If one adopts a real annual discount rate of 5% or more - and there are no significant climate impacts for half a century -these impacts have a present value that is virtually negligible, Within a cost-benefit framework, it then becomes exceedingly difficult to justify any near-term actions other than no-regrets policies, In the greenhouse debate, it is important to draw a clear distinction between prescriptive and descriptive reasoning, A philosopher or an economist may counsel a low or a zero rate of time preference, but this advice does not provide a good description of the collective outcome of individual choices, In particular, it implies an unrealistically rapid increase in the rate of savings and investment.