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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.

Two decades have passed since the Clean Air Act Amendments of 1990 launched a grand experiment in market-based environmental policy: the SO2 cap-and-trade system. That system performed well but created four striking ironies. First, by creating this system to reduce SO2 emissions to curb acid rain, the government did the right thing for the wrong reason. Second, a substantial source of this system's cost-effectiveness was an unanticipated consequence of earlier railroad deregulation. Third, it is ironic that cap-and-trade has come to be demonized by conservative politicians in recent years, since this market-based, cost-effective policy innovation was initially championed and implemented by Republican administrations. Fourth, court decisions and subsequent regulatory responses have led to the collapse of the SO2 market, demonstrating that what the government gives, the government can take away.

The outcome of the December 2011 United Nations climate negotiations in Durban, South Africa, provides an important new opportunity to move toward an international climate policy architecture that is capable of delivering broad international participation and significant global CO2 emissions reductions at reasonable cost. We evaluate one important component of potential climate policy architecture for the post-Durban era: links among independent tradable permit systems for greenhouse gases. Because linkage reduces the cost of achieving given targets, there is tremendous pressure to link existing and planned cap-and-trade systems, and in fact, a number of links already or will soon exist. We draw on recent political and economic experience with linkage to evaluate potential roles that linkage may play in post-Durban international climate policy, both in a near-term, de facto architecture of indirect links between regional, national, and sub-national cap-and-trade systems, and in longer-term, more comprehensive bottom-up architecture of direct links. Although linkage will certainly help to reduce long-term abatement costs, it may also serve as an effective mechanism for building institutional and political structure to support a future climate agreement.