This article presents the synthesis of results from the Stanford Energy Modeling Forum Study 27, an inter-comparison of 18 energy-economy and integrated assessment models. The study investigated the importance of individual mitigation options such as energy intensity improvements, carbon capture and storage (CCS), nuclear power, solar and wind power and bioenergy for climate mitigation. Limiting the atmospheric greenhouse gas concentration

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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This paper synthesizes results of the multi-model Energy Modeling Forum 27 (EMF27) with a focus on climate policy scenarios. The study included two harmonized long-term climate targets of 450 ppm CO 2 -e (enforced in 2100) and 550 pm CO 2 -e (not-to-exceed) as well as two more fragmented policies based on national and regional emissions targets. Stabilizing atmospheric GHG concentrations at 450 and 550 ppm CO 2 -e requires a dramatic reduction of carbon emissions compared to baseline levels. Mitigation pathways for the 450 CO 2 -e target are largely overlapping with the 550 CO 2 -e pathways in the first half of the century, and the lower level is achieved through rapid reductions in atmospheric concentrations in the second half of the century aided by negative anthropogenic carbon flows. A fragmented scenario designed to extrapolate current levels of ambition into the future falls short of the emissions reductions required under the harmonized targets. In a more aggressive scenario intended to capture a break from observed levels of stringency, emissions are still somewhat higher in the second half due to unabated emissions from nonparticipating countries, emphasizing that a phase-out of global emissions in the long term can only be reached with full global participation. A key finding is that a large range of energy-related CO 2 emissions can be compatible with a given long-term target, depending on assumptions about carbon cycle response, non-CO 2 and land use CO 2 emissions abatement, partly explaining the spread in mitigation costs.