Can adaptation and mitigation be complements?

It is often claimed that mitigation of greenhouse gases and adaptation to climate change are complementary strategies. If this means that it will usually be optimal to use both mitigation and adaptation to deal with climate change; and that a perceived increase in the damages caused by climate change should require an increase in both mitigation and adaptation, then simple economic analysis would support such an interpretation. However, complementarity has a more technical meaning in economics, which implies if the costs of mitigation fell, then the optimal response would be to increase the level of both mitigation and adaptation. We develop a range of economic models to explore the relationship between mitigation and adaptation; and show that in general adaptation and mitigation will be substitutes. We also find that it is possible for complementarity to occur in the special case where adaptation costs depend on the amount of mitigation.