Negotiations pursuant to the Durban Platform for Enhanced Action appear likely to lead to a 2015 Paris agreement that embodies a hybrid climate policy architecture, combining top-down elements, such as for monitoring, reporting, and verification, with bottom-up elements, including “nationally determined contributions” from each participating country, detailing what it intends to do to reduce emissions, based on its national circumstances. For such a system to be cost-effective—and thus more likely to achieve significant global emissions reductions—a key feature will be linkages among regional, national, and sub-national climate policies. By linkage, we mean a formal recognition by a greenhouse gas mitigation program in one jurisdiction (a regional, national, or sub-national government) of emission reductions undertaken in another jurisdiction for purposes of complying with the first jurisdiction’s mitigation program. We examine how a future international policy architecture could help facilitate the growth and operation of a robust system of international linkages of regional, national, and sub-national policies. Several design elements merit serious consideration for inclusion in the Paris agreement, either directly or by establishing a process for subsequent international elaboration. At the same time, including detailed linkage rules in the core agreement is not desirable because this could make it difficult for rules to evolve in light of experience.

The last ten years have seen the growth of linkages between many of the world’s cap-and-trade systems for greenhouse gases (GHGs), both directly between systems, and indirectly via connections to credit systems such as the Clean Development Mechanism. If nations have tried to act in their own self-interest, this proliferation of linkages implies that for many nations, the expected benefits of linkage outweighed expected costs. In this paper, we draw on the past decade of experience with carbon markets to test a series of hypotheses about why systems have demonstrated this revealed preference for linking. Linkage is a multi-faceted policy decision that can be used by political jurisdictions to achieve a variety of objectives, and we find evidence that many economic, political, and strategic factors — ranging from geographic proximity to integrity of emissions reductions — influence the decision to link. We also identify some potentially important effects of linkage, such as loss of control over domestic carbon policies, which do not appear to have deterred real-world decisions to link. These findings have implications for the future role that decentralized linkages may play in international climate policy architecture. The Kyoto Protocol has entered what is probably its final commitment period, covering only a small fraction of global GHG emissions. Under the Durban Platform for Enhanced Action, negotiators may now gravitate toward a hybrid system, combining top-down elements for establishing targets with bottom-up elements of pledge-and-review tied to national policies and actions. The incentives for linking these national policies are likely to continue to produce direct connections among regional, national, and sub-national cap-and-trade systems. The growing network of decentralized, direct linkages among these systems may turn out to be a key part of a future hybrid climate policy architecture.