Will Steffen is Director of the Center for Resource and Environmental Studies and Director of the ANU Institute of Environment at the Australian National University and Chief Scientist at the International Geosphere-Biosphere Programme, Stockholm.Jeffrey G. Williamson is Laird Bell Professor of Economics Emeritus at Harvard and Honorary Fellow in the Department of Economics at the University of Wisconsin--Madison. He is the coauthor (with Kevin O'Rourke) of Globalization and History: The Evolution of a Nineteenth-Century Atlantic Economy and (with Timothy J. Hatton) of Global Migration and the World Economy: Two Centuries of Policy and Performance, both published by the MIT Press.