This paper deals with the regulation of greenhouse gases emissions related to climate change. We consider a stylized climate–economy sequential model and use a cost-effectiveness approach. The analytical study is based on a dynamic programming method. It provides both a tolerable ceiling of concentration and, under simple conditions involving the marginal abatement cost and emission functions, optimal and effective abatement rates. In particular, we prove how the cost effective abatement rate increases with time. Through the optimal time to act function, we examine in detail the role played by greenhouse gases absorption, growth and discount rates. We also analyze the paths from an intergenerational equity perspective. Numerical examples illustrate the general statements.