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INTRODUCTION
It is now widely accepted that human activity is partly or largely responsible for climate change in the recent past and in the future, that under a 'business as usual' approach such climate change is likely overall to be extremely damaging to human life and well-being, and that we can mitigate its impact by taking steps to reduce our emissions of greenhouse gases in the short- and medium-term. This raises the question of to what extent we ought to reduce our emissions. Being an ought-question, the question is explicitly normative. Further, it is an ethical question, since the people who stand to be damaged the most by anthropogenic climate change, and stand to benefit the most from any mitigative action, are not the same as those on whom most of the responsibility for mitigation would fall. Those who would 'pay' for the mitigation in question are largely those living (i) now and (ii) in relatively affluent countries; the beneficiaries of mitigation are primarily those in poorer countries (where climate impacts are expected to be the most severe) and those who are not yet born. The question of discounting relates to the temporal aspect of this issue; to a first approximation, it is the question of the extent to which the fact that some anticipated benefit of mitigation would occur a given length of time into the future reduces the value of that benefit for ethical purposes, compared with an otherwise-similar hypothetical benefit occurring now.
The issues here are enormously important, and have rightly attracted an increasing amount of attention. The inevitable consequence of this is that the debate has become increasingly complex, and it can become difficult to see the wood for the trees. This article is a survey of the literature (mostly: the economics literature) on discounting. The emphasis is on understanding discounting from first principles, organizing the issues, and relating the controversies over 'the discount rate' to their foundations in matters of ethical theory. My survey thus emphasizes the conceptual rather than the technical aspects of discounting, but I have not shied away from the use of mathematical notation where this is the appropriate means of expression; I have (however)...