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Anaerobic oxidation of methane (AOM) governs methane consumption at seeps, yet δ13C‐CH4 values in the sulfate‐methane transitions are commonly lower than predicted from preferential 12C utilization by methanotrophic archaea, suggesting the influence of a different methane source. To address this isotope discrepancy, we used high‐resolution ion concentrations and carbon isotope data (CH4 and DIC) of porewaters from the Haima seep, South China Sea, to reconstruct the carbon cycle using a reaction‐transport model. Model simulations demonstrate that in situ methanogenesis cannot explain the extreme 13C depletion. Instead, the shift towards 13C‐depleted CH4 and DIC is primarily driven by isotopic equilibration during AOM, modulated by the carbon isotope fractionation factor and reverse AOM reaction flux under stable conditions. This mechanistic framework reconciles the paradoxical 13C‐depletion in CH4 by emphasizing the role of carbon isotope equilibration during AOM, offering an effective method for tracing the cryptic methane cycle in seep environments.

Methane (CH4) is one of the most important trace gases in the atmosphere owing to its role as an exceedingly effective greenhouse gas and atmospheric pollutant. Better understanding of the global methane cycle and its interactions with the Earth system is therefore necessary for robust future projections of anthropogenic climate change and assessments of multi‐gas mitigation strategies. Here we present a newly developed methane emission‐driven Earth system model to simulate the global methane cycle fully interactively. We provide an evaluation of methane sources and sinks and a full‐cycle methane budget and its change over the historic period. We further evaluate the methane atmospheric abundance and lifetime against available observations. The new methane emission‐driven model simulates all the components of the methane cycle within observational uncertainty. We calculate a total present‐day (2000–2009 decadal average) methane source of 591 Tg(CH4) yr−1 with 197 Tg(CH4) yr−1 coming from wetlands. These sources are nearly balanced by the global methane sinks amounting to 580 Tg (CH4) yr−1; reaction of methane with the hydroxyl radical in the troposphere alone removes 525 Tg(CH4) yr−1. The imbalance between sources and sinks of 11 Tg(CH4) yr−1 represents the atmospheric methane growth rate and is in fairly good agreement with current best estimates of 5.8 Tg(CH4) yr−1 with a range of 4.9–6.6 Tg(CH4) yr−1. At present‐day the model shows a maximum systematic negative‐bias of approximately 200 ppb in the methane surface mole fraction. Plain Language Summary Methane is a very important greenhouse gas. The global methane cycle needs to be understood fully to accurately model the way methane affects climate change. We describe a new version of the UKESM1 Earth system model, UKESM1‐ems, that uses emissions of methane to drive the atmospheric chemistry. In case of emissions from global wetlands, such as bogs, swamps and tundra, the methane emissions are calculated by the model during runtime. Methane emissions react directly to changes in the modeled climate. UKESM1‐ems simulates the global cycle of methane from emissions via oxidation in the atmosphere to uptake at the surface more realistically. We also test the model against measurements from satellites and ground‐based stations to ensure the relevant processes in the model behave accurately. The comparison with observations shows that UKESM1‐ems performs well and represents an improvement in simulating important processes in climate and the Earth system. However, we also found that the methane concentration in the model is too low compared to observations for the period of the twentieth and early 21st century during which human activity, especially the use of fossil fuel, is dominating the methane cycle. Key Points A methane emission‐driven configuration of the UK community Earth system model UKESM1, UKESM1‐ems, has been developed In UKESM1‐ems global wetlands are interactively coupled to the atmosphere at every timestep via methane emissions The UKESM1‐ems performs well simulating the global methane cycle including feedbacks; the global budget compares well with observations

The process of nitrite-dependent anaerobic methane oxidation (n-damo) was recently discovered and shown to be mediated by \"Candidatus Methylomirabilis oxyfera\" (M. oxyfera). Here, evidence for n-damo in three different freshwater wetlands located in southeastern China was obtained using stable isotope measurements, quantitative PCR assays, and 16S rRNA and particulate methane monooxygenase gene clone library analyses. Stable isotope experiments confirmed the occurrence of n-damo in the examined wetlands, and the potential n-damo rates ranged from 0.31 to 5.43 nmol CO2 per gram of dry soil per day at different depths of soil cores. A combined analysis of 16S rRNA and particulate methane monooxygenase genes demonstrated that M. oxyfera-like bacteria were mainly present in the deep soil with a maximum abundance of 3.2 × 107 gene copies per gram of dry soil. It is estimated that ∼0.51 g of CH4 m-2 per year could be linked to the n-damo process in the examined wetlands based on the measured potential n-damo rates. This study presents previously unidentified confirmation that the n-damo process is a previously overlooked microbial methane sink in wetlands, and n-damo has the potential to be a globally important methane sink due to increasing nitrogen pollution.

The global methane (CH4) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH4 over the past decade. Emissions and concentrations of CH4 are continuing to increase, making CH4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular (approximately biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (exploiting atmospheric observations within an atmospheric inverse-modeling framework) and bottom-up models, inventories and data-driven approaches (including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations).For the 2003-2012 decade, global methane emissions are estimated by top-down inversions at 558 TgCH4 yr(exp -1), range 540-568. About 60 of global emissions are anthropogenic (range 50-65%). Since 2010, the bottom-up global emission inventories have been closer to methane emissions in the most carbon-intensive Representative Concentrations Pathway (RCP8.5) and higher than all other RCP scenarios. Bottom-up approaches suggest larger global emissions (736 TgCH4 yr(exp -1), range 596-884) mostly because of larger natural emissions from individual sources such as inland waters, natural wetlands and geological sources. Considering the atmospheric constraints on the top-down budget, it is likely that some of the individual emissions reported by the bottom-up approaches are overestimated, leading to too large global emissions. Latitudinal data from top-down emissions indicate a predominance of tropical emissions (approximately 64% of the global budget, less than 30deg N) as compared to mid (approximately 32%, 30-60deg N) and high northern latitudes (approximately 4%, 60-90deg N). Top-down inversions consistently infer lower emissions in China (approximately 58 TgCH4 yr(exp -1), range 51-72, minus14% ) and higher emissions in Africa (86 TgCH4 yr(exp -1), range 73-108, plus 19% ) than bottom-up values used as prior estimates. Overall, uncertainties for anthropogenic emissions appear smaller than those from natural sources, and the uncertainties on source categories appear larger for top-down inversions than for bottom-up inventories and models. The most important source of uncertainty on the methane budget is attributable to emissions from wetland and other inland waters. We show that the wetland extent could contribute 30-40% on the estimated range for wetland emissions. Other priorities for improving the methane budget include the following: (i) the development of process-based models for inland-water emissions, (ii) the intensification of methane observations at local scale (flux measurements) to constrain bottom-up land surface models, and at regional scale (surface networks and satellites) to constrain top-down inversions, (iii) improvements in the estimation of atmospheric loss by OH, and (iv) improvements of the transport models integrated in top-down inversions. The data presented here can be downloaded from the Carbon Dioxide Information Analysis Center (http://doi.org/10.3334/CDIAC/GLOBAL_ METHANE_BUDGET_2016_V1.1) and the Global Carbon Project.

Aquatic ecosystems play an important role in global methane cycling and many field studies have reported methane supersaturation in the oxic surface mixed layer (SML) of the ocean and in the epilimnion of lakes. The origin of methane formed under oxic condition is hotly debated and several pathways have recently been offered to explain the “methane paradox.” In this context, stable isotope measurements have been applied to constrain methane sources in supersaturated oxygenated waters. Here we present stable carbon isotope signatures for six widespread marine phytoplankton species, three haptophyte algae and three cyanobacteria, incubated under laboratory conditions. The observed isotopic patterns implicate that methane formed by phytoplankton might be clearly distinguished from methane produced by methanogenic archaea. Comparing results from phytoplankton experiments with isotopic data from field measurements, suggests that algal and cyanobacterial populations may contribute substantially to methane formation observed in the SML of oceans and lakes. Plain Language Summary Methane plays an important role in atmospheric chemistry and physics as it contributes to global warming and to the destruction of ozone in the stratosphere. Knowing the sources and sinks of methane in the environment is a prerequisite for understanding the global atmospheric methane cycle but also to better predict future climate change. Measurements of the stable carbon isotope composition of carbon—the ratio between the heavy and light stable isotope of carbon—help to identify methane sources in the environment and to distinguish them from other formation processes. We identified the carbon isotope fingerprint of methane released from phytoplankton including algal and cyanobacterial species. The observed isotope signature improves our understanding of methane cycling in the surface layers of aquatic environments helping us to better estimate methane emissions to the atmosphere. Key Points Stable carbon isotope values of methane emitted from six phytoplankton cultures incubated in the laboratory Isotope fractionation between methane source signature and biomass of widespread algal and cyanobacterial species Isotopic patterns of methane released by phytoplankton may be clearly distinguished from methane formed by methanogenic archaea

A rapid and sustained reduction of methane emissions has been proposed recently as a key strategy to meet the climate targets of the Paris Agreement. The social cost of methane (SCM), which expresses the climate damage cost associated with an additional metric ton of methane emitted, is a metric that can be used to design policies to reduce the emissions of this gas. Here, we extend the DICE-2016R2 model so that it includes an improved carbon cycle and energy balance model as well as methane emissions, methane abatement cost, and an atmospheric methane cycle explicitly to be able to provide consistent estimations of the SCM. We estimate the SCM to lie in the range 880–8100 USD/tCH4 in 2020, with a base case estimate of 4000 USD/tCH4. We find our base case estimate to be larger than the average SCM presented in other studies mainly due to the revised damage function we use. We also estimate the social cost of carbon (SCC) and find that SCM estimates are less sensitive to variations in the social discount rate than the SCC due to the relatively short lifetime of methane. Changes in the parameterization of the damage function have similar relative impacts on both SCM and SCC. Furthermore, we evaluate the ratio of SCM to SCC as an alternative metric to GWP-100 of CH4 to facilitate tradeoffs between these two gases. We find this ratio to lie in the range 7–33 in 2020, with a base case estimate of 21, based on an extensive sensitivity analysis with respect to the discount rate, damage cost, and underlying emission scenarios. We also show that the global warming potential (GWP) and the SCM to SCC ratio are almost the same if the inverse of the effective discounting (in the social cost calculations) is equal to the time horizon used to evaluate the GWP. For comparison, the most widely used GWP, i.e., with a time horizon of 100 years, equals 27, hence in the upper range of the ratio we find using the SCM to SCC ratio.

A combination of physicochemical and radiotracer analysis, high-throughput sequencing of the 16S rRNA, and particulate methane monooxygenase subunit A (pmoA) genes was used to link a microbial community profile with methane, sulfur, and nitrogen cycling processes. The objects of study were surface sediments sampled at five stations in the northern part of the Barents Sea. The methane content in the upper layers (0–5 cm) ranged from 0.2 to 2.4 µM and increased with depth (16–19 cm) to 9.5 µM. The rate of methane oxidation in the oxic upper layers varied from 2 to 23 nmol CH4 L−1 day−1 and decreased to 0.3 nmol L−1 day−1 in the anoxic zone at a depth of 16–19 cm. Sulfate reduction rates were much higher, from 0.3 to 2.8 µmol L−1 day−1. In the surface sediments, ammonia-oxidizing Nitrosopumilaceae were abundant; the subsequent oxidation of nitrite to nitrate can be carried out by Nitrospira sp. Aerobic methane oxidation could be performed by uncultured deep-sea cluster 3 of gamma-proteobacterial methanotrophs. Undetectable low levels of methanogenesis were consistent with a near complete absence of methanogens. Anaerobic methane oxidation in the deeper sediments was likely performed by ANME-2a-2b and ANME-2c archaea in consortium with sulfate-reducing Desulfobacterota. Sulfide can be oxidized by nitrate-reducing Sulfurovum sp. Thus, the sulfur cycle was linked with the anaerobic oxidation of methane and the nitrogen cycle, which included the oxidation of ammonium to nitrate in the oxic zone and denitrification coupled to the oxidation of sulfide in the deeper sediments. Methane concentrations and rates of microbial biogeochemical processes in sediments in the northern part of the Barents Sea were noticeably higher than in oligotrophic areas of the Arctic Ocean, indicating that an increase in methane concentration significantly activates microbial processes.

Drylands cover one-third of the Earth’s surface and are one of the largest terrestrial sinks for methane. Understanding the structure–function interplay between members of arid biomes can provide critical insights into mechanisms of resilience toward anthropogenic and climate-change-driven environmental stressors—water scarcity, heatwaves, and increased atmospheric greenhouse gases. This study integrates in situ measurements with culture-independent and enrichment-based investigations of methane-consuming microbiomes inhabiting soil in the Anza-Borrego Desert, a model arid ecosystem in Southern California, United States. The atmospheric methane consumption ranged between 2.26 and 12.73 μmol m2 h−1, peaking during the daytime at vegetated sites. Metagenomic studies revealed similar soil-microbiome compositions at vegetated and unvegetated sites, with Methylocaldum being the major methanotrophic clade. Eighty-four metagenome-assembled genomes were recovered, six represented by methanotrophic bacteria (three Methylocaldum, two Methylobacter, and uncultivated Methylococcaceae). The prevalence of copper-containing methane monooxygenases in metagenomic datasets suggests a diverse potential for methane oxidation in canonical methanotrophs and uncultivated Gammaproteobacteria. Five pure cultures of methanotrophic bacteria were obtained, including four Methylocaldum. Genomic analysis of Methylocaldum isolates and metagenome-assembled genomes revealed the presence of multiple stand-alone methane monooxygenase subunit C paralogs, which may have functions beyond methane oxidation. Furthermore, these methanotrophs have genetic signatures typically linked to symbiotic interactions with plants, including tryptophan synthesis and indole-3-acetic acid production. Based on in situ fluxes and soil microbiome compositions, we propose the existence of arid-soil reverse chimneys, an empowered methane sink represented by yet-to-be-defined cooperation between desert vegetation and methane-consuming microbiomes.

Soil functional genes in grasslands are crucial for processes like nitrogen fixation, nitrification, denitrification, methane production, and oxidation, integral to nitrogen and methane cycles. However, the impact of global changes on these genes is not well understood. We reviewed 84 studies to examine the effects of nitrogen addition (N), warming (W), increased precipitation (PPT +), decreased precipitation (PPT-), and elevated CO 2 (eCO 2 ) on these functional genes. For nitrogen cycling, global changes mainly boost genes involved in nitrification but reduce those in denitrification, with nirK being the most sensitive. Most nitrogen fixation-related genes did not show a significant response. Among single factors, N and PPT + have the most significant effects. The impact of global changes on nitrogen cycling genes is largely additive, and their interaction with N is particularly influential. For methane cycling, global changes notably affect mcrA , while only PPT + significantly reduces pmoA . The magnitude and duration of global change treatments are more critical than the treatment form for nitrogen cycling genes. For methane cycling, the form and intensity of nitrogen addition, along with treatment duration, affect pmoA abundance. We also identified a competitive relationship between methane oxidation and nitrification and a complex coupling with denitrification. This study provides new insights into microbial responses in nitrogen and methane cycling under global changes, with significant implications for experimental design and management strategies in grassland ecosystems.

Constraining the causes of past atmospheric methane variability is important for understanding links between methane and climate. Abrupt methane changes during the last deglaciation have been intensely studied for this purpose, but the relative importance of high-latitude and tropical sources remains poorly constrained. The methane interpolar concentration difference reflects past geographic emission variability, but existing records suffered from subtle but considerable methane production during analysis. Here, we report an ice-core-derived interpolar difference record covering the Last Glacial Maximum and deglaciation, with substantially improved temporal resolution, chronology and a critical correction for methane production in samples from Greenland. Using box models to infer latitudinal source changes, we show that tropical sources dominated abrupt methane variability of the deglaciation, highlighting their sensitivity to abrupt climate change and rapidly shifting tropical rainfall patterns. Northern extratropical emissions began increasing ~16,000 years ago, probably through wetland expansion and/or permafrost degradation induced by high-latitude warming, and contributed at most 25 Tg yr−1 (45% of the total emission increase) to the abrupt methane rise that coincided with rapid northern warming at the onset of the Bølling–Allerød interval. These constraints on deglacial climate–methane cycle interactions can improve the understanding of possible present and future feedbacks.Abrupt changes in atmospheric methane through the last deglaciation were largely the result of tropical sources responding to shifting rainfall patterns, according to a comparison of precisely dated ice cores in Greenland and Antarctica.