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)....

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Debates surrounding the greenhouse gas (GHG) emissions from land use of biofuels production have created a need to quantify the relative land use GHG intensity of fossil fuels. When contrasting land use GHG intensity of fossil fuel and biofuel production, it is the energy yield that greatly distinguishes the two. Although emissions released from land disturbed by fossil fuels can be comparable or higher than biofuels, the energy yield of oil production is typically 2-3 orders of magnitude higher, (0.33-2.6, 0.61-1.2, and 2.2 5.1 PJ/ha) for conventional oil production, oil sands surface mining, and in situ production, respectively). We found that land use contributes small portions of GHGs to life cycle emissions of California crude and in situ oil sands production ( <0.4% or < 0.4 gCO₂e/MJ crude refinery feedstock) and small to modest portions for Alberta conventional oil (0.1-4% or 0.1-3.4 gCO₂e/MJ) and surface mining of oil sands (0.9-11% or 0.8-10.2 gCO₂e/MJ).Our estimates are based on assumptions aggregated over large spatial and temporal scales and assuming 100% reclamation. Values on finer spatial and temporal scales that are relevant to policy targets need to account for site-specific information, the baseline natural and anthropogenic disturbance.

Methods for the inclusion of land use in life cycle assessment are not well established. Here, we describe an approach that compares land disturbance between spatially compact and diffuse activities that contribute to the life cycle of a single product, in this case synthetic crude from Alberta's oil sands. We compare production using surface mining and in situ extraction technologies. In situ technologies disturb less land per unit of production than surface mining, but the spatial footprint of in situ production is more dispersed-increasing landscape fragmentation-and in situ production requires more natural gas which increases land use due to gas production. We examine both direct and peripheral land use of oil sands development by quantifying land disturbance using a parameterized measure of fragmentation that relies on 'edge effects' with an adjustable buffer zone. Using a life cycle perspective, we show that the land area influenced by in situ technology is comparable to land disturbed by surface mining when fragmentation and upstream natural gas production are considered. The results suggest that land disturbance due to natural gas production can be relatively large per unit energy. This method could be applied to other energy developments, for example, a comparison between coal mining and natural gas production when both fuels are used to generate electricity.

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Expansion of oil sands development results not only in the release of greenhouse gas emissions, but also impacts land and water resources. Though less discussed internationally due to to their inherently local nature, land and water impacts can be severe. Research in key areas is needed to manage oil sands operations effectively; including improved monitoring of ground and surface water quality. The resulting information gap means that such impacts are not well understood. Improved analyses of oil sands products are required that compare land and water use with other transportation fuel pathways and use a regional perspective so local effects can be considered and mitigated.

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