The Paris Agreement culminates a six-year transition towards an international climate policy architecture based on parties submitting national pledges every five years 1 . An important policy task will be to assess and compare these contributions 2,3 . We use four integrated assessment models to produce metrics of Paris Agreement pledges, and show di erentiated e ort across countries: wealthier countries pledge to undertake greater emission reductions with higher costs. The pledges fall in the lower end of the distributions of the social cost of carbon and the cost-minimizing path to limiting warming to 2 • C, suggesting insu cient global ambition in light of leaders' climate goals. Countries' marginal abatement costs vary by two orders of magnitude, illustrating that large e ciency gains are available through joint mitigation e orts and/or carbon price coordination. Marginal costs rise almost proportionally with income, but full policy costs reveal more complex regional patterns due to terms of trade e ects.The pledge and review approach formalized in the Paris Agreement requires a well-functioning transparency regime. Given the discretion left with national governments on the form of their mitigation pledges, or Intended Nationally Determined Contributions (INDCs), assessments are necessary to estimate and compare their impacts. Such comparisons will be of interest to environmental stakeholders who want to pressure those countries with relatively modest mitigation contributions. Business stakeholders may focus on assessments of INDCs' economic impacts, specifically energy price and cost impacts among trade partners.Beyond stakeholder interest, transparency and comparability can promote the stability and facilitate greater ambition of an international climate agreement. Transparent reviews serve to enhance the credibility and likelihood that a party will deliver on its announced pledge, especially with repeating rounds of pledge and review [4][5][6] . Assessments of pledges reveal countries' preferences and interests 1 , enabling more-informed negotiations. International institutions to facilitate transparency-through the collection, analysis and dissemination of information on countries' pledges-can lower the costs of international agreements and enhance their legitimacy 7 . Voluntary pledge and review can result in broad participation 8,9 , as evident in the Paris Agreement. In various contexts, including international trade and common pool resource management, the demonstration of reciprocal actions has resulted in fewer deviations from agreements and positive reactions by members of the agreement 10 .The long-term success of the Paris Agreement is likely to depend on assessments of whether comparable countries undertake comparable mitigation efforts. Such assessments are complicated by the variation in the form of pledges: targets specified in terms of a base year, a forecast, or emissions intensity; peaking year; renewable energy goals; and so on. Evaluating the comparability of mitigation effort highlight...

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A natural outcome of the emerging pledge and review approach to international climate change policy is the interest in comparing mitigation effort among countries. Domestic publics and stakeholders will have an interest in knowing if peer countries are undertaking (or planning to undertake) comparable effort in mitigating their greenhouse gas emissions. Moreover, if the aggregate effort is considered inadequate in addressing the risks posed by climate change, then this will likely prompt a broader interest in identifying those countries where greater effort is arguably warranted based on comparison with their peers. Both assessments require metrics of effort and comparisons among countries. We propose a framework for such an exercise, drawing from a set of principles for designing and implementing informative metrics. We present a template for organizing metrics on mitigation effort, for both ex ante and ex post review. We also provide preliminary assessments of effort along emissions, price, and cost metrics for post-2020 climate policy contributions by China, the European Union, Russia, and the United States. We close with a discussion of the role of academics and civil society in promoting transparency and facilitating the evaluation and comparison of effort.Policy Relevance Statement: Our paper presents a framework for the review of intended nationally determined contributions and the ex post review of contributions under the UNFCCC negotiations. We provide an illustration of this framework with an energy-economic model. Our work focuses on how countries may use the review to compare mitigation effort -planned under INDCs and delivered by implementation of the pledged contributions -to address concerns about equity, efficiency, competitiveness, and the stability of any agreement that arise in international negotiations.

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