A negative discount rate for climate policy?

Ever since the Stern Review on climate change came out, the debate has raged about what the appropriate discount rate should be to evaluate future environmental outcomes. Biologists do not see the point of discounting, but they have a biased view as they advocate preservation at any cost (much like doctors advocate preserving every life at any cost). In any case, computing future benefits of something at an infinite horizon is impossible if there is no discounting and the benefits do not decrease.

Marc Fleurbaey and Stéphane Zuber actually advocate that we should use a negative discount rate. Technically, that it is only possible to calculate the net present value with a negative discount rate only if the periodic values decrease at a faster rate than the absolute value of this negative discount rate. The key here is not to think of the discount rate as a fixed number, but rather as a result from the ratio of marginal utility from successive periods (or generations). If future generations are worse off, the discount rate becomes indeed negative. Whether future generations will indeed be worse off is is difficult to ascertain, but it could happen. But this is not how we should think about this.

The proper measurement is to use the marginal utility of the person alleviating today the impact of climate change versus the marginal utility of the person in the future benefiting from this action in the future. This is a person-to-person calculation. The present one is likely rich, the future one likely poor. Thus the discount rate should results from the comparison of these marginal utilities is negative, especially because the developing economies are likely to suffer the most from climate change.