. He is coauthor with Joseph Boyer of Warming the World and with Paul A. Samuelson of Economics, seventeenth edition.