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)....

No abstract

among islands and regions using nested, mixed-model ANOVA. We screened several potential estimators to find that the Chao 1 procedure provided the most stable values for local species richness. This estimator is the sum of the observed number of species and the quotient a 2 /2b, where a and b equal the number of species represented by one and two colonies, respectively.To analyse the local-regional species richness relationship in each habitat, we used a simple linear regression of the mean local richness per site in a region against habitatspecific regional richness. Linearity is supported by these and supplemental regressions using: (1) log-transformed richness data; (2) local richness standardized to 100 or more colonies per sample 18 ; and (3) the two alternative measures of local species diversity, Fisher's alpha and the Chao 1 estimator of local richness.

Biomass‐burning plumes and haze layers were observed during the ABLE 2A flights in July/August 1985 over the central Amazon Basin. The haze layers occurred at altitudes between 1000 and 4000 m and were usually only some 100 to 300‐m thick but extended horizontally over several 100 km. They could be traced by satellite imaging and trajectory studies to biomass burning at the southern perimeter of the Amazon Basin, with transport times estimated to be 1–2 days. These layers strongly influenced the chemical and optical characteristics of the atmosphere over the eastern Amazon Basin. The concentrations of CO, CO2, O3, and NO were significantly elevated in the plumes and haze layers relative to the regional background. The NO/CO ratio in fresh plumes was much higher than in the aged haze layers, suggesting that more than 80% of the NOx in the haze layers had been converted to nitrate and organic nitrogen species subsequent to emission. The haze aerosol was composed predominantly of organic material, NH4+, K+, NO3−, SO4=, and anionic organic species (formate, acetate, and oxalate). While the concentrations of most aerosol ions were substantially higher in the haze layers than in the regional background aerosol, the ratios between the aerosol ions in the haze layer aerosols were very similar to those in the boundary layer aerosol over the central Amazon region. Simultaneous measurements of trace gas and aerosol species in the haze layers made it possible to derive emission ratios for CO, NOx, NH3, sulfur oxides, and aerosol constituents relative to CO2. Regional and global emission estimates based on these ratios indicate that biomass burning is an important contributor in the global and regional cycles of carbon, sulfur, and nitrogen species. Similar considerations suggest that photochemical ozone production in the biomass‐burning plumes contributes significantly to the regional ozone budget.

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...