An accepted author manuscript of the following article: Mehling, M., Metcalf, G. E., & Stavins, R. N. (2018). Linking climate policies to advance global mitigation: Joining jurisdictions can increase efficiency of mitigation. Science, 359(6379), 997-998. DOI: 10.1126/science.aar5988 Linking climate policies to advance global mitigation : Joining jurisdictions can increase efficiency of mitigation By Michael A. Mehling, 1 Gilbert E. Metcalf, 2 and Robert N. Stavins 3 Submitted to Science Magazine “Policy Forum” [1,976 Words, including acknowledgments and endnotes] The latest round of annual climate negotiations, held last November in Bonn, Germany, under the auspices of the United Nations Framework Convention on Climate Change (UNFCCC), 4 validated that the Paris Agreement 5 (2015) has met one of two necessary conditions for ultimate success. By achieving broad participation including 195 countries accounting for 99% of global greenhouse gas (GHG) emissions, 6 the Paris Agreement dramatically improves upon the 14% of global emissions associated with countries acting under the Kyoto Protocol, 7 the international agreement that it will replace in 2020. But the second necessary condition for ultimate success is adequate collective ambition of the Nationally Determined Contributions (NDCs) countries have individually pledged. That condition has not yet been met. As the negotiators in Bonn began the process of elaborating details of the Paris Agreement, a critical question remains how to incentivize countries to increase ambition over time. The ability to link different climate policies, such that emission reductions undertaken in one jurisdiction can be counted toward the mitigation commitments of another jurisdiction, may help Parties increase their ambition over time. In this essay, we explore options and challenges for facilitating such linkages in light of the considerable heterogeneity that is likely to characterize regional, national, and sub-national efforts to address climate change. The three of us engaged in this topic in Bonn. This article draws in part on our experience there. BACKGROUND. Linkage is important, in part, because it can reduce the costs of achieving a given emissions-reduction objective. 8 Lower costs, in turn, may contribute politically to embracing more ambitious objectives. In a world where the marginal cost of abatement – that is, the cost to reduce an additional ton of emissions – varies widely, linkage improves overall cost-effectiveness by allowing jurisdictions to finance reductions in other jurisdictions with relatively lower costs while allowing the former jurisdictions to count the emission reductions towards targets set in their NDCs. In effect, linkage drives participating jurisdictions toward a common cost of carbon, equalizing the marginal cost of abatement and producing a more efficient distribution of abatement activities. These benefits are potentially significant: a report by the World Bank estimated that international