Complementary Climate Policies for Supply and Demand

The traditional approach to climate policy is to regulate the demand side, for example through an emissions fee. Supply-side regulation has received less attention. The two instruments are perfect substitutes in the first best but we show that they are complements in a second-best setting with free-riding incentives. Demand-side policies alone lower the market price of fossil fuels and raise the gains from trade for a country that defects. For a treaty to be maximally robust, strong, and self-enforcing, it must properly balance supply- and demand-side instruments. The results hold with homogeneous countries and are strengthened by heterogeneity.