The concepts of sustainability that are most widely accepted in the international arena are crucially underwritten by duties to non-overlapping future generations. If such duties are derived directly from the legitimate claims or rights of future people, they suffer from some well-known ontological (e.g. Parfitian) problems. This paper will argue that the extant theoretical literature has focused on these ontological problems to the unfortunate neglect of the equally important (and indeed related) fact that duties so conceived (that is, as duties owed directly to people not yet existing) cannot easily capture and support widely accepted principles of international justice, namely those that combine responsibilities in the past (such as polluter-pays principles) as well as present capacities with future- directed duties. To differentiate responsibilities according to which country did what in the past, direct futural duties must thus be complemented by principles based on the other temporal modalities (past and present), which, however, may be and often are in fact rejected by some negotiating parties, making global governance of environmental issues more intractable. I argue that certain reciprocity-based approaches to intergenerational justice fare much better in that they conceive of duties to future people as the indirect result of what every present generation (even if largely involuntarily) received from its ancestors. Such indirect future-oriented duties take into account how much a society benefited from past usages of a limited resource (such as the carbon-absorptive properties of the atmosphere) to calculate both present capacities and futural duties and to address the problems resulting from such usages. Drawing on phenomenological-ontological accounts of historical time, I argue that intergenerational reciprocity is well supported by the very being of institutionalized societies: it intrinsically belongs to institutions to seek to survive, that is, draw on the past in favour of the future beyond the present.