This lecture defends three main theses: (I) that all decisions about the degree of ambition for emissions mitigation are unavoidably also decisions about how to distribute risk across generations and, more specifically, (II) that the less ambitious the mitigation is, the more inherently objectionable the resulting inter-generational risk distribution is, and (III) that mitigation that is so lacking in ambition that it bequeaths risks that remain unlimited, when the risks could have been limited without inordinate sacrifice, is especially objectionable and constitutes a failure to seize a glorious historic opportunity. This Dewey Lecture in Law and Philosophy was presented on November 8, 2017, by Henry Shue, Senior Research Fellow, Centre for International Studies, and Merton College, University of Oxford.