Controlling multiple substances that jointly contribute to climate warming requires some method to compare the effects of the different gases because the physical properties (radiative effects, and persistence in the atmosphere) of the greenhouse gases are very different. We cast such indices as the solution to a dynamic, general equilibrium cost-benefit problem where the correct indices are the relative shadow values of control on the various substances. We find that use of declining discount rate, as recommended by recent research, suggests that the current physical-based indices adopted in international negotiations overestimate the value of control of short-lived gases and underestimates the value of control of very long-lived species. Moreover, we show that such indices will likely need to be revised over time and this will require attention to the process by which decisions are made to revise them and how revisions are announced. / Includes bibliographical references (p. 18-19). / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/)