Quantifying fossil fuel methane emissions using observations of atmospheric ethane and an uncertain emission ratio

We present a method for estimating fossil fuel methane emissions using observations of methane and ethane, accounting for uncertainty in their emission ratio. The ethane:methane emission ratio is incorporated as a spatially and temporally variable parameter in a Bayesian model, with its own prior distribution and uncertainty. We find that using an emission ratio distribution mitigates bias from using a fixed, potentially incorrect emission ratio and that uncertainty in this ratio is propagated into posterior estimates of emissions. A synthetic data test is used to show the impact of assuming an incorrect ethane:methane emission ratio and demonstrate how our variable parameter model can better quantify overall uncertainty. We also use this method to estimate UK methane emissions from high-frequency observations of methane and ethane from the UK Deriving Emissions linked to Climate Change (DECC) network. Using the joint methane–ethane inverse model, we estimate annual mean UK methane emissions of approximately 0.27 (95 % uncertainty interval 0.26–0.29) Tg yr−1 from fossil fuel sources and 2.06 (1.99–2.15) Tg yr−1 from non-fossil fuel sources, during the period 2015–2019. Uncertainties in UK fossil fuel emissions estimates are reduced on average by 15 % and up to 35 % when incorporating ethane into the inverse model, in comparison to results from the methane-only inversion

Variability and quasi-decadal changes in the methane budget overthe period 2000–2012

Following the recent Global Carbon Project (GCP) synthesis of the decadal methane (CH4/ budget over 2000– 2012 (Saunois et al., 2016), we analyse here the same dataset with a focus on quasi-decadal and inter-annual variability in CH4 emissions. The GCP dataset integrates results from topdown studies (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models (including process-based models for estimating land surface emissions and atmospheric chemistry), inventories of anthropogenic emissions, and data-driven approaches.The annual global methane emissions from top-down studies, which by construction match the observed methane growth rate within their uncertainties, all show an increase in total methane emissions over the period 2000–2012, but this increase is not linear over the 13 years. Despite differences between individual studies, the mean emission anomaly of the top-down ensemble shows no significant trend in total methane emissions over the period 2000–2006, during the plateau of atmospheric methane mole fractions, and also over the period 2008–2012, during the renewed atmospheric methane increase. However, the top-down ensemble mean produces an emission shift between 2006 and 2008, leading to 22 [16–32] Tg CH4 yr1 higher methane emissions over the period 2008–2012 compared to 2002–2006. This emission increase mostly originated from the tropics, with a smaller contribution from mid-latitudes and no significant change from boreal regions. The regional contributions remain uncertain in top-down studies. Tropical South America and South and East Asia seem to contribute the most to the emission increase in the tropics. However, these two regions have only limited atmospheric measurements and remain therefore poorly constrained. The sectorial partitioning of this emission increase between the periods 2002–2006 and 2008–2012 differs from one atmospheric inversion study to another. However, all topdown studies suggest smaller changes in fossil fuel emissions (from oil, gas, and coal industries) compared to the mean of the bottom-up inventories included in this study. This difference is partly driven by a smaller emission change in China from the top-down studies compared to the estimate in the Emission Database for Global Atmospheric Research (EDGARv4.2) inventory, which should be revised to smaller values in a near future. We apply isotopic signatures to the emission changes estimated for individual studies based on five emission sectors and find that for six individual top-down studies (out of eight) the average isotopic signature of the emission changes is not consistent with the observed change in atmospheric 13CH4. However, the partitioning in emission change derived from the ensemble mean is consistent with this isotopic constraint. At the global scale, the top-down ensemble mean suggests that the dominant contribution to the resumed atmospheric CH4 growth after 2006 comes from microbial sources (more from agriculture and waste sectors than from natural wetlands), with an uncertain but smaller contribution from fossil CH4 emissions. In addition, a decrease in biomass burning emissions (in agreement with the biomass burning emission databases) makes the balance of sources consistent with atmospheric 13CH4 observations. In most of the top-down studies included here, OH concentrations are considered constant over the years (seasonal variations but without any inter-annual variability). As a result, the methane loss (in particular through OH oxidation) varies mainly through the change in methane concentrations and not its oxidants. For these reasons, changes in the methane loss could not be properly investigated in this study, although it may play a significant role in the recent atmospheric methane changes as briefly discussed at the end of the paper.Published11135–111616A. Geochimica per l'ambienteJCR Journa

The global methane budget 2000–2017

Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, < 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters. Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning

Proteomic response to linoleic acid hydroperoxide in Saccharomyces cerevisiae

Yeast AP-1 transcription factor (Yap1p) and the enigmatic oxidoreductases Oye2p and Oye3p are involved in counteracting lipid oxidants and their unsaturated breakdown products. In order to uncover the response to linoleic acid hydroperoxide (LoaOOH) and the roles of Oye2p, Oye3p and Yap1p, we carried out proteomic analysis of the homozygous deletion mutants oye3∆, oye2Δ and yap1Δ alongside the diploid parent strain BY4743. The findings demonstrate that deletion of YAP1 narrowed the response to LoaOOH, as the number of proteins differentially expressed in yap1Δ was 70% of that observed in BY4743. The role of Yap1p in regulating the major yeast peroxiredoxin Tsa1p was demonstrated by the decreased expression of Tsa1p in yap1Δ. The levels of Ahp1p and Hsp31p, previously shown to be regulated by Yap1p, were increased in LoaOOH-treated yap1Δ, indicating their expression is also regulated by another transcription factor(s). Relative to BY4743, protein expression differed in oye3Δ and oye2Δ under LoaOOH, underscored by superoxide dismutase (Sod1p), multiple heat shock proteins (Hsp60p, Ssa1p, and Sse1p), the flavodoxin-like protein Pst2p and the actin stabiliser tropomyosin (Tpm1p). Proteins associated with glycolysis were increased in all strains following treatment with LoaOOH. Together, the dataset reveals, for the first time, the yeast proteomic response to LoaOOH, highlighting the significance of carbohydrate metabolism, as well as distinction between the roles of Oye3p, Oye2p and Yap1p

Transcriptomic insights into the molecular response of Saccharomyces cerevisiae to linoleic acid hydroperoxide

Eukaryotic microorganisms are constantly challenged by reactive oxygen species derived endogenously or encountered in their environment. Such adversity is particularly applied to Saccharomyces cerevisiae under harsh industrial conditions. One of the major oxidants to challenge S. cerevisiae is linoleic acid hydroperoxide (LoaOOH). This study, which used genome-wide microarray analysis in conjunction with deletion mutant screening, uncovered the molecular pathways of S. cerevisiae that were altered by an arresting concentration of LoaOOH (75 µM). The oxidative stress response, iron homeostasis, detoxification through PDR transport and direct lipid ß-oxidation were evident through the induction of the genes encoding for peroxiredoxins ( GPX2 , TSA2 ), the NADPH:oxidoreductase ( OYE3 ), iron uptake ( FIT2 , ARN2 , FET3 ), PDR transporters ( PDR5 , PDR15 , SNQ2 ) and ß-oxidation machinery ( FAA2 , POX1 ). Further, we discovered that Gpx3p, the dual redox sensor and peroxidase, is required for protection against LoaOOH, indicated by the sensitivity of gpx3 ∆to a mild dose of LoaOOH (37.5 µM). Deletion of GPX3 conferred a greater sensitivity to LoaOOH than the loss of its signalling partner YAP1 . Deletion of either of the iron homeostasis regulators AFT1 or AFT2 also resulted in sensitivity to LoaOOH. These novel findings for Gpx3p, Aft1p and Aft2p point to their distinct roles in response to the lipid peroxide. Finally, the expression of 89 previously uncharacterised genes was significantly altered against LoaOOH, which will contribute to their eventual annotation

An antioxidant screening assay based on oxidant-induced growth arrest in Saccharomyces cerevisiae

This report describes a biological screening system to measure the antioxidant capacity of compounds using the oxidant-induced growth arrest response of Saccharomyces cerevisiae. Alternative methods using the nonphysiological free radical compounds such as diphenylpicrylhydrazyl and azinobis ethylbenzothiaziline-6-sulphonate (ABTS) only provide an indication of the ability of a compound to scavenge oxidants. In contrast, this yeast-based method can also measure the ability of a compound to induce cellular resistance to the damaging effects of oxidants. The screening assay was established against a panel of six physiologically relevant oxidants ranging from reactive oxygen species (hydrogen peroxide, cumene peroxide, linoleic acid hydroperoxide), to a superoxide-generating agent (menadione), reactive nitrogen species (peroxynitrite) and a thiol-oxidizing agent (diamide). The antioxidants ascorbate and gallic acid displayed scavenging activity and induced the resistance of cells against a broad range of oxidants using this assay. Lipoic acid, which showed no scavenging activity and thus would not be detected as an antioxidant using a nonphysiological screen was, however, identified in this assay as providing resistance to cells against a range of oxidants. This assay is high throughput, in the format of a 96-well microtitre plate, and will greatly facilitate the search for effective antioxidants

Identification of aluminium transport-related genes via genome-wide phenotypic screening of Saccharomyces cerevisiae

Genome-wide screening using gene deletion mutants has been widely carried out with numerous toxicants including oxidants and metal ions. The focus of such studies usually centres on identifying sensitive phenotypes against a given toxicant. Here, we screened the complete collection of yeast gene deletion mutants (5047) with increasing concentrations of aluminium sulphate (0.4, 0.8, 1.6 and 3.2 mM) in order to discover aluminium (Al3+) tolerant phenotypes. Fifteen genes were found to be associated with Al3+ transport because their deletion mutants exhibited Al3+ tolerance, including lem3Δ, hal5Δ and cka2Δ. Deletion of CKA2, a catalytic subunit of tetrameric protein kinase CK2, gives rise to the most pronounced resistance to Al3+ by showing significantly higher growth compared to the wild type. Functional analysis revealed that both molecular regulation and endocytosis are involved in Al3+ transport for yeast. Further investigations were extended to all the four subunits of CK2 (CKA1, CKA2, CKB1 and CKB2) and the other 14 identified mutants under a spectrum of metal ions, including Al3+, Zn2+, Mn2+, Fe 2+, Fe3+, Co3+, Ga3+, Cd 2+, In3+, Ni2+ and Cu2+, as well as hydrogen peroxide and diamide, in order to unravel cross-tolerance amongst metal ions and the effect of the oxidants. Finally, the implication of the findings in Al3+ transport for the other species like plants and humans is discussed

Effects of metal ions and hydrogen peroxide on the phenotype of yeast hom6Δ mutant

HOM6 is a major gene in the aspartate pathway which leads to biosynthesis of threonine and methionine. The phenotypes of the gene deletion mutant (hom6∆) in a variety of cultural conditions have previously provided meaningful insights into the biological roles of HOM6 and its upstream intermediate metabolites. Here, we conducted a survey on a spectrum of metal ions for their effect on the aspartate pathway and broader sulphur metabolism. We show that manganese (Mn2+) promoted the growth of hom6∆ under both anaerobic and aerobic conditions. Unexpectedly, 4 mmol l−1 hydrogen peroxide (H2O2), a dose normally causing temporary cell growth arrest, enhanced the growth of hom6∆ under the anaerobic condition only, while it had no effect on the wild type strain BY4743. We propose that Mn2+ and H2O2 promote the growth of hom6∆ by reducing the accumulation of the toxic intermediate metabolite—aspartate β-semialdehyde, via directing the aspartate pathway to the central sugar metabolism–tricarboxylic acid cycle