Showing papers on "Atmospheric methane published in 2023"

Understanding the potential of Sentinel-2 for monitoring methane point emissions

Abstract: Abstract. The use of satellite instruments to detect and quantify methane emissions from fossil fuel production activities is highly beneficial to support climate change mitigation. Different hyperspectral and multispectral satellite sensors have recently shown potential to detect and quantify point-source emissions from space. The Sentinel-2 (S2) mission, despite its limited spectral design, supports the detection of large emissions with global coverage and high revisit frequency thanks to coarse spectral coverage of methane absorption lines in the shortwave infrared. Validation of S2 methane retrieval algorithms is instrumental in accelerating the development of a systematic and global monitoring system for methane point sources. Here, we develop a benchmarking framework for such validation. We first develop a methodology to generate simulated S2 datasets including methane point-source plumes. These benchmark datasets have been created for scenes in three oil and gas basins (Hassi Messaoud, Algeria; Korpeje, Turkmenistan; Permian Basin, USA) under different scene heterogeneity conditions and for simulated methane plumes with different spatial distributions. We use the simulated methane plumes to validate the retrieval for different flux rate levels and define a minimum detection threshold for each case study. The results suggest that for homogeneous and temporally invariant surfaces, the detection limit of the proposed S2 methane retrieval ranges from 1000 to 2000 kg h−1, whereas for areas with large surface heterogeneity and temporal variations, the retrieval can only detect plumes in excess of 500 kg h−1. The different sources of uncertainty in the flux rate estimates have also been examined. Dominant quantification errors are either wind-related or plume mask-related, depending on the surface type. Uncertainty in wind speed, both in the 10 m wind (U10) and in mapping U10 to the effective wind (Ueff) driving plume transport, is the dominant source of error for quantifying individual plumes in homogeneous scenes. For heterogeneous and temporally variant scenes, the surface structure underlying the methane plume affects the plume masking and can become a dominant source of uncertainty.

A high-resolution satellite-based map of global methane emissions reveals missing wetland, fossil fuel, and monsoon sources

Abstract: Abstract. We interpret space-borne observations from the TROPOspheric Monitoring Instrument (TROPOMI) in a multi-inversion framework to characterize the 2018–2019 global methane budget. Evaluation of the inverse solutions indicates that simultaneous source + sink optimization using methane observations alone remains an ill-posed problem – even with the dense TROPOMI sampling coverage. Employing remote carbon monoxide (CO) and hydroxyl radical (OH) observations with independent methane measurements to distinguish between candidate solutions, we infer from TROPOMI a global methane source of 587 (586–589) Tg yr−1 and sink of 571 Tg yr−1 for our analysis period. We apply a new downscaling method to map the derived monthly emissions to 0.1∘ × 0.1∘ resolution, using the results to uncover key gaps in the prior methane budget. The TROPOMI data point to an underestimate of tropical wetland emissions (a posteriori increase of +13 % [6 %–25 %] or 20 [7–25] Tg yr−1), with adjustments following regional hydrology. Some simple wetland parameterizations represent these patterns as accurately as more sophisticated process-based models. Emissions from fossil fuel activities are strongly underestimated over the Middle East (+5 [2–6] Tg yr−1 a posteriori increase) and over Venezuela. The TROPOMI observations also reveal many fossil fuel emission hotspots missing from the prior inventory, including over Mexico, Oman, Yemen, Turkmenistan, Iran, Iraq, Libya, and Algeria. Agricultural methane sources are underestimated in India, Brazil, the California Central Valley, and Asia. Overall, anthropogenic sources worldwide are increased by +19 [11–31] Tg yr−1 over the prior estimate. More than 45 % of this adjustment occurs over India and Southeast Asia during the summer monsoon (+8.5 [3.1–10.7] Tg in July–October), likely due to rainfall-enhanced emissions from rice, manure, and landfills/sewers, which increase during this season along with the natural wetland source.

Observation-derived 2010-2019 trends in methane emissions and intensities from US oil and gas fields tied to activity metrics

Abstract: Significance The United States accounts for a large share of global methane emissions from the oil/gas industry. Analysis of satellite and surface observations of atmospheric methane reveals larger-than-reported year-to-year variability of 2010 to 2019 US oil/gas methane emissions. This variability reflects trends in oil/gas production rates, number of active wells, and drilling of new wells. Emissions surged after 2017 as production increased. The methane intensity from the US oil/gas industry (methane emitted per unit methane gas produced) decreased steadily after 2010. Extension of this decreasing trend to 2030 (target date of the Global Methane Pledge) would result in a 32% decrease in US oil/gas methane emissions and 15% decrease in total anthropogenic emissions relative to 2019 despite an increase in production.

Evaluation of the methane paradox in four adjacent pre-alpine lakes across a trophic gradient

Abstract: Contrasting the paradigm that methane is only produced in anoxic conditions, recent discoveries show that oxic methane production (OMP, aka the methane paradox) occurs in oxygenated surface waters worldwide. OMP drivers and their contribution to global methane emissions, however, are not well constrained. In four adjacent pre-alpine lakes, we determine the net methane production rates in oxic surface waters using two mass balance approaches, accounting for methane sources and sinks. We find that OMP occurs in three out of four studied lakes, often as the dominant source of diffusive methane emissions. Correlations of net methane production versus chlorophyll-a, Secchi and surface mixed layer depths suggest a link with photosynthesis and provides an empirical upscaling approach. As OMP is a methane source in direct contact with the atmosphere, a better understanding of its extent and drivers is necessary to constrain the atmospheric methane contribution by inland waters.

Reconciling the bottom-up and top-down estimates of the methane chemical sink using multiple observations

Abstract: Abstract. The methane chemical sink estimated by atmospheric chemistry models (bottom-up method) is significantly larger than estimates based on methyl chloroform (MCF) inversions (top-down method). The difference is partly attributable to large uncertainties in hydroxyl radical (OH) concentrations simulated by the atmospheric chemistry models used to derive the bottom-up estimates. In this study, we propose a new approach based on OH precursor observations and a chemical box model. This approach contributes to improving the 3D distributions of tropospheric OH radicals obtained from atmospheric chemistry models and reconciling bottom-up and top-down estimates of the chemical loss of atmospheric methane. By constraining simulated OH precursors with observations, the global mean tropospheric column-averaged air-mass-weighted OH concentration ([OH]trop-M) is ∼10×105 molec. cm−3 (which is 2×105 molec. cm−3 lower than the original model-simulated global [OH]trop-M) and agrees with that obtained by the top-down method based on MCF inversions. With OH constrained by precursor observations, the methane chemical loss is 471–508 Tg yr−1, averaged from 2000 to 2009. The new adjusted estimate is in the range of the latest top-down estimate of the Global Carbon Project (GCP) (459–516 Tg yr−1), contrary to the bottom-up estimates that use the original model-simulated OH fields (577–612 Tg yr−1). The overestimation of global [OH]trop-M and methane chemical loss simulated by the atmospheric chemistry models is caused primarily by the models' underestimation of carbon monoxide and total ozone column, and overestimation of nitrogen dioxide. Our results highlight that constraining the model-simulated OH fields with available OH precursor observations can help improve bottom-up estimates of the global methane sink.

Investigating high methane emissions from urban areas detected by TROPOMI and their association with untreated wastewater

Abstract: Even though methane concentrations have contributed an estimated 23% of climate forcing, part of the recent increases in the global methane background concentrations remain unexplained. Satellite remote sensing has been used extensively to constrain emission inventories, for example with the TROPOspheric Monitoring Instrument which has been measuring methane since November 2017. We have identified enhancements of methane over 61 urban areas around the world and estimate their emissions using a two-dimensional Gaussian model. We show that methane emissions from urban areas may be underestimated by a factor of 3–4 in the Emissions Database for Global Atmospheric Research (EDGAR) greenhouse gas emission inventory. Scaling our results to the 385 urban areas with more than 2 million inhabitants suggests that they could account for up to 22% of global methane emissions. The emission estimates of the 61 urban areas do not correlate with the total or sectoral EDGAR emission inventory. They do however correlate with estimated rates of untreated wastewater, varying from 33 kg person−1 year−1 for cities with zero untreated wastewater to 138 kg person−1 year−1 for the cities with the most untreated wastewater. If this relationship were confirmed by higher resolution remote sensing or in situ monitoring, we estimate that reducing discharges of untreated wastewater could reduce global methane emissions by up to 5%–10% while at the same time yielding significant ecological and human co-benefits.

Quantification of oil and gas methane emissions in the Delaware and Marcellus basins using a network of continuous tower-based measurements

Abstract: Abstract. According to the United States Environmental Protection Agency (US EPA), emissions from oil and gas infrastructure contribute 30 % of all anthropogenic methane (CH4) emissions in the US. Studies in the last decade have shown emissions from this sector to be substantially larger than bottom-up assessments, including the EPA inventory, highlighting both the increased importance of methane emissions from the oil and gas sector in terms of their overall climatological impact and the need for independent monitoring of these emissions. In this study we present continuous monitoring of regional methane emissions from two oil and gas basins using tower-based observing networks. Continuous methane measurements were taken at four tower sites in the northeastern Marcellus basin from May 2015 through December 2016 and five tower sites in the Delaware basin in the western Permian from March 2020 through April 2022. These measurements, an atmospheric transport model, and prior emission fields are combined using an atmospheric inversion to estimate monthly methane emissions in the two regions. This study finds the mean overall emission rate from the Delaware basin during the measurement period to be 146–210 Mg CH4 h−1 (energy-normalized loss rate of 1.1 %–1.5 %, gas-normalized rate of 2.5 %–3.5 %). Strong temporal variability in the emissions was present, with the lowest emission rates occurring during the onset of the COVID-19 pandemic. Additionally, a synthetic model–data experiment performed using the Delaware tower network shows that the presence of intermittent sources is not a significant source of uncertainty in monthly quantification of the mean emission rate. In the Marcellus, this study finds the overall mean emission rate to be 19–28 Mg CH4 h−1 (gas-normalized loss rate of 0.30 %–0.45 %), with relative consistency in the emission rate over time. These totals align with aircraft top-down estimates from the same time periods. In both basins, the tower network was able to constrain monthly flux estimates within ±20 % uncertainty in the Delaware and ±24 % uncertainty in the Marcellus. The results from this study demonstrate the ability to monitor emissions continuously and detect changes in the emissions field, even in a basin with relatively low emissions and complex background conditions.

The role of rice cultivation in changes in atmospheric methane concentration and the Global Methane Pledge

Abstract: Resumption of the increase in atmospheric methane (CH4) concentrations since 2007 is of global concern and may partly have resulted from emissions from rice cultivation. Estimates of CH4 emissions from rice fields and abatement potential are essential to assess the contribution of improved rice management in achieving the targets of the Global Methane Pledge agreed upon by over 100 countries at COP26. However, the contribution of CH4 emissions from rice fields to the resumed CH4 growth and the global abatement potential remains unclear. In this study, we estimated the global CH4 emissions from rice fields to be 27 ± 6 Tg CH4 year−1 in the recent decade (2008–2017) based on the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. The trend of CH4 emissions from rice cultivation showed an increase followed by no significant change and then, a stabilization over 1990–2020. Consequently, the contribution of CH4 emissions from rice fields to the renewed increase in atmospheric CH4 concentrations since 2007 was minor. We summarized the existing low‐cost measures and showed that improved water and straw management could reduce one‐third of global CH4 emissions from rice fields. Straw returned as biochar could reduce CH4 emissions by 12 Tg CH4 year−1, equivalent to 10% of the total reduction of all anthropogenic emissions. We conclude that other sectors than rice cultivation must have contributed to the renewed increase in atmospheric CH4 concentrations, and that optimizing multiple mitigation measures in rice fields could contribute significantly to the abatement goal outlined in the Global Methane Pledge.

Methane emissions are predominantly responsible for record-breaking atmospheric methane growth rates in 2020 and 2021

Abstract: Abstract. The global atmospheric methane growth rates reported by NOAA for 2020 and 2021 are the largest since systematic measurements began in 1983. To explore the underlying reasons for these anomalous growth rates, we use newly available methane data from the Japanese Greenhouse gases Observing SATellite (GOSAT) to estimate methane surface emissions. Relative to baseline values in 2019, we find that a significant global increase in methane emissions of 27.0 ± 11.3 and 20.8 ± 11.4 Tg is needed to reproduce observed atmospheric methane in 2020 and 2021, respectively, assuming fixed climatological values for OH. We see the largest annual increases in methane emissions during 2020 over Eastern Africa (14 ± 3 Tg), tropical Asia (3 ± 4 Tg), tropical South America (5 ± 4 Tg), and temperate Eurasia (3 ± 3 Tg), and the largest reductions are observed over China (−6 ± 3 Tg) and India (−2 ± 3 Tg). We find comparable emission changes in 2021, relative to 2019, except for tropical and temperate South America where emissions increased by 9 ± 4 and 4 ± 3 Tg, respectively, and for temperate North America where emissions increased by 5 ± 2 Tg. The elevated contributions we saw in 2020 over the western half of Africa (−5 ± 3 Tg) are substantially reduced in 2021, compared to our 2019 baseline. We find statistically significant positive correlations between anomalies of tropical methane emissions and groundwater, consistent with recent studies that have highlighted a growing role for microbial sources over the tropics. Emission reductions over India and China are expected in 2020 due to the Covid-19 lockdown but continued in 2021, which we do not currently understand. To investigate the role of reduced OH concentrations during the Covid-19 lockdown in 2020 on the elevated atmospheric methane growth in 2020–2021, we extended our inversion state vector to include monthly scaling factors for OH concentrations over six latitude bands. During 2020, we find that tropospheric OH is reduced by 1.4 ± 1.7 % relative to the corresponding 2019 baseline value. The corresponding revised global growth of a posteriori methane emissions in 2020 decreased by 34 % to 17.9 ± 13.2 Tg, relative to the a posteriori value that we inferred using fixed climatological OH values, consistent with sensitivity tests using the OH climatology inversion using reduced values for OH. The counter statement is that 66 % of the global increase in atmospheric methane during 2020 was due to increased emissions, particularly from tropical regions. Regional flux differences between the joint methane–OH inversion and the OH climatology inversion in 2020 are typically much smaller than 10 %. We find that OH is reduced by a much smaller amount during 2021 than in 2020, representing about 10 % of the growth of atmospheric methane in that year. Therefore, we conclude that most of the observed increase in atmospheric methane during 2020 and 2021 is due to increased emissions, with a significant contribution from reduced levels of OH.

Atmospheric methane: Comparison between methane’s record in 2006-2022 and during glacial terminations.

Abstract: Atmospheric methane’s rapid growth from 2006 to the present is unprecedented in the observational record. Isotopic evidence implies the growth is mainly driven by an increase in biogenically-sourced emissions, both from wetlands and ruminants, and waste. A significant part of methane’s current rise may come not from direct anthropogenic emissions and land use changes, but rather from a combination of natural biogenic feedback responses, occurring in response to the anthropogenic forcing. Although microbial emissions from agricultural and waste have increased between 2006-2020 by about 35 Tg/yr, perhaps 35-40 Tg/yr of the recent net growth in methane emissions may have been driven by natural biogenic processes, especially wetland feedbacks to climate change. Modelling comparison between the biogenic component of methane’s growth and isotopic shift in the 15 years from 2007-2022, and the global-scale climate reorganisations during the transitions from glacial to interglacial periods in the Pleistocene, shows that the modern growth event is comparable to or greater than the scale and speed of methane’s growth and isotopic shift during past glacial/interglacial termination events. It remains possible that current changes are related to decadal- or centennial-scale variability in precipitation and temperature and remain within the range of Holocene variability, or due to direct anthropogenic actions. But, though any current transition will differ greatly from the past glacial-interglacial changes, it is also possible methane’s remarkable growth and isotopic shift that began in 2006 may be a first indicator that a very large-scale reorganisation of the natural climate and biosphere system is under way.

Decreasing seasonal cycle amplitude of methane in the northern high latitudes being driven by lower latitude changes in emissions and transport

Abstract: Abstract. Atmospheric methane (CH4) concentrations are rising which is expected to lead to a corresponding increase in its global seasonal cycle amplitude (SCA), the difference between its seasonal maximum and minimum values. Spatially-varying changes in the SCA could indicate long-term persistent variations in the seasonal sources and sinks but such SCA changes have not been investigated. Here we use surface flask measurements and a 3-D chemical transport model (TOMCAT) to diagnose changes in the SCA of atmospheric CH4 between 1995–2020 and attribute the changes regionally to contributions from different sectors. We find that the observed SCA decreased by 4 ppb (7.6 %) in the northern high latitudes (NHL, 60° N–90° N), whilst globally the SCA increased by 2.5 ppb (6.5 %) during this time period. TOMCAT reproduces the change in the SCA at observation sites across the globe and therefore we use it to attribute regions which are contributing to the changes in the NHL SCA. We find that well-mixed background CH4, likely from emissions originating in, and transported from, more southerly latitudes has the largest impact on the decreasing SCA in the NHL (56.5 % of total contribution to NHL). In addition to the background CH4, recent emissions from Canada, the Middle East and Europe contribute 16.9 %, 12.1 % and 8.4 %, respectively, to the total change in the SCA in the NHL. The regional contributions are driven by increases in summer emissions from the Boreal Plains in Canada, decreases in winter emissions across Europe, and a combination of increases in summer emissions and decreases in winter emissions over the Arabian Peninsula and Caspian Sea in the Middle East. These results highlight that changes in the observed seasonal cycle can be an indicator of changing emission regimes in local and non-local regions, particularly in the NHL where the change is counter-intuitive.

Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption

Abstract: Abstract Although greenhouse gases absorb primarily long-wave radiation, they also absorb short-wave radiation. Recent studies have highlighted the importance of methane short-wave absorption, which enhances its stratospherically adjusted radiative forcing by up to ~ 15%. The corresponding climate impacts, however, have been only indirectly evaluated and thus remain largely unquantified. Here we present a systematic, unambiguous analysis using one model and separate simulations with and without methane short-wave absorption. We find that methane short-wave absorption counteracts ~30% of the surface warming associated with its long-wave radiative effects. An even larger impact occurs for precipitation as methane short-wave absorption offsets ~60% of the precipitation increase relative to its long-wave radiative effects. The methane short-wave-induced cooling is due largely to cloud rapid adjustments, including increased low-level clouds, which enhance the reflection of incoming short-wave radiation, and decreased high-level clouds, which enhance outgoing long-wave radiation. The cloud responses, in turn, are related to the profile of atmospheric solar heating and corresponding changes in temperature and relative humidity. Despite our findings, methane remains a potent contributor to global warming, and efforts to reduce methane emissions are vital for keeping global warming well below 2 °C above preindustrial values.

Groundwater springs formed during glacial retreat are a large source of methane in the high Arctic

Abstract: Abstract Permafrost and glaciers in the high Arctic form an impermeable ‘cryospheric cap’ that traps a large reservoir of subsurface methane, preventing it from reaching the atmosphere. Cryospheric vulnerability to climate warming is making releases of this methane possible. On Svalbard, where air temperatures are rising more than two times faster than the average for the Arctic, glaciers are retreating and leaving behind exposed forefields that enable rapid methane escape. Here we document how methane-rich groundwater springs have formed in recently revealed forefields of 78 land-terminating glaciers across central Svalbard, bringing deep-seated methane gas to the surface. Waters collected from these springs during February–May of 2021 and 2022 are supersaturated with methane up to 600,000 times greater than atmospheric equilibration. Spatial sampling reveals a geological dependency on the extent of methane supersaturation, with isotopic evidence of a thermogenic source. We estimate annual methane emissions from proglacial groundwaters to be up to 2.31 kt across the Svalbard archipelago. Further investigations into marine-terminating glaciers indicate future methane emission sources as these glaciers transition into fully land-based systems. Our findings reveal that climate-driven glacial retreat facilitates widespread release of methane, a positive feedback loop that is probably prevalent across other regions of the rapidly warming Arctic.

Enteric methane output and weight accumulation of Nguni and Bonsmara cows raised under different grazing conditions

Abstract: Abstract Several experts throughout the world have focused a lot of their research on the rise in methane concentrations in the atmosphere and its causes. Cattle are the livestock species that contribute the most to methane emissions, according to research conducted over the previous three decades. A greenhouse gas called enteric methane (CH 4 ) is created by microbial fermentation in the rumen and is released into the atmosphere through a variety of excretory processes. To reduce methane emissions, research on the ways that various breeds of cattle are reared on different grazing regimes should be prioritized. The goal of the current study was to measure the weight gain and intestinal methane emission of Nguni and Bonsmara cows grown under various grazing conditions. Eighty-four cows belonging to the 2 grazing systems were randomly selected and grouped according to three age groups: A (young adult cow, n = 7, 24–48 months), B (adult, n = 7, 60–80 months), and C (old adult, n = 7, 90–120 months) are three different age groups for adult cows. Methane production was higher per head in older animals, with C producing the most, followed by B and A (C > B > A; P 0.0001). In Bonsmara, body condition, body weight (BW), dry matter intake (DMI), and daily methane were all higher ( P 0.05). Nguni had more methane per pound of weight ( P 0.05), although methane per kilogram of body mass ( P > 0.05) was similar across breeds. In the commercial system, body condition, BW, and DMI were higher ( P 0.05). On the other hand, communal grazing resulted in increased daily methane production, methane per DMI, and methane produced per BW. These findings support the notion that breed genetics, grazing system, and age all have an impact on methane levels and performance. So, all these aspects must be taken into account in breeding strategies for traits like methane production that are challenging to assess.

Spatiotemporal Variability of Global Atmospheric Methane Observed from Two Decades of Satellite Hyperspectral Infrared Sounders

Abstract: Methane (CH4) is the second most significant contributor to climate change after carbon dioxide (CO2), accounting for approximately 20% of the contributions from all the well-mixed greenhouse gases. Understanding the spatiotemporal distributions, and the relevant long-term trends are crucial to identifying the sources, sinks, and impacts on climate. Hyperspectral thermal infrared (TIR) sounders, including the Atmospheric Infrared Sounder (AIRS), the Cross-track Infrared Sounder (CrIS), and the Infrared Atmospheric Sounding Interferometer (IASI), have been used to measure global CH4 concentrations since 2002. This study analyzed nearly twenty years of data from AIRS and CrIS and confirmed a significant increase in CH4 concentrations in the mid-upper troposphere (around 400 hPa) from 2003 to 2020, with a total increase of approximately 85 ppb, representing a +4.8% increase in 18 years. The rate of increase was derived using global satellite TIR measurements is consistent with in-situ measurements, indicating a steady increase starting in 2007 and became stronger in 2014. The study also compared CH4 concentrations derived from the AIRS and CrIS against ground-based measurements from NOAA Global Monitoring Laboratory (GML) and found phase shifts in the seasonal cycles in the middle to high latitudes in the northern hemisphere, which is attributed to the influence of stratospheric CH4 that varies at different latitudes. These findings provide insights into the global budget of atmospheric composition and the understanding of satellite measurement sensitivity of CH4.

An Improved Workflow in Mass Balance Approach for Estimating Regional Methane Emission Rate Using Satellite Measurements

Abstract: Satellite-retrieved methane (CH4) concentration data offers a valuable opportunity for large-scale emissions monitoring. However, its widespread adoption remains challenging due to the data volume and varying data quality. A workflow to estimate the methane emission rate of major hydrocarbon plays based on the mass balance principle using publicly available Sentinel-5P satellite data is presented. This workflow estimates the methane emission rate originating from specific regions. The proposed workflow is applied to estimate emissions from the Permian Basin and the Appalachian Basin in the United States. The results are compared against volumes estimated by other means and reported in the literature. The proposed method is easy to implement and offers promising potential for practical and reliable estimates for long-term regional methane emission monitoring purposes.

Supporting Methane Mitigation Efforts by Improving Urban-scale Methane Emission Estimates in Melbourne, Australia. Part 2: Developing the methane observation network for the Melbourne region

Abstract: Methane (CH4) is the second greatest contributor to climate forcing after carbon dioxide (CO2).  Methane has a considerably shorter atmospheric lifetime compared to CO2 (12 yr c.f. 300-1000 yr) but a higher warming potential in the atmosphere (GWP100yr 28, (IPCC, 2014)). Most anthropogenic emissions come from landfills, wastewater treatment plants, leaks in the fossil fuel supply chain and ruminant livestock.  The reduction of anthropogenic methane emissions is key to maintaining the feasibility of the Paris Agreement. The Global Methane Pledge launched at COP26 aims to reduce methane emissions by 30% relative to 2020 by 2030. Urban areas are an ideal target to reduce methane emissions given that they account for around 20% of the total emissions whilst they occupy only 3% of the land surface. Urban methane mitigations plans are proven to have a high impact reducing GHG emissions and bringing co-benefits in public health through improvements in air quality.Australia is a signatory to both the Paris Agreement and the Global Methane Pledge and have an important potential emission reduction in urban areas. Melbourne is the second most populous city in Australia with over 5 million people (around 1/5 of Australia’s population) and it is projected to become the most populated by 2050. A recent study attempted to improve methane emission inventories for Melbourne using an inversion system, global emission data and atmospheric measurements (Shahrokhi , 2022).  Their results showed that current emission datasets do not accurately represent the spatial distribution and total estimates of methane emissions over Melbourne. Hence, an improved emission inventory is required for Melbourne. This will reduce the uncertainty and limitations of current methane emission estimates and support the formulation of effective emissions mitigation plans. Essential to this is the expansion of the Melbourne urban observational network, which is currently too sparse to accurately detect emissions. Here we present our preliminary progress on the development of a comprehensive methane observation network. This project aims to combine different measurement techniques to achieve a better representation of methane mole fraction variability in the Melbourne region, to inform inverse modelling estimates of emissions. We use a combination of mobile and stationary ground observations in key parts of the city to better capture and represent methane emissions. Future work includes the comparison of high precision analysers with low-cost sensors, improvement of source attribution by measurements of methane isotopes and other tracers, and the use of “AirCore” technology to obtain vertical methane profiles. ReferencesIPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.Shahrokhi, N. 2022. Regional Methane Inversion for Melbourne, Australia, using in-situ measurements. Online poster [accessed 10 Jan 2022]. Available from: https://agu2022fallmeeting-agu.ipostersessions.com/default.aspx?s=2F-7B-00-39-98-DC-28-09-C3-90-81-25-D7-44-E2-D2

Investigating national methane sources with satellite retrievals: a case of South Korea

Abstract: Methane is the second largest greenhouse gas after carbon dioxide in its impact on climate change. Atmospheric methane has stagnated from 2000 to 2006, and then began to increase again in 2007, showing the largest increase since observation in 2021(19.94 ppb/yr).As part of UNFCCC’s goals for carbon neutrality, it is necessary to verify each country's GHG’s emissions sources and the verifications using satellite observations and atmospheric models are one of the important approaches.Currently, satellite data have been useful for methane monitoring, particularly the retrievals measured by TROPOMI with a high resolution(~7km) and good spatial coverage.Here we investigated the spatio-temporal characteristics of national methane distribution and the spatial correlation between satellite concentrations and the national emission sources over South Korea to identify the characteristics of high-level methane distributions from August 2018 to July 2019 . During the period, the average concentration of XCH4 in Korea was ~1858 ppb and the monthly mean concentrations of methane in Korea were higher from June to October, which in fact reflected the characteristics of rice paddy and wetlands in monsoon season. The spatial correlation analysis (SDM) found that there are some areas showing specific contributing emissions sources with higher methane levels. There are areas with high correlations with livestock production, fossil fuel uses(gas & oils), wastes(& landfill), and rice paddies, while there are areas with high correlations with complex effects of the four fields or with no clear correlations. Based on our analysis, the spatial correlation analysis with various emission sources and satellite data can provide the information to evaluate the CH4 emissions inventory and give some ideas to manage regional greenhouse gases reduction policies

From Archaea to the atmosphere: linking above and belowground communities to scale methane and isotopic emissions across the Arctic

Abstract: High latitude peatlands are a significant source of atmospheric methane. Production, consumption and emission rates are spatially and temporally heterogeneous, resulting in a wide range of global estimates for the atmospheric budget of methane. Increasing temperatures in Arctic regions cause degradation of underlying permafrost, changing hydrology, vegetation and microbial communities, but the consequences of this for methane cycling, including stable methane isotopes, are poorly understood. We provide evidence of direct linkages between below ground methanogen communities and above ground plant communities that can be remotely sensed and used in model simulations to effectively predict methane and isotopic fluxes across the landscape. Combining remote sensing with biogeochemical modeling can be used to predict methane dynamics, including the fraction derived from hydrogenotrophic versus acetoclastic microbial methanogenesis. Applying this approach across heterogeneous discontinuous permafrost peatlands enables us to accurately predict isotopic emissions, which will help constrain the global role of Arctic methane emissions.

Influence of wind speed and wind direction above the sea surface on the diffusive methane flux and the atmospheric methane concentration at the North Sea

Abstract: The estimations of the diffusive methane flux from the water phase into the atmosphere in coastal waters is relevant for a better estimate of the atmospheric greenhouse-gas budget. Unfortunately, so far, the numerical determination of the fluxes has a high level of uncertainty in coastal waters.To improve the estimation of coastal methane fluxes, not only a high temporal and spatial sampling resolution of the dissolved methane in the water are required. Besides, also the atmospheric methane concentration and the wind speed and wind direction above the surface is important. In most cases, these atmospheric data are obtained from near-by atmospheric and meteorologic monitoring stations. In this study, we measured wind speed, direction and atmospheric methane local directly on board of three research vessel cruising in the southern North Sea within the MOSES project and compared the effects of local versus remote measurements of these data on the flux data. In addition, using the wind direction and speed, we try to assess the origin of the atmospheric methane measured in the study area. Using these “improved” data sets, we discuss if local measurements of auxiliary data provide better insights in the determining factors of the methane flux, and thus also improve the regional aquatic methane budget.

Methanotrophic bacteria in forest soils – should I stay or should I go?

Abstract: Methanotrophic bacteria are capable to uptake methane (CH4) from the atmosphere. They are mainly found in forest soils, making them the most important terrestrial sink for methane. While agricultural soils have partially lost their methane sink function due to the negative effect of nitrogen fertilization on methanotrophy, the methane sink function of forest soils is considered to be intact. Differences in sink capacity of different land use types and short-term factors influencing methane uptake are well studied. Since the most commonly used methodology to measure methane fluxes are energy- and personnel-intensive chamber measurements, there are only few long-term measurements, especially of forest soils. Therefore, little is known about long-term effects and climate change impacts on methane sink function.In the long-term forest environmental monitoring of the Forest Research Institute (FVA-BW), soil air including methane content has been measured at different depth levels at a monthly interval on 13 forest monitoring plots (ICP Forest Level II) in southwestern Germany for more than 20 years. This method, which is well suited for long-term monitoring, allowed continuous sampling since 1998 and 2010, respectively. From the concentration gradients, the methane flux can be determined using the gradient method. To make these calculated fluxes more precise, chamber measurements were carried out over 1.5 years in 2021/2022 in parallel with the collection of gas samples. By comparing the fluxes of both measurement methods, a validation of the long-term measurement series is possible.First evaluations of our sites show so far insignificant changes in methane fluxes over the last 20 years. This contrasts our results with study results (Ni & Groffman, 2018), which report a dramatic 53-89% decrease in methane uptake in forest soils observed at four sites in the USA over the last 20 years. Trend estimates of our monitoring sites and the analysis of significant factors influencing long-term methane trends are presented.References:Ni, X., Groffman P.M.2018. Declines in methane uptake in forest soils. PNAS 115 (34) 8587-.

ISAMO (Iron Salt Atmospheric Methane Oxidation)

Abstract: Methane is a well-mixed greenhouse gas responsible for >1/3 of global warming since pre-industrial times whose atmospheric burden continues to increase with a new record set in 2022. Active chlorine in the atmosphere is poorly constrained and so is its role in the oxidation of methane. This uncertainty propagates into methane source budgets through isotope-constrained top-down models, in which the observed abundance of 13C in tropospheric methane (commonly expressed as δ13C-CH4) is used to constrain the sources of methane using their characteristic δ13C-CH4 values. These models need to account for the change in the observed δ13C-CH4 by the Cl and OH sinks, which shift the observed isotope towards higher δ13C-CH4 values of fossil fuel sources, and away from 13C depleted biological sources. The ISAMO project focuses on the hypothesis that Cl atoms are produced naturally by the action of sunlight on particles containing iron and chloride and these chlorine atoms oxidize atmospheric methane. To study this, we use the sensitive and selective indirect quantification of the concentration of atomic Cl through the strong carbon kinetic isotope effect (KIE) in the CH4 + Cl reaction, which leaves the remaining CH4 enriched in 13C, and producing extremely 13C-depleted CO. We will present field and laboratory observations and global modelling, including CO isotope measurement from flasks samples across the North Atlantic. We show how this mechanism affects 13C depletion in atmospheric CO and how the corresponding 13C enrichment in CH4 affects global methane emission estimates.

Methane emissions from abandoned hydrocarbon wells in Italy: inventory, measurement techniques and the role of mega-emitters

Abstract: Abandoned hydrocarbon (oil and gas) wells (AOG) represent a poorly studied source of atmospheric methane, potentially contributing to total anthropogenic fossil methane emission and related climatic impact. Methane leakage from AOG was measured only in a few countries (U.S.A., Canada, the Netherlands, United Kingdom), and available inventories in other countries are incomplete or need quality checks. Methodologies for gas flux measurement are not standardized. New studies have recently started in Italy in order to inventory onshore AOG, design multiple and versatile techniques for methane flux measurement, which can be adaptable to different typologies of well-heads, and to execute first measurements. Preliminary data revealed the existence of several AOW releasing relevant amounts of methane (orders of 101 ton yr-1), which are up to two orders of magnitude above those typically observed in North America. Contextualization of such “mega-emitters” (their percentage with respect to total AOW, technical conditions, possible existence in other countries) is necessary to assess average emission factors and derive bottom-up methane emission estimates at national and global scale.

Excess methane, ethane, and propane production in Greenland ice core samples and a first characterization of the δ13C-CH4 and δD-CH4 signature

Abstract: Air trapped in polar ice provides unique records of the past atmospheric composition ranging from key greenhouse gases such as methane (CH4) to short-lived trace gases like ethane (C2H6) and propane (C3H8). Interpreting these data in terms of atmospheric changes requires that the analyzed species accurately reflect the past atmospheric composition.Recent comparisons of Greenland CH4 records obtained using different extraction techniques revealed discrepancies in the CH4 concentration for the last glacial. Elevated methane levels (excess methane or CH4(exs)) were detected in dust rich ice core sections measured by discrete melt extraction techniques pointing to an artefact sensitive to the measurement technique. To shed light on the underlying mechanism, we analyzed Greenland ice core samples for methane and other short-chain alkanes (ethane and propane) covering the time interval from 12 to 42 kyr using a classic wet extraction technique. The artefact production happens during the melting and extraction step (in extractu) and reaches 14 to 91 ppb CH4(exs) in dusty ice samples. For the first time in ice core analyses, we document a co-production of excess methane, ethane, and propane (excess alkanes) with the observed concentrations for ethane and propane exceeding, at least by a factor of 10, their past atmospheric concentration. Independent of the produced amounts, excess alkanes were produced in a fixed molar ratio of approximately 14:2:1, indicating a common production. We also discovered that the amount of excess alkanes scales generally with the amount of mineral dust (or Ca2+ as a proxy for mineral dust) within the ice samples. Moreover, applying the Keeling-plot approach we are able to isotopically characterize CH4(exs) revealing a relatively heavy carbon isotopic signature of (-46.4 ± 2.4) ‰ and a light deuterium isotopic signature of (-326 ± 57) ‰ of the excess methane in the samples analyzed.The co-production ratios of excess alkanes and the isotopic composition of excess methane allows us to confine potential formation processes. We discovered that this specific alkane pattern is not in line with an anaerobic methanogenic origin but indicative for abiotic decomposition of organic matter as also found in sediments, soils, and plant leaves. From the present-day state of research little is known about this process and there is urgent need to improve our understanding for future ice core measurements. Moreover, the already existing discrete records of atmospheric CH4 in Greenland ice need to be corrected for excess CH4 contribution (CH4(exs), δ13C-CH4(exs), δD-CH4(exs)) in dust-rich intervals.While the size of the excess methane production has little effect on reconstructed radiative forcing changes of CH4 in the past, it is in the same range as the Inter-Polar Difference (IPD) for CH4. Knowing the empirical relation of excess CH4(exs)/ Ca2+ and CH4(exs)/ C2H6 allows us to derive a first-order correction of existing CH4 data sets to revise previous interpretations of the relative contribution of high latitude northern hemispheric CH4 sources based on the IPD.

Global environmental implications of atmospheric methane removal through chlorine-mediated chemistry-climate interactions

Abstract: Abstract Atmospheric methane is both a potent greenhouse gas and photochemically active, with approximately equal anthropogenic and natural sources. The addition of chlorine to the atmosphere has been proposed to mitigate global warming through methane reduction by increasing its chemical loss. However, the potential environmental impacts of such climate mitigation remain unexplored. Here, sensitivity studies are conducted to evaluate the possible effects of increasing reactive chlorine emissions on the methane budget, atmospheric composition and radiative forcing. Because of non-linear chemistry, in order to achieve a reduction in methane burden (instead of an increase), the chlorine atom burden needs to be a minimum of three times the estimated present-day burden. If the methane removal target is set to 20%, 45%, or 70% less global methane by 2050 compared to the levels in the Representative Concentration Pathway 8.5 scenario (RCP8.5), our modeling results suggest that additional chlorine fluxes of 630, 1250, and 1880 Tg Cl/year, respectively, are needed. The results show that increasing chlorine emissions also induces significant changes in other important climate forcers. Remarkably, the tropospheric ozone decrease is large enough that the magnitude of radiative forcing decrease is similar to that of methane. Adding 630, 1250, and 1880 Tg Cl/year to the RCP8.5 scenario, chosen to have the most consistent current-day trends of methane, will decrease the surface temperature by 0.2, 0.4, and 0.6 °C by 2050, respectively. The quantity and method in which the chlorine is added, its interactions with climate pathways, and the potential environmental impacts on air quality and ocean acidity, must be carefully considered before any action is taken.

Why atmospheric methane surged in 2020?

Abstract: Methane (CH4) levels in the atmosphere increased by 15.1 ± 0.4 ppb in 2020, the highest annual increase from 1984 to 2020, despite a likely decrease in anthropogenic CH4 emissions during COVID-19 confinements. Here, we used bottom-up and top-down methods to quantify the changes in different sources of CH4, and in its atmospheric sink due to the hydroxyl radical (OH) in 2020 compared to 2019. Bottom-up methods showed that, globally, total anthropogenic emissions slightly decreased by ~1.2 Tg CH4 yr-1, fire emissions were lower than in 2019 by ~6.5 Tg CH4 yr-1, and wetland emissions increased by 6.0 ± 2.3 Tg CH4 yr-1. In addition to higher wetland emissions in 2020 than 2019 from bottom-up, we found a decrease of 1.6–1.8% in tropospheric OH concentration relative to 2019, mainly due to lower anthropogenic NOx emissions and associated lower free tropospheric ozone during the confinements. Based on atmospheric CH4 observations from the surface network, and considering the decrease in OH, using top-down inversions, we infer that global net emissions increased by 6.9 ± 2.1 Tg CH4 yr-1 in 2020 relative to 2019, while the global CH4 removal from reaction with OH in the atmosphere decreased in 2020 by 7.5 ± 0.8 Tg CH4 yr-1. Therefore, we attribute the positive growth rate anomaly of atmospheric CH4 in 2020 relative to 2019 to lower OH sink (53 ± 10%) and higher natural emissions (47 ± 16%), mostly from wetlands. Warmer and wetter climate conditions in the Northern Hemisphere promoted wetland emissions, but fires decreased in the Southern Hemisphere, compared to the previous year. Our study highlights that northern microbial emissions of CH4 are highly sensitive to a warmer and wetter climate and could act as a positive feedback in the future. Our study also hints that the global CH4 pledge must be implemented by taking into account NOx emissions trend, whose reduction lengthens the lifetime of atmospheric CH4.