Multiple scientific studies suggest that methane emissions from natural gas systems could be larger than estimated in official inventories, with implications for the use of natural gas in sustainable energy systems. 2 Main Text:Natural gas emits less carbon dioxide during combustion than other fossil fuels and can be flexibly used in a variety of industries. This makes natural gas (NG) a potential "bridge fuel" during the transition to a decarbonized energy system. However, due to the high global warming potential (GWP) of methane (CH 4 ), climate benefits from NG depend on system leakage rates.Several recent estimates of leakage rates have challenged the benefits of fuel switching from coal to NG, a large near-term greenhouse gas (GHG) reduction opportunity (1-3). Policymakers require improved understanding of the leakage rates from NG systems. To this end, we review twenty years of scientific and technical literature on NG emissions. This study presents a first effort to systematically compare emissions estimates at scales ranging from devices (kg/y) to continent-wide atmospheric studies (Tg/y).We first present results from "top-down" studies which measure airborne methane concentrations. We then discuss "bottom-up" studies, which measure device-and facilitylevel leakage rates. We explore differences between study results, and discuss attribution of emissions to natural gas systems. Lastly, we examine implications for GHG emissions policies.Atmospheric studies employ aircraft (1, 4-7), tower (3, 5) and ground (3, 6-9) gas sampling, as well as remote sensing (6, 10,11). All such studies observe atmospheric concentrations, and must infer fluxes by accounting for atmospheric transport. Inference can be made using tracer-tracer correlations (2, 3, 6, 9,11,12), mass-balance (1, 13), and atmospheric modeling and inversion methods (4, 5, 7,14). Strengths and weaknesses exist with each approach (see SI).Figure 1 compiles published estimates of CH 4 leakage at all scales. It includes all known studies which a) performed measurements of emissions at some scale, and b) compared these measurements to inventories or established emissions factors. The ratio of observed emissions to the comparable emissions inventory is plotted on the x-axis, such that ratios >1 imply excess emissions observed relative to those expected.Figure 1 plots estimated CH 4 emissions from atmospheric studies above 10 10 gCH 4 /y. We include all atmospheric studies of CH 4 emissions -not just those that focus on NG -so as to bound emissions from NG. Across years, scales, and methods, these studies systematically find larger CH 4 emissions than predicted by inventories (ratios generally >1). Smaller-scale studies focusing on NG producing (1-3, 8) and consuming regions (2, 6,(9)(10)(11)14) find larger excess CH 4 emissions than national-level studies. This trend may be due to averaging effects of continental-scale atmospheric processes, or due to regional atmospheric studies focusing on areas with air quality problems, such as wintertime ozone (1, 3)....

Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

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