Description:    Although policymaking in response to the climate change threat is essentially a challenge of risk management, most studies of the relation of emissions targets to desired climate outcomes are either deterministic or subject to a limited representation of the underlying uncertainties. Monte Carlo simulation, applied to the MIT Integrated Global System Model (an integrated economic and earth system model of intermediate complexity), is used to analyze the uncertain outcomes that flow from a set of century-scale emissions paths developed originally for a study by the U.S. Climate Change Science Program. The resulting uncertainty in temperature change and other impacts under these targets is used to illustrate three insights not obtainable from deterministic analyses: that the reduction of extreme temperature changes under emissions constraints is greater than the reduction in the median reduction; that the incremental gain from tighter constraints is not linear and depends on the target to be avoided; and that comparing median results across models can greatly understate the uncertainty in any single model. Content Type Journal Article Pages 1-15 DOI 10.1007/s10584-011-0260-0 Authors Mort Webster, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA Andrei P. Sokolov, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA John M. Reilly, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA Chris E. Forest, Department of Meteorology, Pennsylvania State University, University Park, PA, USA Sergey Paltsev, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA Adam Schlosser, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA Chien Wang, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA David Kicklighter, The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA, USA Marcus Sarofim, AAAS Science and Technology Policy Fellow, U.S. Environmental Protection Agency, Washington DC, USA Jerry Melillo, The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA, USA Ronald G. Prinn, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA Henry D. Jacoby, Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA Journal Climatic Change Online ISSN 1573-1480 Print ISSN 0165-0009