Let’s assume that costs to prevent additional accumulation of CO2 (and equivalents) come to 1% of GNP every year forever, and, in accordance with a fair amount of empirical evidence, that the component of the discount rate attributable to the declining marginal utility of consumption is equal to twice the rate of growth of consumption. A straightforward calculation shows that mitigation is better than business as usual – that is, the present value of the benefits exceeds the present value of the costs – for any social rate of time preference less than 8.5%.