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Nuclear plant costs in the U.S. have repeatedly exceeded projections. Here we use data covering five decades and bottom-up cost modeling to identify the mechanisms behind this divergence. We observe that nth-of-a-kind plants have been more, not less expensive than first-of-a-kind plants. Soft factors external to standardized reactor hardware, such as on-site labor supervision, contributed over half of the rapid cost rise from 1976-1987. Relatedly, reactor containment building costs more than doubled from 1976-2017, due only in part to safety regulations. Labor productivity in recent plants is up to thirteen times lower than industry expectations. Our results point to a gap between expected and realized costs stemming from low resilience to time-and site-dependent construction conditions. Prospective models suggest reducing commodity usage and automating construction to increase resilience. More generally, rethinking engineering design to relate design variables to cost change mechanisms could help deliver real-world cost reductions for technologies with demanding on-site construction requirements.

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Reducing carbon dioxide (CO 2 ) emissions through a reliance on natural gas can create a hidden commitment to methane (CH 4 ) leakage mitigation. While the quantity of CH 4 leakage from natural gas has been studied extensively, the magnitude and timing of the CH 4 mitigation required to meet climate policy goals is less well understood. Here we address this topic by examining the case of US electricity under a range of baseline natural gas leakage rate estimates and emissions equivalency metrics for converting CH 4 to CO 2 -equivalent emissions. We find that CH 4 emissions from the power sector would need to be reduced by 30%-90% from today's levels by 2030 in order to meet a CO 2 -equivalent climate policy target while continuing to rely on natural gas. These CH 4 emissions reductions are greater than the required CO 2 reductions under the same policy. Alternatively, expanding carbon-free sources more rapidly could meet the 2030 target without reductions in natural gas leakage rates. The results provide insight on an important policy choice in regions and sectors using natural gas, between emphasizing a natural gas supply chain clean-up effort or an accelerated transition toward carbon-free energy sources.

Growing momentum for decentralized climate policy and the falling costs of low-carbon technologies are creating new climate change mitigation opportunities for subnational actors. Here we discuss how research can best support these subnational efforts to allow limited resources to stretch further. To stimulate this discussion, we identify four research priorities. (1) Innovation mechanisms examines local policy opportunities for technology improvement to achieve high returns on investments. (2) Co-benefits analyzes the non-climate benefits of emissions reductions to highlight how local policies can affect communities directly. (3) Emissions monitoring develops rapid, low-cost, local measurement strategies to allow communities to assess and weigh in on the emissions impacts of local energy systems. (4) Decision levers reframes largescale analyses into more targeted and actionable metrics for local policy decisions. This piece was informed and inspired by a set of interviews we conducted with representatives in business, government, NGOs, and educational institutions actively engaged in local climate action, and by our own research.

Evaluating technology options to mitigate the climate impacts of road transportation can be challenging, particularly when they involve a tradeoff between long-lived emissions (e.g., carbon dioxide) and short-lived emissions (e.g., methane or black carbon). Here we present trends in short- and long-lived emissions for light- and heavy-duty transport globally and in the U.S., EU, and China over the period 2000-2030, and we discuss past and future changes to vehicle technologies to reduce these emissions. We model the tradeoffs between short- and long-lived emission reductions across a range of technology options, life cycle emission intensities, and equivalency metrics. While short-lived vehicle emissions have decreased globally over the past two decades, significant reductions in CO will be required by mid-century to meet climate change mitigation targets. This is true regardless of the time horizon used to compare long- and short-lived emissions. The short-lived emission intensities of some low-CO technologies are higher than others, and thus their suitability for meeting climate targets depends sensitively on the evaluation time horizon. Other technologies offer low intensities of both short-lived emissions and CO.

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The Policy Research Working Paper Series disseminates the findings of work in progress to encourage the exchange of ideas about development issues. An objective of the series is to get the findings out quickly, even if the presentations are less than fully polished. The papers carry the names of the authors and should be cited accordingly. The findings, interpretations, and conclusions expressed in this paper are entirely those of the authors. They do not necessarily represent the views of the International Bank for Reconstruction and Development/World Bank and its affiliated organizations, or those of the Executive Directors of the World Bank or the governments they represent.