This paper explores the relationship between rationality and equity in an intergenerational context of greenhouse gas emission reduction. It is shown that the least-cost trajectory to a constraint on cumulative emissions implies an upward-sloping emission reduction effort, in most cases, whether technological development is exogenous or endogenous (either investments in research, development and demonstration or learning-by-doing). The least-cost trajectory, however, also implies in most cases that generations in the further future face higher relative costs than do generations in the nearer future. Cost-effectiveness thus may well violate intergenerational equity and rationality of future decision makers. More equitable solutions would lead to a relative shift of abatement effort to the near future, although emission reduction would still be increasing over time. In all cases, technological development in the earlier decades is very important.