Showing papers on "Atmospheric methane published in 2018"

Quantifying methane point sources from fine-scale satellite observations of atmospheric methane plumes

Abstract: . Anthropogenic methane emissions originate from a large number of relatively small point sources. The planned GHGSat satellite fleet aims to quantify emissions from individual point sources by measuring methane column plumes over selected ∼ 10 × 10 km 2 domains with ≤ 50 × 50 m 2 pixel resolution and 1 %–5 % measurement precision. Here we develop algorithms for retrieving point source rates from such measurements. We simulate a large ensemble of instantaneous methane column plumes at 50×50 m 2 pixel resolution for a range of atmospheric conditions using the Weather Research and Forecasting model (WRF) in large eddy simulation (LES) mode and adding instrument noise. We show that standard methods to infer source rates by Gaussian plume inversion or source pixel mass balance are prone to large errors because the turbulence cannot be properly parameterized on the small scale of instantaneous methane plumes. The integrated mass enhancement (IME) method, which relates total plume mass to source rate, and the cross-sectional flux method, which infers source rate from fluxes across plume transects, are better adapted to the problem. We show that the IME method with local measurements of the 10 m wind speed can infer source rates with an error of 0.07–0.17 t h - 1 + 5 %–12 % depending on instrument precision (1 %–5 %). The cross-sectional flux method has slightly larger errors (0.07–0.26 t h - 1 + 8 %–12 %) but a simpler physical basis. For comparison, point sources larger than 0.3 t h −1 contribute more than 75 % of methane emissions reported to the US Greenhouse Gas Reporting Program. Additional error applies if local wind speed measurements are not available and may dominate the overall error at low wind speeds. Low winds are beneficial for source detection but detrimental for source quantification.

Unravelling the Identity, Metabolic Potential and Global Biogeography of the Atmospheric Methane‐Oxidizing Upland Soil Cluster α

Abstract: Understanding of global methane sources and sinks is a prerequisite for the design of strategies to counteract global warming. Microbial methane oxidation in soils represents the largest biological sink for atmospheric methane. However, still very little is known about the identity, metabolic properties and distribution of the microbial group proposed to be responsible for most of this uptake, the uncultivated upland soil cluster α (USCα). Here, we reconstructed a draft genome of USCα from a combination of targeted cell sorting and metagenomes from forest soil, providing the first insights into its metabolic potential and environmental adaptation strategies. The 16S rRNA gene sequence recovered was distinctive and suggests this crucial group as a new genus within the Beijerinckiaceae, close to Methylocapsa. Application of a fluorescently labelled suicide substrate for the particulate methane monooxygenase enzyme (pMMO) coupled to 16S rRNA fluorescence in situ hybridisation (FISH) allowed for the first time a direct link of the high-affinity activity of methane oxidation to USCα cells in situ. Analysis of the global biogeography of this group further revealed its presence in previously unrecognized habitats, such as subterranean and volcanic biofilm environments, indicating a potential role of these environments in the biological sink for atmospheric methane.

Declines in methane uptake in forest soils.

Abstract: Forest soils are a sink for atmospheric methane (CH4) and play an important role in modulating the global CH4 budget. However, whether CH4 uptake by forest soils is affected by global environmental change is unknown. We measured soil to atmosphere net CH4 fluxes in temperate forests at two long-term ecological research sites in the northeastern United States from the late 1990s to the mid-2010s. We found that annual soil CH4 uptake decreased by 62% and 53% in urban and rural forests in Baltimore, Maryland and by 74% and 89% in calcium-fertilized and reference forests at Hubbard Brook, New Hampshire over this period. This decrease occurred despite marked declines in nitrogen deposition and increases in atmospheric CH4 concentration and temperature, which should lead to increases in CH4 uptake. This decrease in soil CH4 uptake appears to be driven by increases in precipitation and soil hydrological flux. Furthermore, an analysis of CH4 uptake around the globe showed that CH4 uptake in forest soils has decreased by an average of 77% from 1988 to 2015, particularly in forests located from 0 to 60 °N latitude where precipitation has been increasing. We conclude that the soil CH4 sink may be declining and overestimated in several regions across the globe.

Nongrowing season methane emissions-a significant component of annual emissions across northern ecosystems.

Abstract: Wetlands are the single largest natural source of atmospheric methane (CH4 ), a greenhouse gas, and occur extensively in the northern hemisphere. Large discrepancies remain between "bottom-up" and "top-down" estimates of northern CH4 emissions. To explore whether these discrepancies are due to poor representation of nongrowing season CH4 emissions, we synthesized nongrowing season and annual CH4 flux measurements from temperate, boreal, and tundra wetlands and uplands. Median nongrowing season wetland emissions ranged from 0.9 g/m2 in bogs to 5.2 g/m2 in marshes and were dependent on moisture, vegetation, and permafrost. Annual wetland emissions ranged from 0.9 g m-2 year-1 in tundra bogs to 78 g m-2 year-1 in temperate marshes. Uplands varied from CH4 sinks to CH4 sources with a median annual flux of 0.0 ± 0.2 g m-2 year-1 . The measured fraction of annual CH4 emissions during the nongrowing season (observed: 13% to 47%) was significantly larger than that was predicted by two process-based model ensembles, especially between 40° and 60°N (modeled: 4% to 17%). Constraining the model ensembles with the measured nongrowing fraction increased total nongrowing season and annual CH4 emissions. Using this constraint, the modeled nongrowing season wetland CH4 flux from >40° north was 6.1 ± 1.5 Tg/year, three times greater than the nongrowing season emissions of the unconstrained model ensemble. The annual wetland CH4 flux was 37 ± 7 Tg/year from the data-constrained model ensemble, 25% larger than the unconstrained ensemble. Considering nongrowing season processes is critical for accurately estimating CH4 emissions from high-latitude ecosystems, and necessary for constraining the role of wetland emissions in a warming climate.

Attribution of recent increases in atmospheric methane through 3-D inverse modelling

Abstract: . The atmospheric methane ( CH4 ) growth rate has varied considerably in recent decades. Unexplained renewed growth after 2006 followed 7 years of stagnation and coincided with an isotopic trend toward CH4 more depleted in 13C , suggesting changes in sources and/or sinks. Using surface observations of both CH4 and the relative change of isotopologue ratio ( δ13CH4 ) to constrain a global 3-D chemical transport model (CTM), we have performed a synthesis inversion for source and sink attribution. Our method extends on previous studies by providing monthly and regional attribution of emissions from six different sectors and changes in atmospheric sinks for the extended 2003–2015 period. Regional evaluation of the model CH4 tracer with independent column observations from the Greenhouse Gases Observing Satellite (GOSAT) shows improved performance when using posterior fluxes ( R=0.94 –0.96, RMSE =8.3 –16.5 ppb), relative to prior fluxes ( R=0.60 –0.92, RMSE =48.6 –64.6 ppb). Further independent validation with data from the Total Carbon Column Observing Network (TCCON) shows a similar improvement in the posterior fluxes ( R=0.87 , RMSE =18.8 ppb) compared to the prior fluxes ( R=0.69 , RMSE =55.9 ppb). Based on these improved posterior fluxes, the inversion results suggest the most likely cause of the renewed methane growth is a post-2007 1.8±0.4 % decrease in mean OH, a 12.9±2.7 % increase in energy sector emissions, mainly from Africa–Middle East and southern Asia–Oceania, and a 2.6±1.8 % increase in wetland emissions, mainly from northern Eurasia. The posterior wetland flux increases are in general agreement with bottom-up estimates, but the energy sector growth is greater than estimated by bottom-up methods. The model results are consistent across a range of sensitivity analyses. When forced to assume a constant (annually repeating) OH distribution, the inversion requires a greater increase in energy sector ( 13.6±2.7 %) and wetland ( 3.6±1.8 %) emissions and an 11.5±3.8 % decrease in biomass burning emissions. Assuming no prior trend in sources and sinks slightly reduces the posterior growth rate in energy sector and wetland emissions and further increases the magnitude of the negative OH trend. We find that possible tropospheric Cl variations do not influence δ13CH4 and CH4 trends, although we suggest further work on Cl variability is required to fully diagnose this contribution. While the study provides quantitative insight into possible emissions variations which may explain the observed trends, uncertainty in prior source and sink estimates and a paucity of δ13CH4 observations limit the robustness of the posterior estimates.

Members of the Genus Methylobacter Are Inferred To Account for the Majority of Aerobic Methane Oxidation in Oxic Soils from a Freshwater Wetland

Abstract: Microbial carbon degradation and methanogenesis in wetland soils generate a large proportion of atmospheric methane, a highly potent greenhouse gas. Despite their potential to mitigate greenhouse gas emissions, knowledge about methane-consuming methanotrophs is often limited to lower-resolution single-gene surveys that fail to capture the taxonomic and metabolic diversity of these microorganisms in soils. Here our objective was to use genome-enabled approaches to investigate methanotroph membership, distribution, and in situ activity across spatial and seasonal gradients in a freshwater wetland near Lake Erie. 16S rRNA gene analyses demonstrated that members of the methanotrophic Methylococcales were dominant, with the dominance largely driven by the relative abundance of four taxa, and enriched in oxic surface soils. Three methanotroph genomes from assembled soil metagenomes were assigned to the genus Methylobacter and represented the most abundant methanotrophs across the wetland. Paired metatranscriptomes confirmed that these Old Woman Creek (OWC) Methylobacter members accounted for nearly all the aerobic methanotrophic activity across two seasons. In addition to having the capacity to couple methane oxidation to aerobic respiration, these new genomes encoded denitrification potential that may sustain energy generation in soils with lower dissolved oxygen concentrations. We further show that Methylobacter members that were closely related to the OWC members were present in many other high-methane-emitting freshwater and soil sites, suggesting that this lineage could participate in methane consumption in analogous ecosystems. This work contributes to the growing body of research suggesting that Methylobacter may represent critical mediators of methane fluxes in freshwater saturated sediments and soils worldwide.IMPORTANCE Here we used soil metagenomics and metatranscriptomics to uncover novel members within the genus Methylobacter We denote these closely related genomes as members of the lineage OWC Methylobacter Despite the incredibly high microbial diversity in soils, here we present findings that unexpectedly showed that methane cycling was primarily mediated by a single genus for both methane production ("Candidatus Methanothrix paradoxum") and methane consumption (OWC Methylobacter). Metatranscriptomic analyses revealed that decreased methanotrophic activity rather than increased methanogenic activity possibly contributed to the greater methane emissions that we had previously observed in summer months, findings important for biogeochemical methane models. Although members of this Methylococcales order have been cultivated for decades, multi-omic approaches continue to illuminate the methanotroph phylogenetic and metabolic diversity harbored in terrestrial and marine ecosystems.

Variability in Atmospheric Methane From Fossil Fuel and Microbial Sources Over the Last Three Decades

Abstract: Atmospheric measurements show an increase in CH4 from the 1980s to 1998 followed by a period of near-zero growth until 2007. However, from 2007, CH4 has increased again. Understanding the variability in CH4 is critical for climate prediction and climate change mitigation. We examine the role of CH4 sources and the dominant CH4 sink, oxidation by the hydroxyl radical (OH), in atmospheric CH4 variability over the past three decades using observations of CH4, C2H6, and δ CCH4 in an inversion. From 2006 to 2014, microbial and fossil fuel emissions increased by 36 ± 12 and 15 ± 8 Tg y , respectively. Emission increases were partially offset by a decrease in biomass burning of 3 ± 2 Tg y 1 and increase in soil oxidation of 5 ± 6 Tg y . A change in the atmospheric sink did not appear to be a significant factor in the recent growth of CH4. Plain Language Summary Methane is the second most important greenhouse gas and is responsible for approximately 17% of the direct radiative forcing from all long-lived greenhouse gases. Observations of methane in the atmosphere have shown a dramatic increase from 2007 after a period of relative stability between the late 1990s and early 2000s, but the cause of this increase is still under scientific debate. This study uses atmospheric observations of methane and two related tracers, the isotopic ratio of carbon in methane and ethane, to constrain the sources and sinks of methane over the past three decades. The increase in methane between 2007 and 2014 is likely due to an increase in microbial sources, of 24–48 Tg/y (predominantly natural wetlands and agricultural), as well as fossil fuel sources, of 7–23 Tg/y. In contrast to other recent studies, a reduction in the atmospheric sink of methane was found not to be a significant factor in explaining the recent atmospheric increase.

Extensive loss of past permafrost carbon but a net accumulation into present-day soils.

Abstract: Atmospheric concentrations of carbon dioxide increased between the Last Glacial Maximum (LGM, around 21,000 years ago) and the preindustrial era1. It is thought that the evolution of this atmospheric carbon dioxide (and that of atmospheric methane) during the glacial-to-interglacial transition was influenced by organic carbon that was stored in permafrost during the LGM and then underwent decomposition and release following thaw2,3. It has also been suggested that the rather erratic atmospheric δ13C and ∆14C signals seen during deglaciation1,4 could partly be explained by the presence of a large terrestrial inert LGM carbon stock, despite the biosphere being less productive (and therefore storing less carbon)5,6. Here we present an empirically derived estimate of the carbon stored in permafrost during the LGM by reconstructing the extent and carbon content of LGM biomes, peatland regions and deep sedimentary deposits. We find that the total estimated soil carbon stock for the LGM northern permafrost region is smaller than the estimated present-day storage (in both permafrost and non-permafrost soils) for the same region. A substantial decrease in the permafrost area from the LGM to the present day has been accompanied by a roughly 400-petagram increase in the total soil carbon stock. This increase in soil carbon suggests that permafrost carbon has made no net contribution to the atmospheric carbon pool since the LGM. However, our results also indicate potential postglacial reductions in the portion of the carbon stock that is trapped in permafrost, of around 1,000 petagrams, supporting earlier studies7. We further find that carbon has shifted from being primarily stored in permafrost mineral soils and loess deposits during the LGM, to being roughly equally divided between peatlands, mineral soils and permafrost loess deposits today. Comparing the northern permafrost region of the Last Glacial Maximum with the same area today shows that the soil carbon stock has now increased, suggesting that permafrost carbon made no net contribution to preindustrial atmospheric carbon.

Methane and global environmental change

Abstract: Global atmospheric methane concentrations have continued to rise in recent years, having already more than doubled since the Industrial Revolution. Further environmental change, especially climate change, in the twenty-first century has the potential to radically alter global methane fluxes. Importantly, changes in temperature, precipitation, and net primary production may induce positive climate feedback effects in dominant natural methane sources such as wetlands, soils, and aquatic ecosystems. Anthropogenic methane sources may also be impacted, with a risk of enhanced emissions from the energy, agriculture, and waste sectors. Here, we review the global sources of methane, the trends in fluxes by source and sector, and their possible evolution in response to future environmental change. We discuss ongoing uncertainties in flux estimation and projection, and highlight the great potential for multisector methane mitigation as part of wider global climate change policy.

Improving Characterization of Anthropogenic Methane Emissions in the United States

Abstract: Understanding, quantifying, and tracking atmospheric methane and emissions is essential for addressing concerns and informing decisions that affect the climate, economy, and human health and safety. Atmospheric methane is a potent greenhouse gas (GHG) that contributes to global warming. While carbon dioxide is by far the dominant cause of the rise in global average temperatures, methane also plays a significant role because it absorbs more energy per unit mass than carbon dioxide does, giving it a disproportionately large effect on global radiative forcing. In addition to contributing to climate change, methane also affects human health as a precursor to ozone pollution in the lower atmosphere.Improving Characterization of Anthropogenic Methane Emissions in the United States summarizes the current state of understanding of methane emissions sources and the measurement approaches and evaluates opportunities for methodological and inventory development improvements. This report will inform future research agendas of various U.S. agencies, including NOAA, the EPA, the DOE, NASA, the U.S. Department of Agriculture (USDA), and the National Science Foundation (NSF).

Enhanced response of global wetland methane emissions to the 2015-2016 El Niño-Southern Oscillation event

Abstract: Wetlands are thought to be the major contributor to interannual variability in the growth rate of atmospheric methane (CH4) with anomalies driven by the influence of the El Nino-Southern Oscillation (ENSO). Yet it remains unclear whether (i) the increase in total global CH4 emissions during El Nino versus La Nina events is from wetlands and (ii) how large the contribution of wetland CH4 emissions is to the interannual variability of atmospheric CH4. We used a terrestrial ecosystem model that includes permafrost and wetland dynamics to estimate CH4 emissions, forced by three separate meteorological reanalyses and one gridded observational climate dataset, to simulate the spatio-temporal dynamics of wetland CH4 emissions from 1980-2016. The simulations show that while wetland CH4 responds with negative annual anomalies during the El Nino events, the instantaneous growth rate of wetland CH4 emissions exhibits complex phase dynamics. We find that wetland CH4 instantaneous growth rates were declined at the onset of the 2015-2016 El Nino event but then increased to a record-high at later stages of the El Nino event (January through May 2016). We also find evidence for a step increase of CH4 emissions by 7.8+/-1.6 Tg CH4 per yr during 2007-2014 compared to the average of 2000-2006 from simulations using meteorological reanalyses, which is equivalent to a approx.3.5 ppb per yr rise in CH4 concentrations. The step increase is mainly caused by the expansion of wetland area in the tropics (30 deg S-30 deg N) due to an enhancement of tropical precipitation as indicated by the suite of the meteorological reanalyses. Our study highlights the role of wetlands, and the complex temporal phasing with ENSO, in driving the variability and trends of atmospheric CH4 concentrations. In addition, the need to account for uncertainty in meteorological forcings is highlighted in addressing the interannual variability and decadal-scale trends of wetland CH4 fluxes.

Sediment Characteristics and Methane Ebullition in Three Subarctic Lakes

Abstract: Ebullition (bubbling) from climate-sensitive northern lakes remains an unconstrained source of atmospheric methane (CH4) Although the focus of many recent studies, ebullition is rarely linked to t

Observationally derived rise in methane surface forcing mediated by water vapour trends

Abstract: Atmospheric methane (CH4) mixing ratios exhibited a plateau between 1995 and 2006 and have subsequently been increasing. While there are a number of competing explanations for the temporal evolution of this greenhouse gas, these prominent features in the temporal trajectory of atmospheric CH4 are expected to perturb the surface energy balance through radiative forcing, largely due to the infrared radiative absorption features of CH4. However, to date this has been determined strictly through radiative transfer calculations. Here, we present a quantified observation of the time series of clear-sky radiative forcing by CH4 at the surface from 2002 to 2012 at a single site derived from spectroscopic measurements along with line-by-line calculations using ancillary data. There was no significant trend in CH4 forcing between 2002 and 2006, but since then, the trend in forcing was 0.026 ± 0.006 (99.7% CI) W m2 yr−1. The seasonal-cycle amplitude and secular trends in observed forcing are influenced by a corresponding seasonal cycle and trend in atmospheric CH4. However, we find that we must account for the overlapping absorption effects of atmospheric water vapour (H2O) and CH4 to explain the observations fully. Thus, the determination of CH4 radiative forcing requires accurate observations of both the spatiotemporal distribution of CH4 and the vertically resolved trends in H2O. Observations of the radiative forcing from methane at the Earth’s surface are influenced by absorption effects from water vapour, according to spectroscopic measurements and line-by-line calculations.

Long-term nutrient addition increased CH4 emission from a bog through direct and indirect effects.

Abstract: Peatlands are globally significant sources of atmospheric methane (CH4). While several studies have examined the effects of nutrient addition on CH4 dynamics, there are few long-term peatland fertilization experiments, which are needed to understand the aggregated effects of nutrient deposition on ecosystem functioning. We investigated responses of CH4 flux and production to long-term field treatments with three levels of N (1.6–6.4 g m−2 yr−1 as NH4NO3), potassium and phosphorus (PK, 5.0 g P and 6.3 g K m−2 yr−1 as KH2PO4), and NPK in a temperate bog. Methane fluxes were measured in the field from May to August in 2005 and 2015. In 2015 CH4 flux was higher in the NPK treatment with 16 years of 6.4 g N m−2 yr−1 than in the control (50.5 vs. 8.6 mg CH4 m−2 d−1). The increase in CH4 flux was associated with wetter conditions derived from peat subsidence. Incubation of peat samples, with and without short-term PK amendment, showed that potential CH4 production was enhanced in the PK treatments, both from field application and by amending the incubation. We suggest that changes in this bog ecosystem originate from long-term vegetation change, increased decomposition and direct nutrient effects on microbial dynamics.

2010–2016 methane trends over Canada, the United States, and Mexico observed by the GOSAT satellite: contributions from different source sectors

Abstract: . We use 7 years (2010–2016) of methane column observations from the Greenhouse Gases Observing Satellite (GOSAT) to examine trends in atmospheric methane concentrations over North America and infer trends in emissions. Local methane enhancements above background are diagnosed in the GOSAT data on a 0.5 ∘ × 0.5 ∘ grid by estimating the local background as the low (10th–25th) percentiles of the deseasonalized frequency distributions of the data for individual years. Trends in methane enhancements on the 0.5 ∘ × 0.5 ∘ grid are then aggregated nationally and for individual source sectors, using information from state-of-science bottom-up inventories. We find that US methane emissions increased by 2.5±1.4 % a −1 (mean ± 1 standard deviation) over the 7-year period, with contributions from both oil–gas systems (possibly unconventional oil–gas production) and from livestock in the Midwest (possibly swine manure management). Mexican emissions show a decrease that can be attributed to a decreasing cattle population. Canadian emissions show year-to-year variability driven by wetland emissions and correlated with wetland areal extent. The US emission trends inferred from the GOSAT data account for about 20 % of the observed increase in global methane over the 2010–2016 period.

Bipolar carbon and hydrogen isotope constraints on the Holocene methane budget

Abstract: . Atmospheric methane concentration shows a well-known decrease over the first half of the Holocene following the Northern Hemisphere summer insolation before it started to increase again to preindustrial values. There is a debate about what caused this change in the methane concentration evolution, in particular, whether an early anthropogenic influence or natural emissions led to the reversal of the atmospheric CH4 concentration evolution. Here, we present new methane concentration and stable hydrogen and carbon isotope data measured on ice core samples from both Greenland and Antarctica over the Holocene. With the help of a two-box model and the full suite of CH4 parameters, the new data allow us to quantify the total methane emissions in the Northern Hemisphere and Southern Hemisphere separately as well as their stable isotopic signatures, while interpretation of isotopic records of only one hemisphere may lead to erroneous conclusions. For the first half of the Holocene our results indicate an asynchronous decrease in Northern Hemisphere and Southern Hemisphere CH4 emissions by more than 30 Tg CH4 yr −1 in total, accompanied by a drop in the northern carbon isotopic source signature of about −3 ‰. This cannot be explained by a change in the source mix alone but requires shifts in the isotopic signature of the sources themselves caused by changes in the precursor material for the methane production. In the second half of the Holocene, global CH4 emissions increased by about 30 Tg CH4 yr −1 , while preindustrial isotopic emission signatures remained more or less constant. However, our results show that this early increase in methane emissions took place in the Southern Hemisphere, while Northern Hemisphere emissions started to increase only about 2000 years ago. Accordingly, natural emissions in the southern tropics appear to be the main cause of the CH4 increase starting 5000 years before present, not supporting an early anthropogenic influence on the global methane budget by East Asian land use changes.

Rhizosphere activity and atmospheric methane concentrations drive variations of methane fluxes in a temperate forest soil

Abstract: Aerated soils represent an important sink for atmospheric methane (CH4), due to the effect of methanotrophic bacteria, thus mitigating current atmospheric CH4 increases. Whilst rates of CH4 oxidation have been linked to types of vegetation cover, there has been no systematic investigation of the interaction between plants and soil in relation to the strength of the soil CH4 sink. We used quasi-continuous automated chamber measurements of soil CH4 and CO2 flux from soil collar treatments that selectively include root and ectomycorrhizal (ECM) mycelium to investigate the role of rhizosphere activity as well as the effects of other environmental drivers on CH4 uptake in a temperate coniferous forest soil. We also assessed the potential impact of measurement bias from sporadic chamber measurements in altering estimates of soil CO2 efflux and CH4 uptake. Results show a clear effect of the presence of live roots and ECM mycelium on soil CO2 efflux and CH4 uptake. The presence of ECM hyphae alone (without plant roots) showed intermediate fluxes of both CO2 and CH4 relative to soils that either contained roots and ECM mycelium, or soil lacking root- and ECM mycelium. Regression analysis confirmed a significant influence of soil moisture as well as temperature on flux dynamics of both CH4 and CO2 flux. We further found a surprising increase in soil CH4 uptake during the night, and discuss diurnal fluctuations in atmospheric CH4 (with higher concentrations during stable atmospheric conditions at night) as a potential driver of CH4 oxidation rates. Using the high temporal resolution of our data set, we show that low-frequency sampling results in systematic bias of up-scaled flux estimates, resulting in under-estimates of up to 20% at our study site, due to fluctuations in flux dynamics on diurnal as well as longer time scales.

Spatial–temporal patterns of methane dynamics in Lake Taihu

Abstract: Lakes are important source of atmospheric methane (CH4). Especially for large and shallow lakes, accurate CH4 emission budgets must consider the spatial and seasonal heterogeneity which characterize these ecosystems. CH4 concentrations in sediment and lake water and CH4 fluxes from sediment were analysed in selected open water areas of Lake Taihu, the third largest freshwater lake in China, to test the hypothesis that CH4 concentrations and fluxes differ both spatially and temporally in relation to biological and environmental features. The results showed that Lake Taihu was a source of CH4 throughout the year. The observed CH4 concentrations and fluxes were higher in the north-western and south-eastern areas of the lake, and lower in the central area. This spatial heterogeneity in methane emissions was mainly related to local organic matter content, plant characteristics, deposition rate and sediment features. Seasonally, CH4 showed stronger emissions in summer. These results suggest that the analysis of CH4 dynamics in shallow lakes should consider the habitat heterogeneity and the different sources of organic matter. Furthermore, the CH4 emission budget should at least include high and low temperature periods to decrease temporal bias.

Monitoring Global Tropospheric OH Concentrations using Satellite Observations of Atmospheric Methane

Abstract: Abstract. The hydroxyl radical (OH) is the main tropospheric oxidant and is the largest sink for atmospheric methane. The global abundance of OH has been monitored for the past decades with the methyl chloroform (CH3CCl3) proxy. This approach is becoming ineffective as atmospheric CH3CCl3 concentrations decline. Here we propose that satellite observations of atmospheric methane in the shortwave infrared (SWIR) and thermal infrared (TIR) can provide an effective replacement method. The premise is that the atmospheric signature of the methane sink from oxidation by OH is distinct from that of methane emissions. We evaluate this method in an observing system simulation experiment (OSSE) framework using synthetic SWIR and TIR satellite observations representative of the TROPOMI and CrIS instruments, respectively. The synthetic observations are interpreted with a Bayesian inverse analysis optimizing both gridded methane emissions and global OH concentrations with detailed error accounting, including errors in meteorological fields and in OH distributions. We find that the satellite observations can constrain the global tropospheric OH concentrations with a precision better than 1 % and an accuracy of about 3 % for SWIR and 7 % for TIR. The inversion can successfully separate contributions from methane emissions and OH concentrations to the methane budget and its trend. We also show that satellite methane observations can constrain the interhemispheric difference in OH. The main limitation to the accuracy is uncertainty in the spatial and seasonal distribution of OH.

Small spatial variability in methane emission measured from a wet patterned boreal bog

Abstract: . We measured methane fluxes of a patterned bog situated in Siikaneva in southern Finland from six different plant community types in three growing seasons (2012–2014) using the static chamber method with chamber exposure of 35 min. A mixed-effects model was applied to quantify the effect of the controlling factors on the methane flux. The plant community types differed from each other in their water level, species composition, total leaf area (LAI TOT) and leaf area of aerenchymatous plant species (LAI AER) . Methane emissions ranged from − 309 to 1254 mg m −2 d −1 . Although methane fluxes increased with increasing peat temperature, LAI TOT and LAI AER , they had no correlation with water table or with plant community type. The only exception was higher fluxes from hummocks and high lawns than from high hummocks and bare peat surfaces in 2013 and from bare peat surfaces than from high hummocks in 2014. Chamber fluxes upscaled to ecosystem level for the peak season were of the same magnitude as the fluxes measured with the eddy covariance (EC) technique. In 2012 and in August 2014 there was a good agreement between the two methods; in 2013 and in July 2014, the chamber fluxes were higher than the EC fluxes. Net fluxes to soil, indicating higher methane oxidation than production, were detected every year and in all community types. Our results underline the importance of both LAI AER and LAI TOT in controlling methane fluxes and indicate the need for automatized chambers to reliably capture localized events to support the more robust EC method.

Molecule mapping of HR8799b using OSIRIS on Keck. Strong detection of water and carbon monoxide, but no methane

Abstract: Context. In 2015, Barman et al. (ApJ, 804, 61) presented detections of absorption from water, carbon monoxide, and methane in the atmosphere of the directly imaged exoplanet HR8799b using integral field spectroscopy (IFS) with OSIRIS on the Keck II telescope. We recently devised a new method to analyse IFU data, called molecule mapping, searching for high-frequency signatures of particular molecules in an IFU data cube.Aims. The aim of this paper is to use the molecule mapping technique to search for the previously detected spectral signatures in HR8799b using the same data, allowing a comparison of molecule mapping with previous methods.Methods. The medium-resolution H - and K -band pipeline-reduced archival data were retrieved from the Keck archive facility. Telluric and stellar lines were removed from each spectrum in the data cube, after which the residuals were cross-correlated with model spectra of carbon monoxide, water, and methane.Results. Both carbon monoxide and water are clearly detected at high signal-to-noise, however, methane is not retrieved.Conclusions. Molecule mapping works very well on the OSIRIS data of exoplanet HR8799b. However, it is not evident why methane is detected in the original analysis, but not with the molecule mapping technique. Possible causes could be the presence of telluric residuals, different spectral filtering techniques, or the use of different methane models. We do note that in the original analysis methane was only detected in the K -band, while the H -band methane signal could be expected to be comparably strong. More sensitive observations with the JWST will be capable of confirming or disproving the presence of methane in this planet at high confidence.

Error Budget of the MEthane Remote LIdar missioN and Its Impact on the Uncertainties of the Global Methane Budget

Abstract: MEthane Remote LIdar missioN (MERLIN) is a German-French space mission, scheduled for launch in 2024 and built around an innovative light detecting and ranging instrument that will retrieve methane atmospheric weighted columns MERLIN products will be assimilated into chemistry transport models to infer methane emissions and sinks Here the expected performance of MERLIN to reduce uncertainties on methane emissions is estimated A first complete error budget of the mission is proposed based on an analysis of the plausible causes of random and systematic errors Systematic errors are spatially and temporally distributed on geophysical variables and then aggregated into an ensemble of 32 scenarios Observing System Simulation Experiments are conducted, originally carrying both random and systematic errors Although relatively small (±29 ppb), systematic errors are found to have a larger influence on MERLIN performances than random errors The expected global mean uncertainty reduction on methane emissions compared to the prior knowledge is found to be 32%, limited by the impact of systematic errors The uncertainty reduction over land reaches 60% when the largest desert regions are removed At the latitudinal scale, the largest uncertainty reductions are achieved for temperate regions (84%) and then tropics (56%) and high latitudes (53%) Similar Observing System Simulation Experiments based on error scenarios for Greenhouse Gases Observing SATellite reveal that MERLIN should perform better than Greenhouse Gases Observing SATellite for most continental regions The integration of error scenarios for MERLIN in another inversion system suggests similar results, albeit more optimistic in terms of uncertainty reduction