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.

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 polig 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 a 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. [PUBLICATION ABSTRACT]

A key outcome of the Seventeenth Conference of the Parties (COP-17) of the United Nations Framework Convention on Climate Change (UNFCCC), held in Durban, South Africa, late in 2011 — the Durban Platform for Enhanced Action — represents an important milestone in the history of climate negotiations. This is because it departs from the long-standing and problematic dichotomous division of the world's countries into those with serious emissions-reduction responsibilities and the others — with no such responsibilities whatsoever. That distinction, now apparently abandoned, has prevented meaningful progress for decades. The Durban Platform — by replacing the Berlin Mandate's (1995) division of the world into a set of countries with ambitious responsibilities and another set of countries with no responsibilities — has opened an important window. National delegations from around the world now have a challenging task before them: to identify a new international climate policy architecture that is consistent with the process, pathway, and principles laid out in the Durban Platform, while still being consistent with the UNFCCC. The challenge is to find a way to include all key countries in a structure that brings about meaningful emission reduction on an appropriate timetable at acceptable cost, while recognizing the different circumstances of countries in a way that is more subtle, more sophisticated, and — most important — more effective than the dichotomous distinction of years past.

The 1992 United Nations Framework Convention on Climate Change (UNFCCC) launched a process to confront risks posed by global climate change. It has led to a dichotomy between countries with serious emission-reduction responsibilities and others with no responsibilities whatsoever. This has prevented progress, but recent talks suggest the prospect for a better way forward and an openness to outside-the-box thinking. Scholars and practitioners have a new opportunity to contribute innovative proposals for a future international climate policy architecture.