Through a brief look at the science and economics of climate, the authors show that if climate change turns out to be a serious threat, an effective response will require a substantial and very long-term global effort. Today's focus on near-term emissions reductions will be counter-productive if it delays development of the institutions and policy architectures that would be necessary to mount and sustain such an effort over much of the next century. The authors discuss three legacies that our generation could leave that would make this struggle to devise a global response easier: (1) an international climate agreement that could, if necessary, reduce greenhouse gas emissions substantially, at least cost, while being responsive both to changes in our scientific understanding and to evolving political and economic conditions, (2) enhanced technical options that could, if necessary, ease the task of maintaining economic growth while controlling greenhouse gas emissions, and (3) an international system that could, if necessary, transfer substantial sums to developing countries to assist their participation in an emissions control effort. Building these legacies is a huge challenge, but this task merits at least the same sense of urgency that has motivated pre-Kyoto negotiations about short-term CO2 emissions reductions. / Abstract in HTML and technical report in HTML and PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).