Changes In Methane Emissions Research Articles

Assessing the economic implications of carbon emissions on climate change: Estimating the impact using methane-adjusted DICE model

This study aims to highlight the importance of accurately estimating methane emissions, one of the most dangerous and important greenhouse gases in the context of climate change. By incorporating methane emissions as a variable within the integrated assessment model DICE (Dynamic Integrated Climate-Economy model), we investigate how these emissions influence temperature changes and subsequently impact economic policies, including climate economic policies, carbon pricing, and non-price factors. We use the existing DICE-2020 and DICE-2023 models as references for our analysis. In addition to industrial emissions that can be detected through satellite observations, we address the challenge of estimating natural emissions from wetlands and permafrost, which leak gradually and are difficult to detect. By considering these emissions, we account for their exogenous nature and their divergence from the current situation. Our study reveals that incorporating methane emissions into the DICE model has significant implications for global temperature outcomes and subsequent policy changes. We find that by implementing existing methane reduction policies, which includes cutting the level of methane emissions in half and increase the carbon price in 4 times to 500 USD per ton, it is possible to achieve the more ambitious goal of limiting the global temperature increase to 1.5 °C by 2100 in an optimistic scenario instead of the common target of 2 °C. More pessimistic scenarios that do not imply big change in methane emissions, but the same numerical data for carbon price, still suggests the possibility of keeping the global temperature below 2 °C. The findings of this study emphasize the importance of estimating methane emissions in efforts to mitigate climate change. Recognizing the impact of methane on global temperature change, policymakers can make informed decisions regarding economic policies, carbon pricing, to effectively address the challenges posed by climate change. This research contributes to the field by incorporating methane emissions into the DICE model, providing a more complete understanding of its influence on global climate outcomes and economic implications. Additionally, by highlighting the potential benefits of methane reduction measures, this study provides information on how to achieve more ambitious climate targets.

Fine-Scale Spatial Variability of Greenhouse Gas Emissions From a Subantarctic Peatland Bog.

Peatlands are recognized as crucial greenhouse gas sources and sinks and have been extensively studied. Their emissions exhibit high spatial heterogeneity when measured on site using flux chambers. However, the mechanism by which this spatial variability behaves on a very fine scale remains unclear. This study investigates the fine-scale spatial variability of greenhouse gas emissions from a subantarctic Sphagnum peatland bog. Using a recently developed skirt chamber, methane emissions and ecosystem respiration (as carbon dioxide) were measured at a submeter scale resolution, at five specific 3 × 3 m plots, which were examined across the site throughout a single campaign during the Austral summer season. The results indicated that methane fluxes were significantly less homogeneously distributed compared with ecosystem respiration. Furthermore, we established that the spatial variation scale, i.e., the minimum spatial domain over which notable changes in methane emissions and ecosystem respiration occur, was <0.56 m2. Factors such as ground height relative to the water table and vegetation coverage were analyzed. It was observed that Tetroncium magellanicum exhibited a notable correlation with higher methane fluxes, likely because of the aerenchymatous nature of this species, facilitating gas transport. This study advances understanding of gas exchange patterns in peatlands but also emphasizes the need for further efforts for characterizing spatial dynamics at a very fine scale for precise greenhouse gas budget assessment.

Paving the way for sustainable agriculture: An analysis of evolution and driving forces of methane emissions reduction in China

The agriculture sector in China faces the dual challenge of ensuring food security while reducing methane emissions. This study investigates the factors driving changes in methane emissions from rice cultivation, livestock manure, and enteric fermentation in the agricultural sector between 2010 and 2019. First, we use the extended Logarithmic Mean Divisia Index method to measure four effects—emission source structure, emission intensity, economic activity, and population effects. Second, Tapio's decoupling index is employed to analyze the association between export-induced methane emissions and economic growth over the entire period. The results show that (1) rice cultivation is the primary contributor to the overall increase in national CH4 emissions, particularly after 2014, while CH4 emissions from other sources have decreased; (2) economic activity is the key driver of rising methane emissions, while emission intensity has the opposite impact, albeit with a decreasing contribution; and (3) decoupling states exhibit weak and strong decoupling recently, indicating the preliminary performance of the transformation of farm management. Furthermore, we find heterogeneous and spatially imbalanced performance in CH4 emission mitigation among provinces. Although China is on the right track to reduce CH4 emissions and upgrade the domestic and international agricultural structure, this has not been achieved at the provincial level.

Methane emission and influencing factors of China's oil and natural gas sector in 2020–2060: A source level analysis

The Chinese oil and gas industry requires targeted policies to reduce methane emissions. To achieve this goal, it is necessary to predict future methane emission trends and analyze the factors that influence them. However, changing economic development patterns, insufficient analysis of various factors influencing emissions, and inadequate resolution of methane emission inventories have made these goals difficult to achieve. Accordingly, this study aims to expand the methane emission estimation method to compile source-level emission inventories for future emissions, analyze the factors influencing them, and form a mechanistic understanding of the methane emissions from the local oil and gas industry. The research results indicate that methane emissions deriving from this industry will increase rapidly before 2030, after which they will decline slowly in all scenarios. The production and utilization processes in the natural gas supply chain, i.e., compressors and liquid unloading, include the main sources of methane emissions. Emissions are affected significantly by total production and consumption. Change in the overall supply and demand of natural gas affects change in methane emissions more significantly than adopting new technologies and strengthening facility maintenance, i.e., the overall supply and demand of natural gas are the dominant factors in controlling methane emissions. This study suggests that controlling the total demand for oil and gas should be at the core of the methane emission control policy for the local oil and gas industry. Moreover, equipment maintenance and emission reduction technologies should be used more effectively to reduce total emissions.

Atmospheric methane since the last glacial maximum was driven by wetland sources

Abstract. Atmospheric methane (CH4) has changed considerably in the time between the last glacial maximum (LGM) and the preindustrial (PI) periods. We investigate these changes in transient experiments with an Earth system model capable of simulating the global methane cycle interactively, focusing on the rapid changes during the deglaciation, especially pronounced in the Bølling–Allerød (BA) and Younger Dryas (YD) periods. We consider all relevant natural sources and sinks of methane and examine the drivers of changes in methane emissions as well as in the atmospheric lifetime of methane. We find that the evolution of atmospheric methane is largely driven by emissions from tropical wetlands, while variations in the methane atmospheric lifetime are small but not negligible. Our model reproduces most changes in atmospheric methane very well, with the exception of the mid-Holocene decrease in methane, although the timing of ice-sheet meltwater fluxes needs to be adjusted slightly in order to exactly reproduce the variations in the BA and YD.

Attribution of the 2020 surge in atmospheric methane by inverse analysis of GOSAT observations

Atmospheric methane mixing ratio rose by 15 ppbv between 2019 and 2020, the fastest growth rate on record. We conduct a global inverse analysis of 2019–2020 Greenhouse Gases Observing Satellite observations of atmospheric methane to analyze the combination of sources and sinks driving this surge. The imbalance between sources and sinks of atmospheric methane increased by 31 Tg a−1 from 2019 to 2020, representing a 36 Tg a−1 forcing (direct changes in methane emissions and OH concentrations) on the methane budget away from steady state. 86% of the forcing in the base inversion is from increasing emissions (82 ± 18% in the nine-member inversion ensemble), and only 14% is from decrease in tropospheric OH. Half of the increase in emissions is from Africa (15 Tg a−1) and appears to be driven by wetland inundation. There is also a large relative increase in emissions from Canada and Alaska (4.8 Tg a−1, 24%) that could be driven by temperature sensitivity of boreal wetland emissions.

Climate-driven chemistry and aerosol feedbacks in CMIP6 Earth system models

Abstract. Feedbacks play a fundamental role in determining the magnitude of the response of the climate system to external forcing, such as from anthropogenic emissions. The latest generation of Earth system models includes aerosol and chemistry components that interact with each other and with the biosphere. These interactions introduce a complex web of feedbacks that is important to understand and quantify. This paper addresses multiple pathways for aerosol and chemical feedbacks in Earth system models. These focus on changes in natural emissions (dust, sea salt, dimethyl sulfide, biogenic volatile organic compounds (BVOCs) and lightning) and changes in reaction rates for methane and ozone chemistry. The feedback terms are then given by the sensitivity of a pathway to climate change multiplied by the radiative effect of the change. We find that the overall climate feedback through chemistry and aerosols is negative in the sixth Coupled Model Intercomparison Project (CMIP6) Earth system models due to increased negative forcing from aerosols in a climate with warmer surface temperatures following a quadrupling of CO2 concentrations. This is principally due to increased emissions of sea salt and BVOCs which are sensitive to climate change and cause strong negative radiative forcings. Increased chemical loss of ozone and methane also contributes to a negative feedback. However, overall methane lifetime is expected to increase in a warmer climate due to increased BVOCs. Increased emissions of methane from wetlands would also offset some of the negative feedbacks. The CMIP6 experimental design did not allow the methane lifetime or methane emission changes to affect climate, so we found a robust negative contribution from interactive aerosols and chemistry to climate sensitivity in CMIP6 Earth system models.

Infectious Diseases, Livestock, and Climate: A Vicious Cycle?

Ruminant livestock are a significant contributor to global methane emissions. Infectious diseases have the potential to exacerbate these contributions by elevating methane outputs associated with animal production. With the increasing spread of many infectious diseases, the emergence of a vicious climate–livestock–disease cycle is a looming threat.

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.

Marginal methane emission estimation from the natural gas system

A new cause-based approach was used to estimate the change in methane emissions from the natural gas system resulting from a change in throughput. The analysis builds upon prior work (Mac Kinnon et al., 2018) positing that a cause-based, marginal approach to estimating methane emission impacts of reducing or increasing natural gas use was more accurate than assuming that methane emission vary one-for-one with throughput. The goal of this work is to determine the relationship between methane emissions and changes in throughput both over short time horizons where the gas infrastructure is fixed and over time periods where system expansion (or retirement) and technological improvements via component replacement occur. The results show that methane emissions change with throughput but the relative change in emissions is less than the relative change in throughput. There are many components (emissions sources) in the natural gas system that emit the same amount of methane to the atmosphere regardless of their operational mode; meaning some emissions sources have no or only partial dependence on throughput. As a result, reducing natural gas consumption in the future will not yield a directly proportional reduction in the methane emissions. It is believed that the results of this study will help energy policymakers to understand better the effect of policies aimed at reducing natural gas use on greenhouse gas (GHG) emissions and where such policies should be applied (e.g. system operator or end user).

A model for the accurate estimation of methane emissions in landfills

Landfills are one of the major sources of methane (CH 4 ) emissions. Prediction of CH4 emissions from landfills is important in estimating power generation potential and greenhouse gas (GHG) emissions from landfills. The most widely used landfill gas (LFG) models developed based on the first order decay (FOD) reaction do not take into account changing waste composition and landfill site conditions in methane estimations. The aim of this study was therefore to develop a LFG model for estimation of methane emissions from landfills in Lagos metropolis. Field investigations were carried out to determine waste composition, waste disposal rates and site conditions relevant for methane emissions estimation. Waste composition studies were conducted and waste fractions were divided into rapidly, moderately and slowly degrading. The output of the model was verified with the US EPA Landfill Gas Emission model (Land GEM). Results revealed maximum CH 4 emissions estimated occurred at the end of landfill’s closure. Methane generation potential (𝑳𝒐) and methane generation rate (𝒌) parameters were dependent on waste composition and site conditions. Model verification also showed methane emissions peaked at the end of landfill’s closure for both models and variation in modelling parameters by Land GEM model resulted in significant change in methane emissions. Keywords : Methane emissions, Landfills, Municipal solid waste, landfill gas, Land GEM model

Rapid and reliable assessment of methane impacts on climate

Abstract. It is clear that the most effective way to limit global temperature rise and associated impacts is to reduce human emissions of greenhouse gases, including methane. However, quantification of the climate benefits of mitigation options are complicated by the contrast in the timescales at which short-lived climate pollutants, such as methane, persist in the atmosphere compared to carbon dioxide. Whereas simple metrics fail to capture the differential impacts across all timescales, sophisticated climate models that can address these temporal dynamics are often inaccessible, time-intensive, require special infrastructure, and include high unforced interannual variability that makes it difficult to analyse small changes in forcings. On the other hand, reduced-complexity climate models that use basic knowledge from observations and complex Earth system models offer an ideal compromise in that they provide quick, reliable insights into climate responses, with only limited computational infrastructure needed. They are particularly useful for simulating the response to forcings of small changes in different climate pollutants, due to the absence of internal variability. In this paper, we build on previous evaluations of the freely available and easy-to-run reduced-complexity climate model MAGICC by comparing temperature responses to historical methane emissions to those from a more complex coupled global chemistry–climate model, GFDL-CM3. While we find that the overall forcings and temperature responses are comparable between the two models, the prominent role of unforced variability in CM3 demonstrates how sophisticated models are potentially inappropriate tools for small forcing scenarios. On the other hand, we find that MAGICC can easily and rapidly provide robust data on climate responses to changes in methane emissions with clear signals unfettered by variability. We are therefore able to build confidence in using reduced-complexity climate models such as MAGICC for purposes of understanding the climate implications of methane mitigation.

The Structural and Functional Capacity of Ruminal and Cecal Microbiota in Growing Cattle Was Unaffected by Dietary Supplementation of Linseed Oil and Nitrate.

Microorganisms in the digestive tract of ruminants differ in their functionality and ability to use feed constituents. While cecal microbiota play an important role in post-rumen fermentation of residual substrates undigested in the rumen, limited knowledge exists regarding its structure and function. In this trial we investigated the effect of dietary supplementation with linseed oil and nitrate on methane emissions and on the structure of ruminal and cecal microbiota of growing bulls. Animals were allocated to either a CTL (control) or LINNIT (CTL supplemented with 1.9% linseed and 1.0% nitrates) diet. Methane emissions were measured using the GreenFeed system. Microbial diversity was assessed using amplicon sequencing of microbial genomic DNA. Additionally, total RNA was extracted from ruminal contents and functional mcrA and mtt genes were targeted in amplicon sequencing approach to explore the diversity of functional gene expression in methanogens. LINNIT had no effect on methane yield (g/kg DMI) even though it decreased methane production by 9% (g/day; P < 0.05). Methanobrevibacter- and Methanomassiliicoccaceae-related OTUs were more abundant in cecum (72 and 24%) compared to rumen (60 and 11%) irrespective of the diet (P < 0.05). Feeding LINNIT reduced the relative abundance of Methanomassiliicoccaceae mcrA cDNA reads in the rumen. Principal component analysis revealed significant differences in taxonomic composition and abundance of bacterial communities between rumen and cecum. Treatment decreased the relative abundance of a few Ruminococcaceae genera, without affecting global bacterial community structure. Our research confirms a high level of heterogeneity in species composition of microbial consortia in the main gastrointestinal compartments where feed is fermented in ruminants. There was a parallel between the lack of effect of LINNIT on ruminal and cecal microbial community structure and functions on one side and methane emission changes on the other. These results suggest that the sequencing strategy used here to study microbial diversity and function accurately reflected the absence of effect on methane phenotypes in bulls treated with linseed plus nitrate.

Regional development and climate change mitigation modelling of municipal solid waste emissions in the middle east

AbstractThis study adapts a climate change mitigation model to assess impacts of municipal solid wastes in the Middle East and North Africa region, taking into account the future development of integrated waste management infrastructure involving collection, treatment and disposal. Mitigated and unmitigated disposal scenarios including waste growth reduction are used to estimate disposal methane emission changes at regional and subregional level. This study shows that unmitigated regional emissions by 2050 could rise to 296 542 GgCO2eq from a current level in 2015 of 128 184 GgCO2eq additionally decreasing annually by up to 8.75% falling to 10.16% due to waste characterisation changes as developing countries in the region progress to developed country status. A further annual rise, due to hotter drier climate conditions experienced as desertification, of up to a range of 1.70% increasing to 2.29% is also demonstrated. The findings enable regional planners to better assess and develop plans for implementation of integrated waste management policies.

Wetland methane emissions altered by vegetation disturbance: An interaction between stem clipping and nutrient enrichment

Plant-mediated transport is the dominant means of methane release from vegetated wetlands. Whether aboveground plant disturbances affect the emission of methane is largely unknown. We tested the effects of stem clipping on methane emissions from freshwater wetland mesocosms vegetated with Sagittaria lancifolia, Panicum hemitomon, Echinochloa walteri, or Eleocharis macrostachya under different nutrient regimes. Mesocosms vegetated with E. macrostachya showed an effect of treatments on CH4 emission which was elevated in high nutrient level treatment and suppressed in mid nutrient level treatment as compared to controls when the plant was clipped to 3cm above water line. S. lancifolia and P. hemitomon showed a similar pattern, and E. walteri showed no response to treatments. Mean CH4 emission significantly differed between species, being up to 56% higher in mesocosms vegetated with P. hemitomon. Treatments where plants were clipped below the water line had low initial emission rates, but these increased after three days. Sediment samples were analyzed for patterns in the microbial functional genes mcrA (methanogens) and pmoA (methanotrophs), but no changes in microbial communities in response to treatments were observed. Changes in methane emissions due to plant disturbances in wetlands warrant further investigation, but this study demonstrates that the available nutrient pool must also be considered. Additionally, initial responses to treatments were observed, but these responses tended to dissipate after three days, emphasizing the importance of monitoring CH4 emissions over time rather than only observing initial responses. Our results suggest that nutrient enrichment alone can suppress CH4 emission, however interactions with plant disturbances can be of equal importance.

Contribution of oil and natural gas production to renewed increase in atmospheric methane (2007–2014): top–down estimate from ethane and methane column observations

Abstract. Harmonized time series of column-averaged mole fractions of atmospheric methane and ethane over the period 1999–2014 are derived from solar Fourier transform infrared (FTIR) measurements at the Zugspitze summit (47° N, 11° E; 2964 m a.s.l.) and at Lauder (45° S, 170° E; 370 m a.s.l.). Long-term trend analysis reveals a consistent renewed methane increase since 2007 of 6.2 [5.6, 6.9] ppb yr−1 (parts-per-billion per year) at the Zugspitze and 6.0 [5.3, 6.7] ppb yr−1 at Lauder (95 % confidence intervals). Several recent studies provide pieces of evidence that the renewed methane increase is most likely driven by two main factors: (i) increased methane emissions from tropical wetlands, followed by (ii) increased thermogenic methane emissions due to growing oil and natural gas production. Here, we quantify the magnitude of the second class of sources, using long-term measurements of atmospheric ethane as a tracer for thermogenic methane emissions. In 2007, after years of weak decline, the Zugspitze ethane time series shows the sudden onset of a significant positive trend (2.3 [1.8, 2.8] × 10−2 ppb yr−1 for 2007–2014), while a negative trend persists at Lauder after 2007 (−0.4 [−0.6, −0.1] × 10−2 ppb yr−1). Zugspitze methane and ethane time series are significantly correlated for the period 2007–2014 and can be assigned to thermogenic methane emissions with an ethane-to-methane ratio (EMR) of 12–19 %. We present optimized emission scenarios for 2007–2014 derived from an atmospheric two-box model. From our trend observations we infer a total ethane emission increase over the period 2007–2014 from oil and natural gas sources of 1–11 Tg yr−1 along with an overall methane emission increase of 24–45 Tg yr−1. Based on these results, the oil and natural gas emission contribution (C) to the renewed methane increase is deduced using three different emission scenarios with dedicated EMR ranges. Reference scenario 1 assumes an oil and gas emission combination with EMR = 7.0–16.2 %, which results in a minimum contribution C &gt; 39 % (given as lower bound of 95 % confidence interval). Beside this most plausible scenario 1, we consider two less realistic limiting cases of pure oil-related emissions (scenario 2 with EMR = 16.2–31.4 %) and pure natural gas sources (scenario 3 with EMR = 4.4–7.0 %), which result in C &gt; 18 % and C &gt; 73 %, respectively. Our results suggest that long-term observations of column-averaged ethane provide a valuable constraint on the source attribution of methane emission changes and provide basic knowledge for developing effective climate change mitigation strategies.

Biological implications of longevity in dairy cows: 2. Changes in methane emissions and efficiency with age

Previous studies indicated that absolute CH4 emissions and CH4 yield might increase and that milk production efficiency might decrease with age in cattle. Both would make strategies to increase longevity in dairy cattle less attractive. These aspects were experimentally determined in Brown Swiss cattle distributed continuously across a large age range. Thirty lactating dairy cows (876–3,648 d of age) received diets consisting of hay, corn silage, and grass pellets supplemented with 0 or 5kg of concentrate per day. Twelve heifers (199–778 d of age) received hay only. Cows and heifers were members of herds subjected to the 2 different feeding regimens (with or without concentrate) for the past 10 yr. Methane emissions were measured individually for 2 d in open-circuit respiration chambers, followed by quantifying individual feed intake and milk yield over 8 d. Additional data on digestibility, rumination time, and passage time of feed of all experimental animals were available. Regression analyses were applied to evaluate effects of age and feeding regimen. Body weight, milk yield, and the hay proportion of forage dry matter intake were considered as covariates. Methane emissions per unit of intake, body weight, and milk yield were significantly related to age. Their development in the cows with age was characterized by an increase to maximum at around 2,000 d of age, followed by a decline. This response was not accompanied by corresponding age-related changes in intake, chewing activity, digesta passage time, and digestibility of organic matter, which would have explained shifts in CH4. However, fiber digestibility showed a similar change with age as methane emissions, resulting in quite stable methane emissions per unit of digestible fiber. As expected, methane emissions intensity per unit of milk produced was greater by 8% without concentrate than with concentrate, but no difference was noted in the response to age when the animals were subjected to different feeding regimens. The efficiency of milk production was only marginally influenced by age and diet, and no different response was observed for age in the 2 dietary regimens. In conclusion, life cycle analyses of milk production systems focusing on longevity should consider changing methane yields with age in addition to the variation in environmental costs for replacements of culled cows.

Exploring the influence of ancient and historic megaherbivore extirpations on the global methane budget

Globally, large-bodied wild mammals are in peril. Because "megamammals" have a disproportionate influence on vegetation, trophic interactions, and ecosystem function, declining populations are of considerable conservation concern. However, this is not new; trophic downgrading occurred in the past, including the African rinderpest epizootic of the 1890s, the massive Great Plains bison kill-off in the 1860s, and the terminal Pleistocene extinction of megafauna. Examining the consequences of these earlier events yields insights into contemporary ecosystem function. Here, we focus on changes in methane emissions, produced as a byproduct of enteric fermentation by herbivores. Although methane is ∼ 200 times less abundant than carbon dioxide in the atmosphere, the greater efficiency of methane in trapping radiation leads to a significant role in radiative forcing of climate. Using global datasets of late Quaternary mammals, domestic livestock, and human population from the United Nations as well as literature sources, we develop a series of allometric regressions relating mammal body mass to population density and CH4 production, which allows estimation of methane production by wild and domestic herbivores for each historic or ancient time period. We find the extirpation of megaherbivores reduced global enteric emissions between 2.2-69.6 Tg CH4 y(-1) during the various time periods, representing a decrease of 0.8-34.8% of the overall inputs to tropospheric input. Our analyses suggest that large-bodied mammals have a greater influence on methane emissions than previously appreciated and, further, that changes in the source pool from herbivores can influence global biogeochemical cycles and, potentially, climate.

Accuracy of micrometeorological techniques for detecting a change in methane emissions from a herd of cattle

Micrometeorological techniques are effective in measuring methane (CH4) emission rates at the herd scale, but their suitability as verification tools for emissions mitigation depends on the uncertainty with which they can detect a treatment difference. An experiment was designed to test for a range of techniques whether they could detect a change in weekly mean emission rate from a herd of cattle, in response to a controlled change in feed supply. The cattle were kept in an enclosure and fed pasture baleage, of amounts increasing from one week to the next. Methane emission rates were measured at the herd scale by the following techniques: (1) an external tracer-ratio technique, releasing nitrous oxide (N2O) from canisters on the animals’ necks and measuring line-averaged CH4 and N2O mole fractions with Fourier-transform infra-red (FTIR) spectrometers deployed upwind and downwind of the cattle, (2) a mass-budget technique using vertical profiles of wind speed and CH4 mole fraction, (3) a dispersion model, applied separately to CH4 mole fraction data from the FTIR spectrometers, the vertical profile, and a laser system measuring along four paths surrounding the enclosure. For reference, enteric CH4 emissions were also measured at the animal scale on a daily basis, using an enteric tracer-ratio technique (with SF6 as the tracer). The animal-scale technique showed that mean CH4 emissions increased less than linearly with increasing feed intake. The herd-scale techniques showed that the emission rates followed a diurnal pattern, with the maximum about 2h after the feed was offered. The herd-scale techniques could detect the weekly changes in emission levels, except that the two vertical-profile techniques (mass-budget technique and dispersion model applied to profile) failed to resolve the first step change. The weekly emission rates from the external tracer-ratio technique and the dispersion model, applied to data from either the two FTIR paths or the four laser paths, agreed within ±10% with the enteric tracer-ratio technique. By contrast, the two vertical-profile techniques gave 33–68% higher weekly emission rates. It is shown with a sensitivity study that systematically uneven animal distribution within the enclosure could explain some of this discrepancy. Another cause for bias was the data yield of the vertical-profile techniques being higher at day-time than at night-time, thus giving more weight to times of larger emission rates. The techniques using line-averaged mole fractions were less sensitive to the exact locations of emission sources and less prone to data loss from unsuitable wind directions; these advantages outweighed the lack of a method to calibrate CH4 mole fractions in situ.

Reconciling the changes in atmospheric methane sources and sinks between the Last Glacial Maximum and the pre-industrial era

We know from the ice record that the concentration of atmospheric methane, [CH4], at the Last Glacial Maximum (LGM) was roughly half that in the pre-industrial era (PI), buthow much of the difference was source-driven, and how much was sink-driven, remains uncertain. Recent developments include: a higher estimate of the LGM-PI change in methane emissions from wetlands―the dominant, natural methane source; and the possible recycling of OH consumed in isoprene oxidation―the principal methane sink. Here, in view of these developments, we use an atmospheric chemistry-transport model to re-examine the main factors affecting OH during this period: changes in air temperature and emissions of non-methane volatile organic compounds from vegetation. We find that their net effect was negligible(with and without an OH recycling mechanism), implyingthe change in [CH4] was almost entirely source driven―a conclusion that, though subject to significant uncertainties,can be reconciled with recent methane source estimates.