Under cross-pressure to pledge ambitious emission cuts and deliver concrete policy action, climate policymakers must navigate the tension between ambition and implementation prospects. Achieving the Paris Agreement’s long-term targets is possible only if countries make highly ambitious climate pledges. However, very ambitious pledges might engender widespread implementation failure. Devoid of enforcement mechanisms, the Paris Agreement risks an “ambition trap” whereby policymakers pledge ever more ambitious targets without the willingness or capability to ensure these targets’ implementation. Arguing that the difficulties of implementing highly ambitious pledges might threaten the long-term credibility of international climate cooperation, we report two main empirical findings. First, the ambitiousness of existing nationally determined contributions (NDCs) is inversely related to implementation likelihood, indicating a trade-off between pledges’ ambition and implementation prospects. Second, a conjoint experiment in five major democracies shows that the public is (far) more concerned with emission targets’ implementation likelihood than with their stringency (ambitiousness). Our findings suggest that maintaining the Paris Agreement’s long-term credibility requires aligning NDCs’ ambitiousness with feasible implementation. In short, emission targets must be ambitious, yet realistic.

As a quintessential long-term policy problem, climate change poses two major challenges. The first is to develop, under considerable uncertainty, a plan for allocating resources over time to achieve an effective policy response. The second is to implement this plan, once arrived at, consistently over time. We consider the second of these two challenges, arguing that it consists of three interrelated, commitment problems—the time inconsistency problem, the domestic politics problem, and the anarchy problem. We discuss each of these commitment problems in some detail, explore how they relate to climate policy, and suggest institutional designs that may help limit their adverse impact. While each of these commitment problems is difficult to tackle on its own, climate change requires us to cope with all of them at once. This is likely one major reason why we have so far made only modest headway on this vital issue.

Arild Underdal's work on the Law of the Least Ambitious Program (LLAP) is a significant contribution to our understanding of the logic of international collaboration. The LLAP, however, applies only under particular conditions. After comparing the law to the joint decision trap and the veto player concept, we discuss four observations that tend to limit the law's domain. First, while the LLAP is intended to apply to decision-making under unanimity, in a number of international bodies decisions are made by some kind of majority voting. Second, the LLAP assumes that the alternative to collective agreement is individual decision-making, yet in practice the relevant alternative (the “reversion rule”) is often the continuation of some pre-existing collaborative arrangement. Third, whereas the LLAP assumes that the unanimity rule invariably favors the least ambitious program, there are interesting cases where this assumption does not hold. Finally, the LLAP does not take into account that the outcome of international decision-making not only depends on the decision rule and the reversion rule, but also on the voting sequence.

The United States, the world's largest emitter of greenhouse gases, is not going to ratify the Kyoto Protocol in the foreseeable future. Yet, a number of countries have decided to stay on the Kyoto track. Four main explanations for this apparent puzzle are considered. The first is that remaining Annex I countries still expect the Kyoto Protocol to reduce global warming sufficiently to outweigh the economic costs of implementation. The second is that the parties, by implementing the treaty, hope to induce non-parties to follow suit at some later stage. A third hypothesis is that EU climate institutions have generated a momentum that has made a change of course difficult. Finally, Kyoto's persistence may be linked to the European Union's desire to stand forth as an international leader in the field of climate politics. We conclude that the first two explanations have little explanatory power, but find the latter two more promising.

In international regimes research, one of the most important questions is how effective regimes are in delivering what they were established and designed to achieve. Perhaps the most explicit and rigorous formula for measuring regime effectiveness is the so-called Oslo-Potsdam solution. This formula has recently been criticized by Oran Young, himself one of the founding fathers of regime analysis. The present article reviews and responds to his critique and provides several extensions of the Oslo-Potsdam solution. Our response may be summarized in three points. First, we recognize that difficult problems remain unsolved. Second, we argue that for some of the most profound problems there is no escape; we need to engage in counterfactual reasoning, and we need some notion of the “best” solution achievable (such as the “collective optimum”). Finally, we would welcome efforts to further develop and refine the Oslo-Potsdam formula as well as alternative approaches.