Measurements of methane emissions at natural gas production sites in the United States

Engineering estimates of methane emissions from natural gas production have led to varied projections of national emissions. This work reports direct measurements of methane emissions at 190 onshore natural gas sites in the United States (150 production sites, 27 well completion flowbacks, 9 well unloadings, and 4 workovers). For well completion flowbacks, which clear fractured wells of liquid to allow gas production, methane emissions ranged from 0.01 Mg to 17 Mg (mean = 1.7 Mg; 95% confidence bounds of 0.67–3.3 Mg), compared with an average of 81 Mg per event in the 2011 EPA national emission inventory from April 2013. Emission factors for pneumatic pumps and controllers as well as equipment leaks were both comparable to and higher than estimates in the national inventory. Overall, if emission factors from this work for completion flowbacks, equipment leaks, and pneumatic pumps and controllers are assumed to be representative of national populations and are used to estimate national emissions, total annual emissions from these source categories are calculated to be 957 Gg of methane (with sampling and measurement uncertainties estimated at ±200 Gg). The estimate for comparable source categories in the EPA national inventory is ∼1,200 Gg. Additional measurements of unloadings and workovers are needed to produce national emission estimates for these source categories. The 957 Gg in emissions for completion flowbacks, pneumatics, and equipment leaks, coupled with EPA national inventory estimates for other categories, leads to an estimated 2,300 Gg of methane emissions from natural gas production (0.42% of gross gas production).Methane is the primary component of natural gas and is also a greenhouse gas (GHG). In the US national inventories of GHG emissions for 2011, released by the Environmental Protection Agency (EPA) in April 2013 (1), 2,545 Gg of CH₄ emissions have been attributed to natural gas production activities. These published estimates of CH₄ emissions from the US natural gas industry are primarily based on engineering estimates along with average emission factors developed in the early 1990s (2, 3). During the past two decades, however, natural gas production processes have changed significantly, so the emission factors from the 1990s may not reflect current practices. This work presents direct measurements of methane emissions from multiple sources at onshore natural gas production sites incorporating operational practices that have been adopted or become more prevalent since the 1990s.Horizontal drilling and hydraulic fracturing are among the practices that have become more widely used over the past two decades. During hydraulic fracturing, materials that typically consist of water, sand and, additives, are injected at high pressure into low-permeability formations. The injection of the hydraulic fracturing fluids creates channels for flow in the formations (often shale formations), allowing methane and other hydrocarbon gases and liquids in the formation to migrate to the production well. The well and formation is partially cleared of liquids in a process referred to as a completion flowback, after which the well is placed into production. Production of natural gas from shale formations (shale gas) accounts for 30% of US natural gas production, and this percentage is projected to grow to more than 50% by 2040 (4).Multiple analyses of the environmental implications of gas production using hydraulic fracturing have been performed, including assessments of water contamination (5–8), criteria air pollutant and air toxics releases (9–11), and greenhouse gas emissions (11–18). Greenhouse gas emission analyses have generally been based on either engineering estimates of emissions or measurements made 100 m to a kilometer downwind of the well site. This work reports direct on-site measurements of methane emissions from natural gas production in shale gas production regions.Methane emissions were measured directly at 190 natural gas production sites in the Gulf Coast, Midcontinent, Rocky Mountain, and Appalachian production regions of the United States. The sites included 150 production sites with 489 wells, all of which were hydraulically fractured. In addition to the 150 production sites, 27 well completion flowbacks, 9 well unloadings, and 4 well workovers were sampled; the sites were operated by nine different companies. The types of sources that were targeted for measurement account for approximately two-thirds of methane emissions from all onshore and offshore natural gas production, as estimated in the 2011 national greenhouse gas emission inventory (1). A summary of the scope of the study, along with a rationale for the inclusion or exclusion of sources for direct measurement efforts, is provided in SI Appendix. Sampling was conducted from May 2012 through December 2012 at sites throughout the United States (see SI Appendix for a map and for the number of sampling sites in each region). All nine companies that participated in the study provided sites for sampling, and at least three companies provided sites in each of the regions (SI Appendix).The data presented in this report represent hundreds of measurements of methane emissions from several types of onshore natural gas production activities; however, the sites sampled still represent a small fraction of the total number of sites nationwide (SI Appendix.

Table 1.

Comparison of sample set size to emission source populations

Evaluating a groundwater supply contamination incident attributed to Marcellus Shale gas development

High-volume hydraulic fracturing (HVHF) has revolutionized the oil and gas industry worldwide but has been accompanied by highly controversial incidents of reported water contamination. For example, groundwater contamination by stray natural gas and spillage of brine and other gas drilling-related fluids is known to occur. However, contamination of shallow potable aquifers by HVHF at depth has never been fully documented. We investigated a case where Marcellus Shale gas wells in Pennsylvania caused inundation of natural gas and foam in initially potable groundwater used by several households. With comprehensive 2D gas chromatography coupled to time-of-flight mass spectrometry (GCxGC-TOFMS), an unresolved complex mixture of organic compounds was identified in the aquifer. Similar signatures were also observed in flowback from Marcellus Shale gas wells. A compound identified in flowback, 2-n-Butoxyethanol, was also positively identified in one of the foaming drinking water wells at nanogram-per-liter concentrations. The most likely explanation of the incident is that stray natural gas and drilling or HF compounds were driven ∼1–3 km along shallow to intermediate depth fractures to the aquifer used as a potable water source. Part of the problem may have been wastewaters from a pit leak reported at the nearest gas well pad—the only nearby pad where wells were hydraulically fractured before the contamination incident. If samples of drilling, pit, and HVHF fluids had been available, GCxGC-TOFMS might have fingerprinted the contamination source. Such evaluations would contribute significantly to better management practices as the shale gas industry expands worldwide.Horizontal drilling and high-volume hydraulic fracturing (HVHF) are used in combination to extract natural gas, condensate, and oil from shale reservoirs in the United States at rates affecting the world economy (1–4). In the shale gas-rich Marcellus Formation, such slick water HVHF began in 2004, leading to >8,000 Marcellus wells drilled in Pennsylvania (PA) alone as of October 2014. Nearly 70% of these have been hydraulically fractured using large volumes of water and sand with relatively small volumes of gels, acids, biocide, and other compounds (5, 6). The fast rate of such shale development in the northeastern United States has led to several cases of water resource impacts, including surface discharges of contaminants as well as subsurface gas migration (6–12). Although media reports of incidents are common, published reports are few (10).The most useful evidence for incidents links contaminants directly to the source with a high degree of certainty. To evaluate impacts, a “multiple lines of evidence” approach (13–16) is generally necessary, including (i) time series analyses of natural gas and organic and inorganic compound concentrations, (ii) comparisons of natural gas isotopic compositions between gas well annular gas and groundwater, (iii) assessments of gas well construction, (iv), chronology of events, (v) hydrogeologic characterization, and (vi) geospatial relationships.Here we provide data for a contamination incident from PA where the regulator (PA Department of Environmental Protection, PADEP) concluded that stray natural gas derived from nearby Marcellus Shale gas wells contaminated the aquifer used by at least three households in southeastern Bradford County, PA (Fig. 1). In addition to gas, the well waters were also observed to foam (Fig. 1C), but no cause was determined. To investigate this and other contaminants present, we demonstrate an investigative approach to identify unique organic unresolved complex mixtures (UCMs) and a target compound linked to shale gas-related contamination (2-n-Butoxyethanol, 2-BE).Open in a separate windowFig. 1.(A) Study area showing the communities of Wyalusing and Sugar Run located on Susquehanna river (dark grey), gas wells (Shirley, Welles 1–5 well pads labeled as W1 through W5), domestic water wells not impacted by gas drilling activities (B1–B3), and notable geologic features (thrust fault surface expression, regional joint orientation, axis of syncline). (B) Expanded view of tributary of Sugar Run creek (blue line) showing domestic water Wells 1–6 impacted by gas drilling activities. Wells 2, 3, and 5 (triangles) are original impacted wells. Wells 1, 4, and 6 (squares) are replacement wells provided by gas company that also showed contamination. Brown lines are elevation contours (m-msl). Black squares are structures and lines are roads. (C) Foam emitted during purging of domestic water Well 2 in Spring, 2012.     

Political influences on greenhouse gas emissions from US states

Starting at least in the 1970s, empirical work suggested that demographic (population) and economic (affluence) forces are the key drivers of anthropogenic stress on the environment. We evaluate the extent to which politics attenuates the effects of economic and demographic factors on environmental outcomes by examining variation in CO₂ emissions across US states and within states over time. We find that demographic and economic forces can in part be offset by politics supportive of the environment—increases in emissions over time are lower in states that elect legislators with strong environmental records.What drives human impacts on the environment? Why do geopolitical units such as nation-states or the states and provinces within them differ in the stress they place on the environment? From the initial scientific debates on these questions to the most recent reviews, economic and demographic factors are identified as the dominant driving forces of environmental impact (1–4). Even as the emissions scenarios and representative emissions pathways that drive climate models have become more sophisticated, population and affluence are still at their core (5). However, a diverse set of theories and supporting empirical evidence suggest that the effects of economic activity and population size might be mitigated by other factors. These arguments, including ecological modernization, treadmill of production, environmental Kuznets curve, world systems, neo-structuralism, and commons management theories, are complex and subtle (6–11). However, the importance of politics is a common theme in each of them, as well as the focus of a substantial literature in its own right. Some approaches focus on how institutions, including laws and treaties, shape the actions of individuals, organizations and nations (9, 12–15). Others emphasize the dynamics of political power (6–8, 16–20). As Shwom argues, who has power influences what polices, programs, and institutions are in place to moderate or exacerbate how human actions influence the environment–political factors are part of what are commonly called the driving forces of environmental change (21).We use greenhouse gas emissions of US states to examine the potential moderating role of politics on the more frequently examined drivers of environmental stress. We make this choice for several reasons. First, greenhouse gas emissions are a critical environmental stressor that influences the climate—a global commons. Explaining variation in emissions is thus an appropriate challenge for a theory of anthropogenic environmental change. Second, in the absence of strong US national policy on climate change, states have varied substantially in the actions they have taken to limit emissions, potentially reflecting differences in the distribution of political power and political ideology (22). Third, the concepts we wish to address can be operationalized with well measured variables available for all 50 states over a reasonable span of time.     

Direct measurements of methane emissions from abandoned oil and gas wells in Pennsylvania

Abandoned oil and gas wells provide a potential pathway for subsurface migration and emissions of methane and other fluids to the atmosphere. Little is known about methane fluxes from the millions of abandoned wells that exist in the United States. Here, we report direct measurements of methane fluxes from abandoned oil and gas wells in Pennsylvania, using static flux chambers. A total of 42 and 52 direct measurements were made at wells and at locations near the wells (“controls”) in forested, wetland, grassland, and river areas in July, August, October 2013 and January 2014, respectively. The mean methane flow rates at these well locations were 0.27 kg/d/well, and the mean methane flow rate at the control locations was 4.5 × 10⁻⁶ kg/d/location. Three out of the 19 measured wells were high emitters that had methane flow rates that were three orders of magnitude larger than the median flow rate of 1.3 × 10⁻³ kg/d/well. Assuming the mean flow rate found here is representative of all abandoned wells in Pennsylvania, we scaled the methane emissions to be 4–7% of estimated total anthropogenic methane emissions in Pennsylvania. The presence of ethane, propane, and n-butane, along with the methane isotopic composition, indicate that the emitted methane is predominantly of thermogenic origin. These measurements show that methane emissions from abandoned oil and gas wells can be significant. The research required to quantify these emissions nationally should be undertaken so they can be accurately described and included in greenhouse gas emissions inventories.Abandoned oil and gas wells provide a potential pathway for subsurface migration and emissions to the atmosphere of methane and other fluids (1). According to one recent study, there are an estimated 3 million abandoned oil and gas wells throughout the United States (2). Methane emissions from these wells are assumed to be the second largest potential contribution to total US methane emissions above US Environmental Protection Agency estimates and are not included in any emissions inventory (2). There is a lack of empirical studies that can be used to estimate the methane emission potential of these wells (2).Methane is a greenhouse gas (GHG) and its oxidation produces ozone (O₃) that degrades air quality and adversely impacts human health, agricultural yields, and ecosystem productivity (3). Therefore, it is important to understand methane emission sources so that appropriate mitigation strategies can be developed and implemented.Efforts to improve estimates of methane emissions to the atmosphere from oil and gas production in the United States are being driven, in part, by growth in unconventional production. Estimates of methane emissions from activities on producing oil and gas sites, including well completion, routine maintenance, and equipment leaks, are used to develop bottom–up estimates (4, 5). Overall, a comparison of bottom–up and top–down estimates indicate that there may be missing sources in bottom–up estimates (2, 6–8, 9). Here, we focus on one missing source: abandoned oil and gas wells.There is no regulatory requirement to monitor or account for methane emissions from abandoned wells in the United States. Methane leakage through abandoned wells linked to recent growth in unconventional oil and gas production is being studied as a groundwater contamination issue (10–14), but no direct evidence for leakage through abandoned wells to groundwater aquifers currently exists. Abandoned wells have been connected to subsurface methane accumulations that have caused explosions, which are major concerns in urban areas with oil and gas development or natural gas storage reservoirs, as well as in coal mines (15, 16). Therefore, existing monitoring is focused on detecting large concentrations. The result is a lack of information to quantify methane emissions from abandoned oil and gas wells.To characterize abandoned oil and gas wells’ potential as a significant methane source, we made first-of-a-kind direct measurements of methane flow rates from 19 wells in various locations across McKean and Potter counties in Pennsylvania (PA) (Fig. 1). The measured wells were selected mainly based on logistical and legal access (Supporting Information). As of January 17, 2014, only 1 of the 19 wells was on the PA Department of Environmental Protection’s (DEP’s) list of abandoned and orphaned wells. (Orphaned wells can be defined as abandoned wells with no responsible party available, other than the state.) The DEP database provides information on the well status (abandoned, plugged, or orphan) and well type (gas, oil, combined oil and gas, or undetermined) but does not provide other information such as well age and depth. No additional information on the measured wells is available. This is indicative of the general scarcity of available information on this class of old wells in PA. Given the lack of records on the wells we measured, no distinction was made between oil and gas wells; the wells were simply categorized as plugged or unplugged, based on surface evidence of cementing and/or presence of a marker. With this criterion, 5 of the 19 measured wells (26%) were classified as plugged.Open in a separate windowFig. 1.The 19 measured wells are located in McKean County and Potter County in Pennsylvania. There are 12,127 abandoned, orphaned, and plugged wells on the Pennsylvania DEP’s website (as of January 17, 2014), with 4,273 in McKean County and 188 in Potter County. The map shows the DEP wells that are in the region of our field study. Note that only the western portion of Potter County is shown in the detailed map.In addition to methane, we also analyzed the collected samples for ethane, propane, n-butane, and carbon isotopes of methane, to provide insight on the potential sources of the emitted methane. This work provides previously unavailable data on methane leakage rates and other emissions from abandoned oil and gas wells.     

From the Cover: Methane emissions from natural gas infrastructure and use in the urban region of Boston,Massachusetts

Methane emissions from natural gas delivery and end use must be quantified to evaluate the environmental impacts of natural gas and to develop and assess the efficacy of emission reduction strategies. We report natural gas emission rates for 1 y in the urban region of Boston, using a comprehensive atmospheric measurement and modeling framework. Continuous methane observations from four stations are combined with a high-resolution transport model to quantify the regional average emission flux, 18.5 ± 3.7 (95% confidence interval) g CH₄⋅m⁻²⋅y⁻¹. Simultaneous observations of atmospheric ethane, compared with the ethane-to-methane ratio in the pipeline gas delivered to the region, demonstrate that natural gas accounted for ∼60–100% of methane emissions, depending on season. Using government statistics and geospatial data on natural gas use, we find the average fractional loss rate to the atmosphere from all downstream components of the natural gas system, including transmission, distribution, and end use, was 2.7 ± 0.6% in the Boston urban region, with little seasonal variability. This fraction is notably higher than the 1.1% implied by the most closely comparable emission inventory.Atmospheric methane (CH₄) is an important greenhouse gas (1) and major contributor to elevated surface ozone concentrations worldwide (2). Current atmospheric CH₄ concentrations are 2.5 times greater than preindustrial levels due to anthropogenic emissions from both biological and fossil fuel sources. The growth rate of CH₄ in the atmosphere slowed beginning in the mid-1980s and plateaued in the mid-2000s, but growth has resumed since 2007. The factors responsible for the observed global increase and interannual trends, and the spatiotemporal distribution of sources, remain uncertain (3).Losses of natural gas (NG) to the atmosphere are a significant component of anthropogenic CH₄ emissions (3), with important implications for resource use efficiency, worker and public safety, air pollution, and human health (4), and for the climate impact of NG as a large and growing source of energy. A major focus area of the US Climate Action Plan is reduction of CH₄ emissions (5), but implementation requires identification of dominant source types, locations, and magnitudes. A recent review and synthesis of CH₄ emission measurements in North America, spanning scales of individual components to the continent, found that inventory methods consistently underestimate CH₄ emissions, that fossil fuels are likely responsible for a large portion of the underestimate, and that significant fugitive emissions may be occurring from all segments of the NG system (6).The present study quantifies CH₄ fluxes from NG in the urbanized region centered on Boston. Elevated CH₄ concentrations in urban environments have been documented around the world for decades (7) (SI Appendix, Table S1) and attributed to a variety of anthropogenic source types. Recent studies of urbanized regions in California, using diverse atmospheric observing and modeling approaches, consistently found that CH₄ emission rates were larger than those estimated by regional bottom-up inventories (8–12). In Boston, elevated CH₄ concentrations have been observed at street level and attributed to >3,000 NG pipeline leaks from antiquated infrastructure (13), but associated CH₄ emission rates were not quantitatively assessed.In this study, we combine four key quantities in an atmosphere-based analytical framework: (i) atmospheric CH₄ enhancements above background (ΔCH₄) were determined from measurements at a network of continuous monitoring stations, inside and upwind of the urban core (Fig. 1), for 12 mo in 2012–2013; (ii) the NG fraction of the observed ΔCH₄ was quantified for cool and warm seasons by measuring atmospheric ethane (C₂H₆), a tracer of thermogenic CH₄, and comparing ratios of C₂H₆ and CH₄ in the atmosphere and in the pipeline gas flowing through the region; (iii) total CH₄ emissions were derived from an atmospheric transport model, which quantitatively links surface fluxes with observed ΔCH₄ using assimilated meteorology; and (iv) the fraction of delivered NG lost to the atmosphere was estimated by comparing CH₄ emissions to spatially explicit data on NG consumption. The result encompasses NG losses from the entire urbanized region, including emissions from NG transmission, storage, distribution, end use, and liquefied NG importation.Open in a separate windowFig. 1.Location of two city [Boston University (BU), 29-m height; Copley Square (COP), 215-m height] and two peripheral [Harvard Forest (HF); Nahant (NHT)] measurement stations (black points) in Boston, and the surrounding area, overlaid on a map of the number of housing units with NG per square kilometer (14). The 90-km radius circle delineates the ∼18,000-km² land area for which CH₄ emissions and the NG loss rate were calculated. The magenta and purple contours enclose 50% of the average footprint (sensitivity area) of the BU and COP afternoon measurements, respectively. The two city sites are difficult to distinguish at this scale because the horizontal distance between them is ∼2 km. The influence area is ∼80% larger for COP than BU because the former station is higher. See SI Appendix, Table S2, for additional measurement site location information.     

Toward a better understanding of protein folding pathways.

Experimental observations of how unfolded proteins refold to their native three-dimensional structures contrast with many popular theories of protein folding mechanisms. The available experimental evidence (ignoring slow cis-trans peptide bond isomerization) is largely consistent with the following general scheme: under folding conditions, unfolded protein molecules rapidly equilibrate between different conformations prior to complete refolding. This rapid prefolding equilibrium favors certain compact conformations that have somewhat lower free energies than the other unfolded conformations. Some of the favored conformations are important for productive folding. The rate-limiting step occurs late in the pathway and involves a high-energy, distorted form of the native conformation; there appears to be a single transition state through which essentially all molecules refold. Consequently, proteins are not assembled via a large number of independent pathways, nor is folding initiated by a nucleation event in the unfolded protein followed by rapid growth of the folded structure. The known folding pathways involving disulfide bond formation follow the same general principles. An exceptional folding mechanism for reduced ribonuclease A proposed by Scheraga et al. (Scheraga, H.A., Konishi, Y., Rothwarf, D.M. & Mui, P.W. (1987) Proc. Natl. Acad. Sci. USA 84, 5740-5744) is shown to result from experimental shortcomings, an incorrect kinetic analysis, and a failure to consider the kinetics of unfolding.     

From the Cover: Majority of US urban natural gas emissions unaccounted for in inventories

Across many cities, estimates of methane emissions from natural gas (NG) distribution and end use based on atmospheric measurements have generally been more than double bottom-up estimates. We present a top-down study of NG methane emissions from the Boston urban region spanning 8 y (2012 to 2020) to assess total emissions, their seasonality, and trends. We used methane and ethane observations from five sites in and around Boston, combined with a high-resolution transport model, to calculate methane emissions of 76 ± 18 Gg/yr, with 49 ± 9 Gg/yr attributed to NG losses. We found no significant trend in the NG loss rate over 8 y, despite efforts from the city and state to increase the rate of repairing NG pipeline leaks. We estimate that 2.5 ± 0.5% of the gas entering the urban region is lost, approximately three times higher than bottom-up estimates. We saw a strong correlation between top-down NG emissions and NG consumed on a seasonal basis. This suggests that consumption-driven losses, such as in transmission or end-use, may be a large component of emissions that is missing from inventories, and require future policy action. We also compared top-down NG emission estimates from six US cities, all of which indicate significant missing sources in bottom-up inventories. Across these cities, we estimate NG losses from distribution and end use amount to 20 to 36% of all losses from the US NG supply chain, with a total loss rate of 3.3 to 4.7% of NG from well pad to urban consumer, notably larger than the current Environmental Protection Agency estimate of 1.4% [R. A. Alvarez et al., Science 361, 186–188 (2018)].

Atmospheric methane (CH₄) is the second-most important greenhouse gas (GHG) after carbon dioxide (CO₂); the Intergovernmental Panel on Climate Change (IPCC) estimates that it was responsible for ∼20% of global anthropogenic direct radiative forcing from 2000 to 2010 (1). Oil and natural gas (NG) systems are estimated to account for 31% of US anthropogenic methane emissions (2). NG emissions have increased over the last decade, as it has become an increasingly important energy source in the United States due to advances in extraction technology, reduction in cost, and its promotion as having lower CO₂-equivalent emissions relative to other fossil fuels.Densely populated urban areas are well positioned to effect change in GHG emissions, as they have concentrated population, infrastructure, and emissions along with, in many cases, political will to implement emission reductions policies (3). Pipelines, transmission infrastructure, household and commercial appliances, meters, stationary combustion, and service leaks are thought to be the most significant source types for urban NG emissions (2). Bottom-up inventories estimate that distribution and end use contribute 6% of US emissions from the NG supply chain (2), but that estimate is highly uncertain. Urban NG emissions have been reported for several cities including Boston, Indianapolis, Washington, DC, Los Angeles, New York City, and Philadelphia using top-down methods based on tower, aircraft, and remote sensing measurements (e.g., refs. 4–8). Top-down studies consistently estimate distribution and end use NG emissions to be significantly (two to 10 times) larger than the bottom-up estimates. The large gap between bottom-up and top-down analyses indicates that there are likely large missing sources in inventories, and it is unclear from which sector those emissions originate. In order to implement effective GHG mitigation policies, it is necessary to understand the dominant sources of CH₄ emissions from NG distribution and end use.We present an 8-y top-down study of NG emissions from the Boston urban region to assess the NG loss rate over time and investigate the potential missing sources of NG emissions in inventories. We also assess the impact of the COVID-19 shutdown on CH₄ emissions during April 2020. This is one of only a few long-term, top-down studies of urban CH₄ emissions and is in an east-coast city with older, leak-prone infrastructure. Previous studies in other cities have not found any statistically significant trend in emissions over time (7, 9–11). Boston has set a target of becoming carbon neutral by 2050 (12), and Massachusetts has been working to reduce NG leaks, with several laws and regulations implemented between 2014 and 2019 requiring timelines for utilities to report and repair large leaks based on their size (13, 14); our study will assess whether these efforts have produced a measurable change in NG emissions.To put this study in context with other cities and evaluate current knowledge of CH₄ emissions from NG distribution and end use, we then compare top-down NG emission estimates from four US cities with new studies estimating bottom-up emissions from pipeline leaks and in-house losses. By updating top-down versus bottom-up comparisons with the latest available data, we assess the current understanding of the total carbon footprint of NG, the state of the NG budget, and how well we can constrain urban end-use emissions.     

Shale gas development impacts on surface water quality in Pennsylvania

Concern has been raised in the scientific literature about the environmental implications of extracting natural gas from deep shale formations, and published studies suggest that shale gas development may affect local groundwater quality. The potential for surface water quality degradation has been discussed in prior work, although no empirical analysis of this issue has been published. The potential for large-scale surface water quality degradation has affected regulatory approaches to shale gas development in some US states, despite the dearth of evidence. This paper conducts a large-scale examination of the extent to which shale gas development activities affect surface water quality. Focusing on the Marcellus Shale in Pennsylvania, we estimate the effect of shale gas wells and the release of treated shale gas waste by permitted treatment facilities on observed downstream concentrations of chloride (Cl⁻) and total suspended solids (TSS), controlling for other factors. Results suggest that (i) the treatment of shale gas waste by treatment plants in a watershed raises downstream Cl⁻ concentrations but not TSS concentrations, and (ii) the presence of shale gas wells in a watershed raises downstream TSS concentrations but not Cl⁻ concentrations. These results can inform future voluntary measures taken by shale gas operators and policy approaches taken by regulators to protect surface water quality as the scale of this economically important activity increases.     

The role of concrete in life cycle greenhouse gas emissions of US buildings and pavements

Concrete is a critical component of deep decarbonization efforts because of both the scale of the industry and because of how its use impacts the building, transportation, and industrial sectors. We use a bottom-up model of current and future building and pavement stocks and construction in the United States to contextualize the role of concrete in greenhouse gas (GHG) reductions strategies under projected and ambitious scenarios, including embodied and use phases of the structures’ life cycle. We show that projected improvements in the building sector result in a reduction of 49% of GHG emissions in 2050 relative to 2016 levels, whereas ambitious improvements result in a 57% reduction in 2050, which is 22.5 Gt cumulative saving. The pavements sector shows a larger difference between the two scenarios with a 14% reduction of GHG emissions for projected improvements and a 65% reduction under the ambitious scenario, which is ∼1.35 Gt. This reduction occurs despite the fact that concrete usage in 2050 in the ambitious scenario is over three times that of the projected scenario because of the ways in which concrete lowers use phase emissions. Over 70% of future emissions from new construction are from the use phase.

Concrete is the most extensively used building material in the world because it possesses a unique combination of attributes—strength, versatility, and durability—for a relatively low cost using raw materials found all over the world. It is used in nearly every element of our built environment including buildings, pavements, bridges, and water and energy systems. This ubiquity in infrastructure has also made concrete use tightly linked to achieving societal sustainability goals. Thacker et al. (1) found that infrastructure, which makes extensive use of concrete, either directly or indirectly influences the attainment of every United Nations Sustainable Development Goal.On a weight-normalized basis, concrete has a lower carbon and energy footprint than nearly all materials used in the built environment (2). Nevertheless, the cement and concrete sectors are deservedly under scrutiny regarding their environmental footprint because of the sheer scale of production (3). Greenhouse gas (GHG) emissions from the production of cement (the primary driver of GHG emissions for concrete) account for a little over 1% of the total US GHG emissions footprint (4). Thus, the challenge of sustainable development is manifest in microcosm in the use of concrete: accomplishing societal goals while minimizing environmental impacts.There is no question that we need to reduce the emissions associated with cement and concrete production. However, the mitigation solutions for products made with concrete extends beyond the cement and concrete production value chains. Materials dictate the modes of manufacture and constrain the operational performance of the products into which they are fashioned (5). Concrete is a prime example of this phenomenon. Forming the backbone of large, complex, long-lived systems, changes in the use of concrete can positively or negatively impact the in-use performance and GHG emissions of these systems for decades.In this systems context, we seek to evaluate the cost and effectiveness of a range of strategies for reducing the GHG footprint of two important systems—buildings and pavements—including both changes in cement and concrete production and changes in system design, maintenance, and operations. Using this comprehensive model, we also evaluate the relative contribution of embodied and operational emissions as these systems undergo significant change and explore whether GHG emissions reductions are possible in these systems even if there is increased use of concrete. Mapping these changes for buildings and pavements is challenging, because the impact of system structure is influenced by local context, the role of extant stock and its evolution, and the long timeframe that needs to be considered. To overcome these challenges, we develop and apply spatially and temporally heterogenous, life cycle models of the buildings and pavements systems. We limit our analysis to the United States because of the extent of data required for modeling. In the United States, these systems account for over 60% of apparent cement usage according to data from the industry and the building, transportation, and industry sectors account for 90% of all GHG emissions (4). As such, changes in the structural components of these systems can provide influential leverage in meeting climate targets.     

From the Cover: Mode of action uncovered for the specific reduction of methane emissions from ruminants by the small molecule 3-nitrooxypropanol

Ruminants, such as cows, sheep, and goats, predominantly ferment in their rumen plant material to acetate, propionate, butyrate, CO₂, and methane. Whereas the short fatty acids are absorbed and metabolized by the animals, the greenhouse gas methane escapes via eructation and breathing of the animals into the atmosphere. Along with the methane, up to 12% of the gross energy content of the feedstock is lost. Therefore, our recent report has raised interest in 3-nitrooxypropanol (3-NOP), which when added to the feed of ruminants in milligram amounts persistently reduces enteric methane emissions from livestock without apparent negative side effects [Hristov AN, et al. (2015) Proc Natl Acad Sci USA 112(34):10663–10668]. We now show with the aid of in silico, in vitro, and in vivo experiments that 3-NOP specifically targets methyl-coenzyme M reductase (MCR). The nickel enzyme, which is only active when its Ni ion is in the +1 oxidation state, catalyzes the methane-forming step in the rumen fermentation. Molecular docking suggested that 3-NOP preferably binds into the active site of MCR in a pose that places its reducible nitrate group in electron transfer distance to Ni(I). With purified MCR, we found that 3-NOP indeed inactivates MCR at micromolar concentrations by oxidation of its active site Ni(I). Concomitantly, the nitrate ester is reduced to nitrite, which also inactivates MCR at micromolar concentrations by oxidation of Ni(I). Using pure cultures, 3-NOP is demonstrated to inhibit growth of methanogenic archaea at concentrations that do not affect the growth of nonmethanogenic bacteria in the rumen.Since the agricultural and industrial revolution 200 y ago, the methane concentration in the atmosphere has increased from less than 0.6 to 1.8 ppm. The present concentration is only 0.45% of that of CO₂, but because methane has a 28- to 34-fold higher global warming potential than CO₂ on a 100-y horizon, it contributes significantly to global warming (1). On the other hand, the lifetime of atmospheric methane is relatively short relative to CO₂. Accordingly, the climate response to reductions of methane emissions will be relatively rapid. Thus, measures targeting methane emissions are considered paramount to mitigate climate change (2).One of the main anthropogenic sources of atmospheric methane are ruminants (cattle, sheep, goats), the number of which has grown in parallel with the world population. Presently, there are about 1.5 billion cattle, 1.1 billion sheep, and 0.9 billion goats raised by humans (3). Ruminants emit about 100 million tons of methane per year, which corresponds to ∼20% of global methane emissions (4).In the rumen (Fig. 1), plant material is fermented by anaerobic bacteria, protozoa, fungi, and methanogenic archaea in a trophic chain, predominantly yielding acetate, propionate, butyrate, CO₂, and methane with H₂ as intermediate (5, 6). Whereas organic acids are absorbed and metabolized by the animals, methane escapes the rumen into the atmosphere via eructation and breathing of the animals. The generation of methane by methanogenic archaea in the intestine of domestic ruminants lessens feed efficacy, as up to 12% of the gross energy ingested by the animal is lost this way (7).Fig. 1.Methane formation in the rumen of a dairy cow and its inhibition by 3-nitrooxypropanol (3-NOP). The H₂ concentration in the rumen fluid is near 1 µM (≙140 Pa = 0.14% in the gas phase).Methane (CH₄) formation is the main H₂ sink in the rumen. It is formed by methanogenic archaea at the bottom of the trophic chain mainly from carbon dioxide (CO₂) and hydrogen (H₂) (Fig. 1). However, the methane eructated by ruminants contains only minute amounts of H₂; the concentration of dissolved H₂ in the rumen is near 1 µM (8), equivalent to a H₂ partial pressure of near 140 Pa. Because at 1 µM, H₂ formation from most substrates in the rumen is exergonic (9), the low H₂ concentration indicates that H₂ is consumed in the rumen by the methanogens more rapidly than it is formed by other microorganisms (10). The H₂ concentration increases substantially only when methane formation from H₂ and CO₂ is specifically inhibited by more than 50% (10, 11). Already a small increase in the H₂ concentration (8) leads to both down-regulation of H₂-generating pathways (12) and up-regulation of H₂-neutral and H₂-consuming pathways such as propionate formation, resulting in additional energy supply to the host animal (13–15). Thus, the H₂ concentration stays constant, although its consumption by methanogens is partially inhibited in the rumen.The amount of methane formation per unit of ingested feedstuff can differ significantly between individual animals as it is a heritable trait (16). Understanding these differences has been the scientific motivation to pursue the development of selective inhibitors of methanogenesis that are nontoxic to animals (17, 18). Only recently, a compound has been described that apparently can both substantially decrease CH₄ and increase propionate productions in the rumen without compromising animal performance and health (19). It is the small molecule 3-nitrooxypropanol (3-NOP) (chemical structure shown in Fig. 1) that has been found to persistently decrease enteric methane emissions from sheep (20), dairy cows (21), and beef cattle (22) without apparent negative side effects (19). 3-NOP, given to high-producing dairy cows at 60 mg/kg feed dry matter (Fig. 1), not only decreased methane emissions by 30% but also increased body weight gain significantly without negatively affecting feed intake nor milk production and composition (19).Methane formation in methanogenic archaea is catalyzed by methyl-coenzyme M reductase (MCR), involving methyl-coenzyme M and coenzyme B as substrates (Fig. 2A). MCR is a nickel enzyme in which the nickel is bound in a tetrapyrrole derivative named cofactor F₄₃₀ (23, 24). This nickel-containing cofactor has to be in the Ni(I) oxidation state for the enzyme to be active. Because the redox potential E^o′ of the F₄₃₀(Ni²⁺)/ F₄₃₀(Ni¹⁺) couple is −600 mV, the enzyme is very susceptible to inactivation by oxidants (23, 24). MCR has been well characterized by high-resolution X-ray structures (25–27) and EPR spectroscopy (28) with either substrates or products bound.Fig. 2.Binding of 3-NOP to methyl-coenzyme M reductase (MCR) as suggested by molecular docking. The crystal structure of inactive isoenzyme I from M. marburgensis was used in the docking experiments (25). (A) MCR-catalyzed reaction. CH₃-S-CoM, methyl-coenzyme ...The molecular shape of 3-NOP (Fig. 1) is similar to that of methyl-coenzyme M (Fig. 2A). This fact and the moderate oxidation potential of 3-NOP suggested that inhibition of methanogenesis in ruminants is achieved by targeting the active site of MCR, for which we now provide experimental evidence. We start by describing how the development of 3-NOP was facilitated by molecular modeling.     

Climate change and health costs of air emissions from biofuels and gasoline

Environmental impacts of energy use can impose large costs on society. We quantify and monetize the life-cycle climate-change and health effects of greenhouse gas (GHG) and fine particulate matter (PM_2.5) emissions from gasoline, corn ethanol, and cellulosic ethanol. For each billion ethanol-equivalent gallons of fuel produced and combusted in the US, the combined climate-change and health costs are $469 million for gasoline, $472–952 million for corn ethanol depending on biorefinery heat source (natural gas, corn stover, or coal) and technology, but only $123–208 million for cellulosic ethanol depending on feedstock (prairie biomass, Miscanthus, corn stover, or switchgrass). Moreover, a geographically explicit life-cycle analysis that tracks PM_2.5 emissions and exposure relative to U.S. population shows regional shifts in health costs dependent on fuel production systems. Because cellulosic ethanol can offer health benefits from PM_2.5 reduction that are of comparable importance to its climate-change benefits from GHG reduction, a shift from gasoline to cellulosic ethanol has greater advantages than previously recognized. These advantages are critically dependent on the source of land used to produce biomass for biofuels, on the magnitude of any indirect land use that may result, and on other as yet unmeasured environmental impacts of biofuels.     

Global metaanalysis of the nonlinear response of soil nitrous oxide (N2O) emissions to fertilizer nitrogen

Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) that also depletes stratospheric ozone. Nitrogen (N) fertilizer rate is the best single predictor of N₂O emissions from agricultural soils, which are responsible for ∼50% of the total global anthropogenic flux, but it is a relatively imprecise estimator. Accumulating evidence suggests that the emission response to increasing N input is exponential rather than linear, as assumed by Intergovernmental Panel on Climate Change methodologies. We performed a metaanalysis to test the generalizability of this pattern. From 78 published studies (233 site-years) with at least three N-input levels, we calculated N₂O emission factors (EFs) for each nonzero input level as a percentage of N input converted to N₂O emissions. We found that the N₂O response to N inputs grew significantly faster than linear for synthetic fertilizers and for most crop types. N-fixing crops had a higher rate of change in EF (ΔEF) than others. A higher ΔEF was also evident in soils with carbon >1.5% and soils with pH <7, and where fertilizer was applied only once annually. Our results suggest a general trend of exponentially increasing N₂O emissions as N inputs increase to exceed crop needs. Use of this knowledge in GHG inventories should improve assessments of fertilizer-derived N₂O emissions, help address disparities in the global N₂O budget, and refine the accuracy of N₂O mitigation protocols. In low-input systems typical of sub-Saharan Africa, for example, modest N additions will have little impact on estimated N₂O emissions, whereas equivalent additions (or reductions) in excessively fertilized systems will have a disproportionately major impact.Nitrous oxide (N₂O) is a major greenhouse gas (GHG) with a global warming potential ∼300-fold that of CO₂ over a 100-y time period (1). Additionally, N₂O is the largest stratospheric ozone-depleting substance and is projected to remain so for the remainder of this century (2). N₂O emissions from agricultural soils, produced predominantly by the microbial processes of nitrification (oxidation of ammonium to nitrate) and denitrification (reduction of nitrate via N₂O to N₂) (3), constitute ∼50% of global anthropogenic N₂O emissions (1), primarily as a result of the addition of synthetic nitrogen (N) fertilizers and animal manure to soil (4). The total input of N to soil, and its subsequent availability, is a robust predictor of N₂O fluxes and has been used to construct most national GHG inventories using an N₂O emission factor (EF) approach (5).The N₂O EF is the percentage of fertilizer N applied that is transformed into fertilizer-induced emissions, which is calculated for Intergovernmental Panel on Climate Change (IPCC) GHG inventories as the difference in emission between fertilized and unfertilized soil under otherwise identical conditions. Global EFs for fertilizer-induced direct N₂O emissions have been determined by Eichner (6), Bouwman (7, 8), Mosier et al. (9), Bouwman et al. (4, 10), and Stehfest and Bouwman (11). The current global mean value, derived from over 1,000 field measurements of N₂O emissions, is ∼0.9% (10, 11). This value for fertilizer-induced emissions is an approximate average of emissions induced by synthetic fertilizer (1.0%) and animal manure (0.8%), and it has been rounded by the IPCC (5) to 1% due to uncertainties and the inclusion of other N inputs, such as crop residues (12) and soil organic matter mineralization (1). In short, for every 100 kg of N input, 1.0 kg of N in the form of N₂O is estimated to be emitted directly from soil.A 1% EF assumes a linear relationship between N input and N₂O emissions that is indifferent to biological thresholds that might occur, for example, when the availability of soil inorganic N exceeds crop N demands. Because the vast majority of studies on N₂O emissions from crops have examined a single fertilizer input (many without a zero N control), there is no power in these studies for detecting such thresholds. However, results from a growing number of field experiments with multiple N fertilizer rates indicate that emissions of N₂O respond in an exponentially increasing manner to increasing N inputs across a range of fertilizer formulations, climates, and soil types (e.g., refs. 13–16), suggesting that EFs are not constant but increase monotonically with N additions.Incorporating this knowledge into large-scale N₂O models could help to close the gap between bottom-up and top-down estimates of fertilizer N₂O contributions to regional and global fluxes (17, 18). Bottom-up estimates rely on the extrapolation of flux chamber measurements in individual ecosystems to larger regions, including the globe. Grace et al. (19), for example, used a nonlinear N₂O emission function to model total direct N₂O emissions from the US north central region between 1964 and 2005. Their estimate was equivalent to an EF of 1.75% over this period, which is substantially higher than the global default IPCC value of 1%. More recently, Griffis et al. (20) used tall tower eddy covariance measurements to estimate an overall US north central region EF of 1.8% for contemporary fluxes.Top-down estimates are based on changes in atmospheric N₂O concentrations over time that are assigned to changes in human activities known to affect N₂O fluxes. Global top-down estimates of N₂O from anthropogenic sources of reactive N, including animal manure (21), yield an overall EF of 4 ± 1% (17, 18). Although bottom-up models are in broad agreement (22), there are large uncertainties and the agreement breaks down at regional and subregional scales (23). The use of EFs that vary with N input may help to reconcile this difference and guide policies that are urgently needed to curb the projected 20% increase in agricultural N₂O emissions expected by 2030 (23).Nonlinear EFs also hold implications for estimated N₂O fluxes from low-input cropping systems typical of those in sub-Saharan Africa. If N₂O emissions are not much affected by fertilizer N added to meet crop needs precisely, then modest fertilizer additions will have little impact on estimated N₂O emissions. Impacts will be far larger, on the other hand, where N is added at rates that exceed crop N needs.Response curves for N₂O flux as a function of N input have recently become more common. McSwiney and Robertson (13), for example, reported an exponentially increasing N₂O response to N fertilizer along a nine-point fertilizer N gradient for nonirrigated corn in Michigan. In their study, N₂O fluxes following fertilization more than doubled (20 vs. >50 g of N₂O-N ha⁻¹⋅d⁻¹) at N inputs greater than 100 kg⋅ha⁻¹, the level at which yield was maximized. Hoben et al. (15) documented a similar response for five on-farm sites in Michigan under corn-soybean rotations with six fertilizer N inputs (0–225 kg⋅ha⁻¹). Others (14, 16), but not all (24), have since found similar patterns for multiple N-input levels. Kim et al. (25) documented 18 published instances with nonlinear responses to four or more levels.Here, we test the generality of these findings globally. Although there are very few N₂O response studies with a sufficient number of N-input levels to characterize a nonlinear response with precision, we located over 200 studies with two or more N inputs in addition to a zero N control, which allows the determination of two or more EFs for the same site-year. We then evaluated the presence and direction of a slope, and calculated ∆EF as the percent change in EF per unit of additional N input (measured in kilograms per hectare). Here, we report the results of a metaanalysis of this global dataset and investigate the potential interaction of ∆EF with other factors, such as crop type, fertilizer type, and available environmental factors. We also test for possible biases in sampling factors, including the duration and number of measurements, flux chamber area, number of samples per flux measurement, and numbers of replicates. We then compare results with published EF determinations, including those used as a basis for current IPCC tier 1 methodologies (4, 10) and carbon credit markets (26, 27).     

Natural gas of radiolytic origin: An overlooked component of shale gas

Natural gas is an important fossil energy source that has historically been produced from conventional hydrocarbon reservoirs. It has been interpreted to be of microbial, thermogenic, or, in specific contexts, abiotic origin. Since the beginning of the 21^st century, natural gas has been increasingly produced from unconventional hydrocarbon reservoirs including organic-rich shales. Here, we show, based on a careful interpretation of natural gas samples from numerous unconventional hydrocarbon reservoirs and results from recent irradiation experiments, that there is a previously overlooked source of natural gas that is generated by radiolysis of organic matter in shales. We demonstrate that radiolytic gas containing methane, ethane, and propane constitutes a significant end-member that can account for >25% of natural gas mixtures in major shale gas plays worldwide that have high organic matter and uranium contents. The consideration of radiolytic gas in natural gas mixtures provides alternative explanations for so-called carbon isotope reversals and suggests revised interpretations of some natural gas origins. We submit that considering natural gas of radiolytic origin as an additional component in uranium-bearing shale gas formations will lead to a more accurate determination of the origins of natural gas.

Natural gas is extracted from conventional and unconventional hydrocarbon reservoirs to satisfy current energy demands. Three different origins of natural gas have been distinguished in previous literature including microbial, thermogenic, and abiotic (1). Some researchers also advocate for a low-temperature geocatalytic origin of some natural gases (ref. 2 and references therein). Microbial, thermogenic, and geocatalytic gases are derived from organic matter either by the action of microorganisms or due to elevated temperatures during burial of organic-rich sediments or through geocatalytic generation of nonmicrobial gases at low temperatures. Abiotic processes (3) do not involve organic matter but produce gases through gas–water–mineral interactions in the subsurface by reaction of native H₂ with CO₂ (4). The composition and isotopic signatures of natural gas components are frequently used to determine the origin and maturity of the natural gas (Fig. 1). Natural gases of microbial origin consist mostly of methane that is depleted in ¹³C (δ¹³C ranging between less than −90 and −50‰) (5). In contrast, thermogenic gases from shale gas reservoirs typically contain methane, ethane, propane, and higher n-alkanes with δ¹³C of methane varying between −75 and −20‰ (5) dependent on maturity. Geocatalytic gases mostly consist of methane with δ¹³C between −58 and −41‰ (14). Abiotic gases have a wide range of molecular compositions, and their methane is frequently enriched in ¹³C (δ¹³C ranging between −50 and +10‰) (5).Open in a separate windowFig. 1.Revised Bernard plot after Milkov and Etiope (5). Dark blue line indicates a thermogenic maturation line according to ref. 6 with % R_o increasing from 0.6 to 3. Black dashed lines indicate mixing of radiolytic (R) and thermogenic (T) gas components; the green dashed line indicates the mixing of radiolytic gas with a mixture of primary biogenic gas (e.g., methyl type fermentation) and/or secondary microbial gas (both marked as B in the legend). The brown dashed line indicates mixing of radiogenic (R) and microbial gas derived by CO₂ reduction. CR, CO₂ reduction; F, methyl-type fermentation; SM, secondary microbial; OA, oil-associated (midmature) thermogenic gas; LMT, late mature thermogenic gas after Milkov et al. (1). (Inset) Data points from the Woodford Shale with maturities (7) increasing toward lower dryness values. Data are from radiolytic gases (8) and Barnett and Fayetteville (9), Antrim (10), New Albany (11), Woodford (7), Colorado Group (12), and Alum (13) shales.With the onset of the shale gas revolution early in the 21st century facilitated by horizontal drilling technologies combined with high-volume hydraulic fracturing, natural gas has been increasingly produced in recent years from unconventional hydrocarbon reservoirs such as shales with high organic matter content. Such shales are often associated with high contents of radioactive elements (15–17), and hence, the organic matter they contain is exposed to significant radiation doses over geologic time spans. Naturally occurring radioactive isotopes such as ²³⁸U, ²³⁵U, ²³²Th, ²³⁰Th, and ⁴⁰K and their radioactive daughter products emit α- and β-particles and γ-rays that have penetration depths into the organic matter ranging from <100 µm for α-particles (18) and 1 to 5 mm for β-particles to >50 m for γ-rays (19). Potassium (K) and thorium (Th) are usually associated with detrital minerals. The concentration of radioactive ⁴⁰K is too low in shales to produce significant irradiation of surrounding matter since ⁴⁰K constitutes only 0.012% of all K isotopes (20), while concentrations of radioactive thorium can reach 20 ppm (e.g., refs. 21–23) and uranium (U) up to 2,600 ppm (24). Uranium is typically directly associated with organic matter in black shales (25–28), which will absorb most of the energy released during the U decay. For example, organic carbon in the Alum Shale in Europe with, on average, 100 ppm of U absorbed a 10⁸- to 10⁹-Gy radiation dose over the 500 Ma since its deposition (18, 29, 30).High radiation doses can cause changes in the structure and properties of organic matter (31–35). For the fossil organic matter, kerogen, the most notable changes are an increase of aromaticity, of degree of condensation, and of vitrinite reflectance and a decrease in bitumen content (29, 36–42). Ionizing radiation causes polymerization, cross-linking, dealkylation, and aromatization of organic matter (29, 30, 43, 44) and has been shown to produce short-chain alkanes such as methane, ethane, and propane (8). Additionally, experimental irradiation of organic matter showed the importance of mineral surface area and a presence of clay minerals (44) in disintegration of organic matter and formation of radiolytic products including gases. Furthermore, during irradiation, ⁻H^· radicals form in large quantities (45), which might facilitate radiolytic formation of alkanes.Laboratory-based irradiation experiments (8, 18) with organic matter and crude oils have revealed the formation of radiolytic gas that is mainly composed of H₂ (56 to 96 vol. %), while around 2% of the newly formed gas is composed of methane, ethane, and propane with a linear positive relationship between the radiation dosage and the amount of radiolytic H₂ and alkanes produced (19). These radiolytic hydrocarbons are derived from organic matter but neither through microbial nor through temperature-driven reactions, and they have been found to be depleted in ¹³C (δ¹³C of methane less than −65‰, δ¹³C of ethane less than −45‰, and δ¹³C of propane less than −37‰) (8).We note that previous laboratory-based irradiation experiments using shales and fossil organic matter have not used α-particle irradiation, which mostly occurs in U-rich rocks. Thus, the findings and conclusions presented in this paper are based on the assumption that isotopic signatures of radiolytic gases produced during gamma-ray irradiation in laboratory experiments are equivalent to those resulting from alpha radiation in the geosphere. This is supported by similarities observed between irradiated organic matter in laboratory experiments and in nature. Experiments that used gamma rays from a ⁶⁰Co source (18) demonstrated that irradiated organic matter in shales became slightly enriched in ¹³C requiring that the radiolytic gaseous products are depleted in ¹³C. The slight ¹³C enrichment of irradiated organic matter is also observed in natural U-rich rocks (37, 46–48), and thus, radiolytic hydrocarbons formed in such rocks are also expected to be depleted in ¹³C. This indirectly supports the notion that α-radiation in nature causes formation of ¹³C-depleted radiolytic gases in a very similar fashion to that of gamma radiation in laboratory experiments. However, laboratory data are currently scarce, and future experiments with α-particle irradiation of organic shales as well as controlled temperature parameters within the reaction chamber are needed.This study investigates whether radiolytic methane, ethane, and propane (also referred to as “light alkanes” in the subsequent text) constitute a previously overlooked component of natural gas, especially in organic-rich shale gas plays. We demonstrate that light alkanes derived from the irradiation of kerogen and oil make a nonnegligible contribution to natural gas mixtures from unconventional hydrocarbon reservoirs. By using an isotopic maturation-mixing model on a large set of natural gas data, we quantify the effect of the admixture of light alkanes of radiolytic origin to gases of thermogenic and microbial origin. We also demonstrate that the resulting isotope signatures can lead to misinterpretation of gas origin and maturation levels, and we provide an alternative explanation of the so-called isotope reversals in natural gas from unconventional hydrocarbon reservoirs. We conclude that radiolytic gas derived from organic matter constitutes a previously not recognized type of natural gas that needs to be considered especially in organic-rich unconventional hydrocarbon reservoirs that frequently contain uranium (U) in substantial quantities (15, 16).     

Nitrous oxide emissions from agricultural soils challenge climate sustainability in the US Corn Belt

Agricultural landscapes are the largest source of anthropogenic nitrous oxide (N₂O) emissions, but their specific sources and magnitudes remain contested. In the US Corn Belt, a globally important N₂O source, in-field soil emissions were reportedly too small to account for N₂O measured in the regional atmosphere, and disproportionately high N₂O emissions from intermittent streams have been invoked to explain the discrepancy. We collected 3 y of high-frequency (4-h) measurements across a topographic gradient, including a very poorly drained (intermittently flooded) depression and adjacent upland soils. Mean annual N₂O emissions from this corn–soybean rotation (7.8 kg of N₂O–N ha⁻¹⋅y⁻¹) were similar to a previous regional top-down estimate, regardless of landscape position. Synthesizing other Corn Belt studies, we found mean emissions of 5.6 kg of N₂O–N ha⁻¹⋅y⁻¹ from soils with similar drainage to our transect (moderately well-drained to very poorly drained), which collectively comprise 60% of corn–soybean-cultivated soils. In contrast, strictly well-drained soils averaged only 2.3 kg of N₂O–N ha⁻¹⋅y⁻¹. Our results imply that in-field N₂O emissions from soils with moderately to severely impaired drainage are similar to regional mean values and that N₂O emissions from well-drained soils are not representative of the broader Corn Belt. On the basis of carbon dioxide equivalents, the warming effect of direct N₂O emissions from our transect was twofold greater than optimistic soil carbon gains achievable from agricultural practice changes. Despite the recent focus on soil carbon sequestration, addressing N₂O emissions from wet Corn Belt soils may have greater leverage in achieving climate sustainability.

Nitrous oxide (N₂O) is a powerful greenhouse gas with 298 times the warming potential of carbon dioxide (CO₂) over 100 y (1), and N₂O is also the leading contributor to stratospheric ozone depletion (2). Agricultural soils are currently the primary anthropogenic source of N₂O (3), as a consequence of increased application of synthetic nitrogen (N) fertilizer and manure over the past century (4–6). Without efforts to reduce emissions, atmospheric N₂O will continue to rise along with demand for agricultural products, threatening our ability to mitigate climate change and ozone depletion (3). As such, accurate estimates of N₂O emissions from agricultural soils are essential for understanding the scope of anthropogenic N₂O emissions and developing effective solutions. This topic is particularly urgent, given the recent policy emphasis on climate mitigation through soil management, which has often tended to emphasize practices thought to increase soil carbon (C) (7, 8). However, N₂O emissions can determine whether agricultural systems are net sources or sinks of greenhouse gases (9).Despite decades of measurements, the sources and magnitudes of agricultural N₂O emissions remain difficult to estimate. Most published N₂O measurements have been made by using chambers placed over the soil surface (bottom-up measurements), providing snapshots of emissions with typically coarse spatial and temporal resolution (6, 10). Alternatively, N₂O emissions can be estimated over broad spatial extents using atmospheric data (top-down measurements). In some cases, bottom-up and top-down N₂O measurements may disagree markedly (11, 12), implying poor characterization of spatial or temporal variation and the possibility of significant unmeasured sources. In the midwestern US Corn Belt, the dominant agricultural region of North America, a 6-y tall tower study estimated mean regional emissions of 411 Gg of N₂O–N y⁻¹, which would correspond to about 6.9 kg of N₂O–N ha⁻¹⋅y⁻¹ if all emissions were derived directly from the 59.8 million ha of cropland in this study region (11, 13)_. However, bottom-up measurements of soils under corn and soybean cultivation near the tower were much lower, suggesting that direct cropland N₂O emissions could not account for the top-down regional flux (11, 13). Stream emissions of N₂O resulting from nitrate leached from agricultural soils have been termed “indirect” emissions and may comprise a major source of agriculturally related N₂O (14). Thus, extremely high N₂O emissions from low-order and intermittent headwater streams were proposed to explain the apparent discrepancy (15).However, long-term N₂O measurements from elsewhere in the Corn Belt showed direct N₂O emissions (6.5 kg of N₂O–N ha⁻¹⋅y⁻¹) from upland agricultural soils under typical corn–soybean management (16, 17) that were similar to a top-down regional estimate (11). Importantly, these direct N₂O emissions were more than threefold greater than those estimated by the current accounting methods proposed by the Intergovernmental Panel on Climate Change (IPCC) (14, 17), whereby N₂O emissions are estimated as proportions of agroecosystem N inputs and N pools according to “emissions factors” (EFs) defined from literature syntheses (SI Appendix, Table S1). Resolving the controversy regarding the magnitude of direct N₂O emissions, along with their biophysical controls, is key for informing climate-mitigation policy in the Corn Belt and elsewhere. For example, if low-order and intermittent streams dominate regional emissions, N₂O mitigation could involve spatially targeted in-field or edge-of-field practices at terrestrial–aquatic interfaces to retain excess N or remove it via complete denitrification (18–20). Alternatively, if soils are the dominant N₂O source, effective mitigation would likely require widespread changes in field-scale agricultural practices, such as reduced fertilizer inputs (21).Addressing the sources and controls on agricultural N₂O emissions has been hampered by limited temporal and spatial grain of bottom-up measurements, which are often conducted on a weekly (or less frequent) basis and restricted to the crop-growing season (9, 10). Recent work employing automated high-frequency (subdaily) measurements revealed that extreme emissions events, such as spring freeze/thaw cycles or intense storms, may dominate annual emissions, but may be missed by manual sampling (22–25). Consequently, the coarse temporal resolution of most bottom-up N₂O datasets may bias annual emissions totals in unpredictable ways (26, 27). The few studies that assessed biases related to sampling frequency yielded conflicting results: Some suggested little difference between subdaily and weekly sampling (28, 29), whereas others found discrepancies of up to several kilograms of N₂O–N ha⁻¹⋅y⁻¹ (26, 27)—a bias exceeding total annual N₂O emissions of some agroecosystems (30).Apart from high temporal variation, N₂O emissions are also acutely sensitive to spatial variation in soil and hydrological characteristics (31, 32), which may have influenced discrepancies in direct N₂O emissions among previous Corn Belt studies. Soil moisture has long been known to regulate rates of microbial nitrification and denitrification and the proportion of N₂O produced by both processes (33). Much of the Corn Belt contains soils with poor drainage characteristics (Fig. 1), where subsurface drainage tile improves crop production (34). Even where drainage infrastructure is present, however, excess water frequently limits crop production during wet years, especially in temporarily ponded topographic depressions (35). Increased soil moisture and nutrient transport from the surrounding landscape may promote denitrification and N₂O emissions in low-lying areas (36, 37). However, the relationship between N₂O emissions and moisture is complex, as emissions may also be suppressed by soil saturation and because other biogeochemical characteristics that vary with topography may alter the magnitude or timing of emissions (38–40). Measurements at high spatiotemporal resolution are needed to test the hypothesis that low-lying areas contribute disproportionately to site and regional-scale N₂O emissions and whether failure to account for these features may help explain the proposed bottom-up/top-down discrepancies (37).Open in a separate windowFig. 1.Map of the Corn Belt with soils shaded by drainage class. Corn- and soybean-cultivated areas are shaded with color; red circles indicate study locations that were included in our literature synthesis. The white star indicates the location of our high-frequency chamber measurements. A map with smaller spatial extent to show finer-scale drainage features in the Central Iowa study region is presented in SI Appendix, Fig. S10.Here, to test whether direct soil emissions or poorly drained landscape features could explain high top-down regional N₂O emissions from the US Corn Belt, we used a custom automated chamber system tolerant of wet operating conditions (41) to measure N₂O fluxes at subdaily (4-h) resolution in a central Iowa field over 3 y (>20,000 individual N₂O flux measurements). Measurements spanned a >100-m topographic gradient of upland to lowland soils, which varied greatly in moisture dynamics and ponding. We also synthesized existing regional measurements from representative corn–soybean agricultural systems to address three questions:

• 1.Does the coarse sampling frequency of typical manual chamber measurements bias annual N₂O emissions from Corn Belt agroecosystems? The relatively infrequent sampling interval of manual chamber-based measurements may contribute to uncertainty in emission estimates by failing to accurately capture episodic emissions (23), and many studies have not captured the spring-thaw period (6, 25). We resampled our high-frequency chamber data to test how sampling frequency impacts cumulative emission estimates and quantified contributions from spring thaw.
• 2.Are topographic depressions, a common feature of poorly drained Corn Belt landscapes (42), a disproportionate and neglected source of N₂O? We measured N₂O fluxes across a topographic gradient spanning moderately well to very poorly drained (intermittently flooded) soils to test impacts of topography, moisture, and intermittent flooding on N₂O emissions. Hereafter, moderately well to very poorly drained soil drainage classes are referred to as drainage-impaired, as they are not strictly well-drained.
• 3.Have differences in ecosystem characteristics among study sites affected our understanding of direct emissions at regional scale? N₂O emissions can vary threefold across studies conducted in the Corn Belt with similar agricultural management and climate (9). Poor soil drainage has been linked to greater N₂O emissions globally (10). However, we are not aware of an explicit test of the role of soil-drainage characteristics in controlling regional Corn Belt N₂O emissions. We tested the relationship of climate, soil, and management characteristics with N₂O emissions using a compilation of published measurements.

We posit that the collective answer to these questions will provide the basis for improved understanding of the mechanisms and methods needed to accurately characterize Corn Belt N₂O emissions and inform their effective mitigation.     

Towards a better understanding and new therapeutics of osteopetrosis

Lack of or dysfunction in osteoclasts result in osteopetrosis, a group of rare but often severe, genetic disorders affecting skeletal tissue. Increase in bone mass results in skeletal malformation and bone marrow failure that may be fatal. Many of the underlying defects have lately been characterized in humans and in animal models of the disease. In humans, these defects often involve mutations in genes expressing proteins involved in the acidification of the osteoclast resorption compartment, a process necessary for proper bone degradation. So far, the only cure for children with severe osteopetrosis is allogeneic hematopoietic stem cell (HSC) transplantation but without a matching donor this form of therapy is far from optimal. The characterization of the genetic defects opens up the possibility for gene replacement therapy as an alternative. Accordingly, HSC-targeted gene therapy in a mouse model of infantile malignant osteopetrosis was recently shown to correct many aspects of the disease.