Methane emissions from the U.S. oil and natural gas supply chain were estimated by using ground-based, facility-scale measurements and validated with aircraft observations in areas accounting for ~30% of U.S. gas production. When scaled up nationally, our facility-based estimate of 2015 supply chain emissions is 13 ± 2 teragrams per year, equivalent to 2.3% of gross U.S. gas production. This value is ~60% higher than the U.S. Environmental Protection Agency inventory estimate, likely because existing inventory methods miss emissions released during abnormal operating conditions. Methane emissions of this magnitude, per unit of natural gas consumed, produce radiative forcing over a 20-year time horizon comparable to the CO from natural gas combustion. Substantial emission reductions are feasible through rapid detection of the root causes of high emissions and deployment of less failure-prone systems.
Published estimates of methane emissions from atmospheric data (top-down approaches) exceed those from source-based inventories (bottom-up approaches), leading to conflicting claims about the climate implications of fuel switching from coal or petroleum to natural gas. Based on data from a coordinated campaign in the Barnett Shale oil and gas-producing region of Texas, we find that top-down and bottom-up estimates of both total and fossil methane emissions agree within statistical confidence intervals (relative differences are 10% for fossil methane and 0.1% for total methane). We reduced uncertainty in top-down estimates by using repeated mass balance measurements, as well as ethane as a fingerprint for source attribution. Similarly, our bottom-up estimate incorporates a more complete count of facilities than past inventories, which omitted a significant number of major sources, and more effectively accounts for the influence of large emission sources using a statistical estimator that integrates observations from multiple ground-based measurement datasets. Two percent of oil and gas facilities in the Barnett accounts for half of methane emissions at any given time, and high-emitting facilities appear to be spatiotemporally variable. Measured oil and gas methane emissions are 90% larger than estimates based on the US Environmental Protection Agency's Greenhouse Gas Inventory and correspond to 1.5% of natural gas production. This rate of methane loss increases the 20-y climate impacts of natural gas consumed in the region by roughly 50%. methane emissions | oil and gas emissions | greenhouse gas footprint | natural gas supply chain | Barnett Shale M ethane (CH 4 ), the principal component of natural gas, is a powerful greenhouse gas. Although natural gas emits less carbon dioxide (CO 2 ) per unit of energy than coal or oil when burned, CH 4 losses during the production, processing, transportation, and use of natural gas reduce its climate advantage compared with other fossil fuels. For example, if CH 4 losses are large enough (e.g., ∼3% of production), new natural gas power plants can cause greater climate damage than new coal plants for decades or longer (∼1% when comparing natural gas to diesel freight trucks) (1).The lack of current data on CH 4 emissions, magnified by intense public concern over the broader environmental implications of shale gas development, has stimulated significant research to improve estimates of CH 4 emissions (2-18). A recurring theme in recent literature is that "top-down" (TD) approaches produce estimates that are significantly higher than those from "bottomup" (BU) approaches. Concerns about available inventories and divergent TD and BU estimates create confusion regarding policy formulation and leave room for conflicting claims about the greenhouse gas implications of increased use of natural gas.TD approaches for estimating total CH 4 emissions at the regional or larger scale include airborne mass balance (2-4, 19), atmospheric transport models (5,6,(20)(21)(22)(23), and enhancem...
Fugitive losses from natural gas distribution systems are a significant source of anthropogenic methane. Here, we report on a national sampling program to measure methane emissions from 13 urban distribution systems across the U.S. Emission factors were derived from direct measurements at 230 underground pipeline leaks and 229 metering and regulating facilities using stratified random sampling. When these new emission factors are combined with estimates for customer meters, maintenance, and upsets, and current pipeline miles and numbers of facilities, the total estimate is 393 Gg/yr with a 95% upper confidence limit of 854 Gg/yr (0.10% to 0.22% of the methane delivered nationwide). This fraction includes emissions from city gates to the customer meter, but does not include other urban sources or those downstream of customer meters. The upper confidence limit accounts for the skewed distribution of measurements, where a few large emitters accounted for most of the emissions. This emission estimate is 36% to 70% less than the 2011 EPA inventory, (based largely on 1990s emission data), and reflects significant upgrades at metering and regulating stations, improvements in leak detection and maintenance activities, as well as potential effects from differences in methodologies between the two studies.
[1] Recent studies have suggested that CH 4 emissions in Los Angeles and other large cities may be underestimated. We utilized stable isotopes ( 13 C and D) and radiocarbon ( 14 C) to investigate sources of CH 4 in Los Angeles, California. First, we made measurements of d 13 C and dD of various CH 4 sources in urban areas. Fossil fuel CH 4 sources (oil refineries, power plants, traffic, and oil drilling fields) had d13 C values between À45 and À30‰ and dD values between À275 and À100‰, whereas biological CH 4 (cows, biofuels, landfills, sewage treatment plants, and cattle feedlots) had d13 C values between À65 and À45‰ and dD values between À350 and À275‰. We made high-altitude observations of CH 4 concentration using continuous tunable laser spectroscopy measurements combined with isotope analyses ( 13 C, 14 C, and D) of discrete samples to constrain urban CH 4 sources. Our data indicate that the dominant source of CH 4 in Los Angeles has a d 13 C value of approximately À41.5‰ and a dD value between À229 and À208‰. D 14 C of CH 4 in urban air samples ranged from +262 to +344‰ (127.1 to 134.9 pMC), depleted with respect to average global background CH 4 . We conclude that the major source of CH 4 in Los Angeles is leakage of fossil fuels, such as from geologic formations, natural gas pipelines, oil refining, and/or power plants. More research is needed to constrain fluxes of CH 4 from natural gas distribution and refining, as this flux may increase with greater reliance on natural gas and biogas for energy needs.
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