Atmospheric Methane Burden Research Articles

Fate of methane in canals draining tropical peatlands.

Tropical wetlands and freshwaters are major contributors to the growing atmospheric methane (CH4) burden. Extensive peatland drainage has lowered CH4 emissions from peat soils in Southeast Asia, but the canals draining these peatlands may be hotspots of CH4 emissions. Alternatively, CH4 oxidation (consumption) by methanotrophic microorganisms may attenuate emissions. Here, we used laboratory experiments and a synoptic survey of the isotopic composition of CH4 in 34 canals across West Kalimantan, Indonesia to quantify the proportion of CH4 that is consumed and therefore not emitted to the atmosphere. We find that CH4 oxidation mitigates 76.4 ± 12.0% of potential canal emissions, reducing emissions by ~70 mg CH4 m-2 d-1. Methane consumption also significantly impacts the stable isotopic fingerprint of canal CH4 emissions. As canals drain over 65% of peatlands in Southeast Asia, our results suggest that CH4 oxidation significantly influences landscape-scale CH4 emissions from these ecosystems.

δ13C methane source signatures from tropical wetland and rice field emissions.

The atmospheric methane (CH4) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH4 emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new δ13CCH4 signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. Long-term monitoring of δ13CCH4 in ambient air has been conducted at the Chacaltaya observatory, Bolivia and Southern Botswana. Both long-term records are dominated by biogenic CH4 sources, with isotopic signatures expected from wetland sources. From the longer-term Bolivian record, a seasonal isotopic shift is observed corresponding to wetland extent suggesting that there is input of relatively isotopically light CH4 to the atmosphere during periods of reduced wetland extent. This new data expands the geographical extent and range of measurements of tropical wetland and rice δ13CCH4 sources and hints at significant seasonal variation in tropical wetland δ13CCH4 signatures which may be important to capture in future global and regional models.This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

Temporospatial Analysis of methane trend in China over the last decade — Estimation of Anthropogenic Methane Emission and Contributions from Different Anthropogenic Sources

China’s atmospheric methane burden has been on the rise in the past decades. However, the relationship between trends and major anthropogenic sources of methane emissions in China has not been well understood. The temporal and spatial distribution of methane concentration in China is investigated using ten years of methane column data (2010-2019) from the Greenhouse Gases Observing Satellite (GOSAT). High-value regions of methane enhancement are found in the northern provinces of China (northeast Xinjiang, northwest Gansu, inner Mongolia, and the border region of Heilongjiang and Jilin). Based on 2010-2018 China Official Statistical Yearbook data and the latest research about methane emission factors, the major anthropogenic methane emissions in China through 2010-2017 are estimated. With Principal Component Analysis (PCA) method on data of estimated anthropogenic emissions and deseasonalized year-averaged methane concentrations in China, it is found that the top three anthropogenic sources with the greatest impact on methane emissions are coal mining, sanitary landfill, and domestic animal rumination. They could have a potential connection with methane enhancement trend high value regions.

Methane Emissions in a Chemistry-Climate Model: Feedbacks and Climate Response.

Understanding the past, present, and future evolution of methane remains a grand challenge. Here we have used a hierarchy of models, ranging from simple box models to a chemistry‐climate model (CCM), UM‐UKCA, to assess the contemporary and possible future atmospheric methane burden. We assess two emission data sets for the year 2000 deployed in UM‐UKCA against key observational constraints. We explore the impact of the treatment of model boundary conditions for methane and show that, depending on other factors, such as CO emissions, satisfactory agreement may be obtained with either of the CH4 emission data sets, highlighting the difficulty in unambiguous choice of model emissions in a coupled chemistry model with strong feedbacks. The feedbacks in the CH4‐CO‐OH system, and their uncertainties, play a critical role in the projection of possible futures. In a future driven by large increases in greenhouse gas forcing, increases in tropospheric temperature drive, an increase in water vapor, and, hence, [OH]. In the absence of methane emission changes this leads to a significant decrease in methane compared to the year 2000. However, adding a projected increase in methane emissions from the RCP8.5 scenario leads to a large increase in methane abundance. This is modified by changes to CO and NOx emissions. Clearly, future levels of methane are uncertain and depend critically on climate change and on the future emission pathways of methane and ozone precursors. We highlight that further work is needed to understand the coupled CH4‐CO‐OH system in order to understand better future methane evolution.

Reduced net methane emissions due to microbial methane oxidation in a warmer Arctic

Methane emissions from organic-rich soils in the Arctic have been extensively studied due to their potential to increase the atmospheric methane burden as permafrost thaws1–3. However, this methane source might have been overestimated without considering high-affinity methanotrophs (HAMs; methane-oxidizing bacteria) recently identified in Arctic mineral soils4–7. Herein we find that integrating the dynamics of HAMs and methanogens into a biogeochemistry model8–10 that includes permafrost soil organic carbon dynamics3 leads to the upland methane sink doubling (~5.5 Tg CH4 yr−1) north of 50 °N in simulations from 2000–2016. The increase is equivalent to at least half of the difference in net methane emissions estimated between process-based models and observation-based inversions11,12, and the revised estimates better match site-level and regional observations5,7,13–15. The new model projects doubled wetland methane emissions between 2017–2100 due to more accessible permafrost carbon16–18. However, most of the increase in wetland emissions is offset by a concordant increase in the upland sink, leading to only an 18% increase in net methane emission (from 29 to 35 Tg CH4 yr−1). The projected net methane emissions may decrease further due to different physiological responses between HAMs and methanogens in response to increasing temperature19,20. Models overestimate Arctic methane emissions compared to observations. Incorporating microbial dynamics into biogeochemistry models helps reconcile this discrepancy; high-affinity methanotrophs are an important part of the Arctic methane budget and double previous estimates of methane sinks.

Methane Mitigation: Methods to Reduce Emissions, on the Path to the Paris Agreement

AbstractThe atmospheric methane burden is increasing rapidly, contrary to pathways compatible with the goals of the 2015 United Nations Framework Convention on Climate Change Paris Agreement. Urgent action is required to bring methane back to a pathway more in line with the Paris goals. Emission reduction from “tractable” (easier to mitigate) anthropogenic sources such as the fossil fuel industries and landfills is being much facilitated by technical advances in the past decade, which have radically improved our ability to locate, identify, quantify, and reduce emissions. Measures to reduce emissions from “intractable” (harder to mitigate) anthropogenic sources such as agriculture and biomass burning have received less attention and are also becoming more feasible, including removal from elevated‐methane ambient air near to sources. The wider effort to use microbiological and dietary intervention to reduce emissions from cattle (and humans) is not addressed in detail in this essentially geophysical review. Though they cannot replace the need to reach “net‐zero” emissions of CO2, significant reductions in the methane burden will ease the timescales needed to reach required CO2reduction targets for any particular future temperature limit. There is no single magic bullet, but implementation of a wide array of mitigation and emission reduction strategies could substantially cut the global methane burden, at a cost that is relatively low compared to the parallel and necessary measures to reduce CO2, and thereby reduce the atmospheric methane burden back toward pathways consistent with the goals of the Paris Agreement.

Very Strong Atmospheric Methane Growth in the 4 Years 2014–2017: Implications for the Paris Agreement

AbstractAtmospheric methane grew very rapidly in 2014 (12.7 ± 0.5 ppb/year), 2015 (10.1 ± 0.7 ppb/year), 2016 (7.0 ± 0.7 ppb/year), and 2017 (7.7 ± 0.7 ppb/year), at rates not observed since the 1980s. The increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1,775 ppb in 2006 to 1,850 ppb in 2017. Simultaneously the13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, now trending negative for more than a decade. The causes of methane's recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and subtropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at >5 ppb/year in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained.

An assessment of natural methane fluxes simulated by the CLASS-CTEM model

Abstract. Natural methane emissions from wetlands and fire, and soil uptake of methane, simulated using the Canadian Land Surface Scheme and Canadian Terrestrial Ecosystem (CLASS-CTEM) modelling framework, over the historical 1850–2008 period, are assessed by using a one-box model of atmospheric methane burden. This one-box model also requires anthropogenic emissions and the methane sink in the atmosphere to simulate the historical evolution of global methane burden. For this purpose, global anthropogenic methane emissions for the period 1850–2008 were reconstructed based on the harmonized representative concentration pathway (RCP) and Emission Database for Global Atmospheric Research (EDGAR) data sets. The methane sink in the atmosphere is represented using bias-corrected methane lifetimes from the Canadian Middle Atmosphere Model (CMAM). The resulting evolution of atmospheric methane concentration over the historical period compares reasonably well with observation-based estimates (correlation = 0.99, root mean square error = 35 ppb). The modelled natural emissions are also assessed using an inverse procedure where the methane lifetimes required to reproduce the observed year-to-year increase in atmospheric methane burden are calculated based upon the specified global anthropogenic and modelled natural emissions that we have used here. These calculated methane lifetimes over the historical period fall within the uncertainty range of observation-based estimates. The present-day (2000–2008) values of modelled methane emissions from wetlands (169 Tg CH4 yr−1) and fire (27 Tg CH4 yr−1), methane uptake by soil (29 Tg CH4 yr−1), and the budget terms associated with overall anthropogenic and natural emissions are consistent with estimates reported in a recent global methane budget that is based on top-down approaches constrained by observed atmospheric methane burden. The modelled wetland emissions increase over the historical period in response to both increases in precipitation and in atmospheric CO2 concentration. This increase in wetland emissions over the historical period yields evolution of the atmospheric methane concentration that compares better with observation-based values than the case when wetland emissions are held constant over the historical period.

Carbon isotopic signature of coal-derived methane emissions to the atmosphere: from coalification to alteration

Abstract. Currently, the atmospheric methane burden is rising rapidly, but the extent to which shifts in coal production contribute to this rise is not known. Coalbed methane emissions into the atmosphere are poorly characterised, and this study provides representative δ13CCH4 signatures of methane emissions from specific coalfields. Integrated methane emissions from both underground and opencast coal mines in the UK, Australia and Poland were sampled and isotopically characterised. Progression in coal rank and secondary biogenic production of methane due to incursion of water are suggested as the processes affecting the isotopic composition of coal-derived methane. An averaged value of −65 ‰ has been assigned to bituminous coal exploited in open cast mines and of −55 ‰ in deep mines, whereas values of −40 and −30 ‰ can be allocated to anthracite opencast and deep mines respectively. However, the isotopic signatures that are included in global atmospheric modelling of coal emissions should be region- or nation-specific, as greater detail is needed, given the wide global variation in coal type.

Organic matter mineralisation in the hypolimnion of an eutrophic Maar lake

Constraints on hypolimnetic methane production in productive lakes are of interest owing to the importance of methane emissions from lakes for the atmospheric methane burden. We studied carbon fluxes and terminal electron accepting processes in the hypolimnion of eutrophic Meerfelder Maar, Germany. Carbon fluxes from epilimnion and sediments were estimated using sediment traps, porewater analysis and incubation techniques. The contribution of various redox processes to overall dissolved inorganic carbon (DIC) production was quantified from a seasonal hypolimnetic mass balance. Carbon sedimentation rates were 22 mmol m−2 d−1 from the epilimnion and 20–34 mmol m−2 d−1 from the hypolimnion. Anaerobic respiration accounted for a DIC production of 21 mmol m−2 d−1, and was thus capable of using a substantial part of primary production. The diffusive C flux from the sediment was small in comparison, although potential respiration rates in incubation experiments reached 63 – 75 mmol m−2 d−1. Ebullition of methane occurred, but could not be quantified. Following depletion of dissolved oxygen (DO) and nitrate, iron and sulphate reduction and methanogenesis proceeded concurrently in the hypolimnion. Estimated hypolimnetic DIC production decreased in the order methanogenesis (2267 mmol m−2) > oxic respiration (880 mmol m−2) > sulphate reduction (575mmolm−2) > denitrification (159 mmol m−2) > acetogenesis (157 mmol m−2) > iron reduction (19 – 144 mmol m−2). Methane production thus dominated respiration and was not inhibited by sulphate and iron reduction. It also strongly accelerated with increased carbon sedimentation rates in September, which apparently eased limiting factors on the process. Hypolimnetic methane production is thus likely an important process in similar lakes, even in the presence of other electron acceptors, and will contribute to methane emissions during fall turnover.

Livestock methane emission and its perspective in the global methane cycle

Over the past three centuries, the atmospheric methane burden has grown 2.5-fold, reaching levels unprecedented in at least 650 000 years. Agricultural expansion has played a large part in this anthropogenic signal, with enterically fermented methane emitted by farmed ruminant livestock accounting for about one quarter of all anthropogenic emissions. This paper summarises the range of measurements that give confidence in estimates of the emission per animal and per unit feed intake and in their extrapolation to national and global emission inventories, while noting also some of the inherent uncertainties. Global emissions are discussed in the context of the evolving global methane cycle.

Atmospheric methane levels off: Temporary pause or a new steady‐state?

The globally‐averaged atmospheric methane abundance determined from an extensive network of surface air sampling sites was constant at ∼1751 ppb from 1999 through 2002. Assuming that the methane lifetime has been constant, this implies that during this 4‐year period the global methane budget has been at steady state. We also observed a significant decrease in the difference between northern and southern polar zonal annual averages of CH4 from 1991 to 1992. Using a 3‐D transport model, we show that this change is consistent with a decrease in CH4 emissions of ∼10 Tg CH4 from north of 50°N in the early‐1990s. This decrease in emissions may have accelerated the global methane budget towards steady state. Based on current knowledge of the global methane budget and how it has changed with time, it is not possible to tell if the atmospheric methane burden has peaked, or if we are only observing a persistent, but temporary pause in its increase.

Stratospheric ozone depletion at northern mid latitudes in the 21st century: The importance of future concentrations of greenhouse gases nitrous oxide and methane

There is evidence that the halogen loading of the atmosphere has peaked and stratospheric ozone levels are expected to recover to pre‐1980 levels this century. However, N2O concentrations in the atmosphere are increasing, resulting in increasing levels of NOx in the stratosphere. In addition, the growth rate in the atmospheric methane burden has declined in recent years, leading to the suggestion that methane emissions have stabilized. A 2‐D chemical transport model is used to calculate stratospheric ozone from 2000 to 2100 for a range of IPCC scenarios. The model predicts that mid‐latitude stratospheric ozone will recover only partially towards pre‐1980 levels over the next 50 years, but will then decline, largely due to increases in stratospheric NOx. If greenhouse gas mitigation strategies result in lower future methane levels, mid‐latitude stratospheric ozone levels in 2100 are predicted to be lower than current values, particularly in late summer and autumn.

Continuing decline in the growth rate of the atmospheric methane burden

The global atmospheric methane burden has more than doubled since pre-industrial times1,2, and this increase is responsible for about 20% of the estimated change in direct radiative forcing due to anthropogenic greenhouse-gas emissions. Research into future climate change and the development of remedial environmental policies therefore require a reliable assessment of the long-term growth rate in the atmospheric methane load. Measurements have revealed that although the global atmospheric methane burden continues to increase2 with significant interannual variability3,4, the overall rate of increase has slowed2,5. Here we present an analysis of methane measurements from a global air sampling network that suggests that, assuming constant OH concentration, global annual methane emissions have remained nearly constant during the period 1984–96, and that the decreasing growth rate in atmospheric methane reflects the approach to a steady state on a timescale comparable to methane's atmospheric lifetime. If the global methane sources and OH concentration continue to remain constant, we expect average methane mixing ratios to increase slowly from today's 1,730 nmol mol−1 to ∼1,800 nmol mol−1, with little change in the contribution of methane to the greenhouse effect.

Landfill CH 4: Rates, fates, and role in global carbon cycle

Published estimates for worldwide landfill methane emissions range from 9 to 70 Tg yr −1. Field and laboratory studies suggest that maximum methane yields from landfilled refuse are about 0.06 to 0.09 m 3 (dry kg) −1 refuse, depending on moisture content and other variables, such as organic loading, buffering capacity, and nutrients in landfill microenvironments. Methane yields may vary by more than an order of magnitude within a given site. Fates for landfill methane include (1) direct or delayed emission to the atmosphere through landfill cover materials or surface soils; (2) oxidation by methanotrophs in cover soils, with resulting emission of carbon dioxide; or (3) recovery of methane followed by combustion to produce carbon dioxide. The percent methane assigned to each pathway will vary among field sites and, for individual sites, through time. Nevertheless, a general framework for a landfill methane balance can be developed by consideration of landfill age, engineering and management practices, cover soil characteristics, and water balance. Direct measurements of landfill methane emissions are sparse, with rates between 10 −6 and 10 −8 g cm −2 s −1; very high rates of 400 kg m −2 yr −1 have been measured at a semiarid unvegetated site. The proportion of landfill carbon that is ultimately converted to methane and carbon dioxide is problematical; the literature suggests that, at best, 25% to 40% of refuse carbon can be converted to biogas carbon. Cellulose contributes the major portion of the methane potential. Routine excavation of nondecomposed cellulosic materials after one or two decades of landfill burial suggests that uniformly high conversion rates are rarely attained at field sites. For a longer-term viewpoint, considering archaeologic and geologic preservation of organic carbon through anaerobic burial, one can speculate that widespread landfilling practices in developed and developing countries may be providing a measurable sink for organic carbon, as well as increasing the atmospheric methane burden.

2<sub>ν3</sub>帯Q枝吸収による大気メタン量の測定

2<sub>ν3</sub>帯Q枝吸収による大気メタン量の測定