Rising methane: is warming feeding warming?

This talk reports on three years of continuous monitoring of carbon dioxide (CO₂) and methane (CH₄) emissions in two contrasting wetland areas of the Okavango Delta, Botswana: a perennial swamp and a seasonal floodplain. Both sites were net sinks of atmospheric C in 2018, but the radiative budget of the perennial swamp was strongly positive in 2018. Both ecosystems were sensitive to drought, which switched these sinks of atmospheric CO₂ into sources in 2019. This phenomenon was particularly strong at the seasonal floodplain, due to a sharp decrease in gross primary productivity. Similarly, drought caused a marked decrease in CH₄ emissions at the seasonal floodplain, but emissions from the perennial swamp were unaffected. 

This study demonstrates that complex and divergent processes can coexist within the same landscape, and that meteorological anomalies can significantly perturb the balance of the individual terms of the GHG budget. Given the contrasting effects of drought on the CO₂ and CH₄ flux terms, it is crucial to evaluate an ecosystem’s complete carbon budget instead of treating these GHGs in isolation.

Data-poor tropical wetlands constitute an important source of atmospheric CH4 in the world. The group studied CH₄ fluxes using closed chambers along a soil moisture gradient in a tropical seasonal swamp in the Okavango Delta, Botswana, the sixth largest tropical wetland in the world. The objective of the study was to assess net CH₄ fluxes and controlling environmental factors in the Delta’s seasonal floodplains. Net CH₄ emissions from seasonal floodplains in the wetland were estimated at 0.072 ± 0.016 Tg.a^-1. Microbial CH₄ oxidation of approximately 2.817 × 10-3 ± 0.307 × 10^-3 Tg.a^-1 in adjacent dry soils of the occasional floodplains accounted for the sink of 4% of the total soil CH₄ emissions from seasonal floodplains. The observed microbial CH₄ sink in the Delta’s dry soils is, therefore, comparable to the global average sink of 4–6%. Soil water content (SWC) and soil organic matter were the main environmental factors controlling CH₄ fluxes in both the seasonal and occasional floodplains. The optimum SWC for soil CH₄ emissions and oxidation in the Delta were estimated at 50% and 15%, respectively. Electrical conductivity and pH were poorly correlated (r² ≤ 0.11, p < 0.05) with CH₄ fluxes in the Delta’s seasonal floodplain. 

The natural ecosystems of tropical Africa represent a significant store of carbon, playing an important but uncertain role in the global atmospheric budgets of greenhouse gases (GHGs). Recent studies using satellite data have concluded that unusually heavy rainfall over East Africa in late 2019 led to additional methane emissions equivalent to over a quarter of the growth in global CH₄ emissions in 2019, and that the tropical Africa region dominates net carbon emission across the tropics. These conclusions, as well as those of other studies looking at the tropical African carbon and methane cycles, rely on the accuracy of satellite datasets and atmospheric transport models over a region where there is a lack of independent observations to confirm their robustness and validity.

This presentation shows the first ground-based observations of GHG column concentrations over tropical East Africa, obtained using a portable spectrometer set up in Jinja, Uganda. The instrument was operated remotely and near-continuously for a three-month period in early 2020, by housing it inside an automated weatherproof enclosure. This dataset helps to demonstrate the value that these ground-based GHG column measurements can provide towards our understanding of the carbon and methane budget for the tropical East Africa region.

Airborne measurements of methane (CH₄) were recorded over three major wetland areas in Zambia in February 2019 during the MOYA (Methane Observations and Yearly Assessments) ZWAMPS field campaign. Enhancements of up to 600 ppb CH₄ were measured over the Bangweulu (11°36’ S, 30°05’ E), Kafue (15°43’ S, 27°17’ E), and Lukanga (14°29’ S, 27°47’ E) wetlands. Three independent approaches were used to quantify the methane emission fluxes; airborne mass balance, airborne eddy covariance, and atmospheric inversion modelling, with results ranging between 5.0 and 27.9 mg CH₄ m^-2 hr^-1 across the three wetlands. In contrast, fluxes simulated by 19 process-based wetland models (WetCHARTs and GCP suites) were a factor of 10 smaller on average (0.6 – 3.9 mg CH₄ m^-2 hr^-1). Independent total-column CH₄ observations (TROPOMI and GOSAT) were also compared with measurements. These results identify globally significant fluxes of methane from tropical African wetlands, which are not currently accounted for in global methane budgets.

Methane (CH₄) mole fractions from the large semi-seasonal Llanos de Moxos wetlands in northern Bolivia were measured using an aircraft platform at the peak of the wetland extent during the late rainy season in early March 2019. These wetlands cover an area larger than the Netherlands and Belgium combined, and consist of tropical savannah with annual flooded and dry seasons. Methane measurements were made during upwind and downwind flights over the wetland extent to allow box modelling and inversion modelling of the region to determine the methane fluxes from the surface. Grab samples of atmosphere were also taken periodically to allow analysis of ¹³C_CH4 to provide source attribution of the CH₄ enhancement. 

Daily fluxes of CH₄ were predicted to be at least 150 mg CH₄/m²/day during the measurement campaign, and potentially significantly larger. These emission fluxes are at least a factor of 4 larger than previous Amazon basin studies. Isotopic analysis confirmed that the wetlands are the only likely source of the wide area methane enhancement measured. These Bolivian results show that seasonal wetlands in the tropics are major sources of CH₄ and potentially important drivers of methane’s interannual variability.

The last two decades have seen a time of unprecedented change in the hydrology of East Africa. The early 2000s were characterized by drought and the water levels of Lake Victoria receding. In contrast, recent widespread flooding has resulted in lake levels rising to their highest recorded level and the displacement of thousands living round its edge. What does this mean for methane? Water availability is one of the major drivers of methane emissions in this region, with the seasonal cycle of methane emissions linked to the seasonal movement of the inter-tropical convergence zone and associated rainfall. This talk will examine what satellites can tell us, both about the changes in hydrology and changes in methane enhancements and emissions over East Africa. Satellite data have helped identify a substantial increase in methane emissions between 2010–2015 from the Sudd wetlands of South Sudan. More recent data have shown above average seasonal emissions following extreme rainfall in East Africa in 2019. Finally, the recent satellite record will be discussed, examining the potential role of East Africa in the high global methane growth rate of 2020.  

NERC MOYA/ZWAMPS flights sampled strong methane fluxes over wetlands in the Nile, Congo and Zambezi basins, and Bolivian southern Amazonia, and over fires in African woodland, cropland, and savannah grassland. δ¹³C_CH4 isotopic signatures were in the range -55 to -49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely -60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar δ¹³C_CH4 signatures were measured over the Amazonian wetlands of NE Bolivia (around -59‰) and the overall δ¹³C_CH4 signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was -59 ± 2‰. In smoke from tropical dry C3 forest fires in Senegal, δ¹³C_CH4 values were around -28‰.  African tropical C4 grass fire δ¹³C_CH4 values were -16 to -12‰. For African cattle, δ¹³C_CH4 values were around -60 to -50‰. Methane from urban landfills in Zambia and Zimbabwe had  δ¹³C_CH4  around  -37 to -36‰. Campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. These new isotopic values will help place isotopic constraints on global methane budget models.

Atmospheric methane (CH₄) concentrations have been rising globally since 2007 with 2020 exhibiting the largest year-on-year growth in the observational record of the past 40 years. The mechanisms driving this growth trend remain a subject of controversy and many recent studies have utilized observations of atmospheric methane isotopologues (δ¹³C-CH₄ and δ²H-CH₄) in their analyses to quantify source and sink fluxes of CH₄. This work presents spatially-resolved source signatures of δ¹³C-CH₄ and δ²H-CH₄ from wetlands and discusses some of the limitations in modelling methane isotopic ratios in the atmosphere. This study derives a δ¹³C-CH₄ source signature distribution based on differences between four main isotopically differentiated wetland ecosystems (high-latitude bogs and fens, and low-latitude C3- and C4-dominated wetlands) and a δ²H-CH₄ source signature distribution based on the signature of meteoric waters (ie precipitation). The analysis shows how this information can be used to better simulate atmospheric isotopic levels for use in inverse modelling and demonstrates how incorrect assumptions about the spatial distribution of source signatures can lead to erroneous simulations of trend, inter-hemispheric difference and seasonal cycle. This analysis highlights the need to robustly account for uncertainties in source signatures when conducting modelling studies.

Approximately 30% of global CH₄ emissions are thought to derive from wetlands. Because of its vast area and its monsoonal, seasonally very wet, climate Amazonia is one of the largest wetland areas and its emissions may be sensitive to a changing climate. A unique dataset of 590 measured vertical profiles of CH₄ and CO measured at four locations in Amazonia from 2010–2018 permits us to investigate Amazonia's methane budget, it's spatial and temporal variation and flux controls. Based on an air column budgeting technique the group estimates that Amazonia emits 46.2±10.3 TgCH₄yr^-1 with non-fire sources (mainly wetlands) dominating emissions. There are distinct regional patterns. Emissions from southern regions are strongly seasonal and vary synchronously with flood levels. In contrast the northwest-central Amazon emissions are nearly aseasonal, consistent with weak precipitation seasonality. Furthermore there is a strong east-west difference with large emissions concentrated in eastern Amazonia. A recent atmospheric transport inversion study using total-air-column methane retrieved from space confirms this pattern. Why there is such a strong east-west difference is currently unclear. Finally while the analysis of the vertical profiles does not suggest a clear change in emissions over the past decade the atmospheric transport modelling study suggests there is an increase.

Most work on the emission of methane in wetlands over the past several decades has focused on exchanges at the soil surface, aquatic surfaces or from emergent, low canopy herbaceous plants. The woody surfaces of the trees that dominate productivity in these ecosystems, which were previously thought to be neutral in the exchange of methane, are now known to be important mediators of this trace gas, from the soils where it is produced to the atmosphere. This presents opportunities in helping to quantify and account for prior gaps in regional budgets but also new complexity and uncertainty with work needed to understand hydrological, species and within and between-tree variability as well as the relationship between tree fluxes and the soils in which trees grow. In this talk, the group demonstrate the importance of understanding tree methane exchange for the Amazon methane budget and present new findings that show how riparian trees can remain sources of methane when not flooded with implications for broader tropical methane budgets. The talk concludes by highlighting future areas of importance in this growing area of ecosystem biosphere/atmosphere exchange research at a time when forest expansion is being promoted.

Removal of CH₄ by the hydroxyl radical (OH) is part of a much broader role played by this critically important atmospheric cleansing agent. However, our limits to understanding the OH spatial distribution and whether it is increasing or decreasing are seen in the range of estimates coming from recent atmospheric chemistry papers and this year’s IPCC Working Group I report. An early focus on CH₄ – CO – OH chemistry identified reasons for a decrease in OH, whereas more detailed treatments of atmospheric chemistry are now seeing that changes in NOx and water vapour, that increase it, are probably dominant. 

This overview will summarise how use of ¹³C and ¹⁴C isotopic data for both CH₄ and CO plays a key role in determining both global and regional changes in OH. Advances in understanding the solar cycles of ¹⁴C production together with over 30 years of Southern Hemisphere data show a significant decrease in δ¹³CO and no increase in Δ¹⁴CO despite increases in ¹⁴C production. This supports a decrease in OH but also requires careful treatment of trends in atmospheric transport as most ¹⁴CO production occurs in the lower stratosphere whereas most removal is in the troposphere.

Coal seam gas (CSG), coal mining and agriculture in a ~40,000 km² southeast portion of the Surat Basin accounts for 3% of Australia’s reportable methane emissions. Mobile ground-based surveys and airborne measurements of the atmospheric methane mole fraction in 2018 and 2019 have identified regions where the baseline data for the development of custom methane inventories is good or poor. In regions of disparate CSG and agricultural point sources, this study shows strong agreement between airborne measurements of methane emissions and the bottom-up inventory. Compared to most international studies, methane emissions in the Surat Basin CSG fields are low per unit of gas produced. However, the CSG emission loss rate determined from this research is two to three times higher than previous inventory and atmospheric inverse estimates for the region. For this study, neither the mass balance results nor the collected isotope data align with the bottom-up inventory for many portions of the agricultural districts. The results highlight the need for improved data transparency globally and the development of seasonal inventories to be able to close the gap between top-down versus bottom-up methane emission estimates at the regional scale. The workflow can be easily adapted to constrain inventories worldwide.

Agriculture is the largest single source of global anthropogenic methane (CH₄) emissions, with ruminants the dominant contributor. Livestock CH₄ emissions are projected to grow another 30% by 2050 under current policies, yet few countries have set targets or are implementing policies to reduce emissions in absolute terms. The reason for this limited ambition may be linked not only to the underpinning role of livestock for nutrition and livelihoods in many countries but also diverging perspectives on the importance of mitigating these emissions, given the short atmospheric lifetime of CH₄. Here, Dr Gonzalez Fischer shows that in mitigation pathways that limit warming to 1.5°C, which include cost-effective reductions from all emission sources, the contribution of future livestock CH₄ emissions to global warming in 2050 is about one-third of that from future net carbon dioxide emissions. Future livestock CH₄ emissions, therefore, significantly constrain the remaining carbon budget and the ability to meet stringent temperature limits. The talk reviews options to address livestock CH₄ emissions through more efficient production, technological advances and demand side changes, and their interactions with land-based carbon sequestration. The talk concludes that bringing livestock into mainstream mitigation policies, while recognizing their unique social, cultural and economic roles, would make an important contribution towards reaching the temperature goal of the Paris Agreement and is vital for a limit of 1.5°C. 

Gaseous emissions of methane (CH₄) and carbon dioxide (CO₂) from the subglacial environment under Greenland Ice Sheet (GrIS) were discovered recently. The understanding of mechanisms and magnitudes of emissions, and the origin of the gases, is extremely sparse and hence it is yet to be determined how important it is for the panarctic carbon budget. In order to constrain the source magnitude and the origin of the subglacial CH₄ and CO₂ the group measured the in situ net gaseous emissions, dissolved concentrations and isotopic composition of gases (¹³C and ²H) at the onset, near maximum and at the end of the melt season in 2018 and 2019. They found a close correlation between gaseous and dissolved CH₄ and CO₂, respectively, indicating that degassing of CH₄ and CO₂ from the subglacial meltwater is the main source of these gases in the subglacial air. The diurnal variability of in situ mole fractions of CH₄ and CO₂ in subglacial air was related to meltwater runoff showing that the net emission magnitude is directly related to glacial hydrology. The isotopic signature of CH₄ in the subglacial air indicated that subglacial CH₄ likely originated from microbial acetate fermentation, and it remained constant at maximum and waning stage of melt. Isotopic signatures of subglacial gaseous CO₂ indicate mixed subglacial sources from microbial oxidation of CH₄, remineralization of organic carbon in sediments and possibly influenced by removal of CO₂ by chemical weathering.

In addition, Professor Röckmann will briefly highlight the successful implementation of in situ IRMS measurements at various locations in Europe, and their use to constrain local and regional emission inventories.

According to the latest IPCC report, the Arctic is warming at more than twice the global rate. Increases in extreme heat events, fire regimes, and permafrost thaw have accompanied this warming, all of which can contribute to increased methane (CH₄) emissions. Sub-sea CH₄ seepage is also anticipated to increase in a warmer world, while the Arctic is projected to reach practically ice-free conditions in summer at least once before 2050, with unclear effects on ocean emissions and weather patterns. Here, the authors present estimates of recent Arctic Ocean CH₄ fluxes from cross-disciplinary observations in the ocean and the atmosphere combined with state-of-the-art atmospheric models. They demonstrate that the present-day flux is likely small, at least for active seeps around Svalbard, but that ocean-atmosphere fluxes are observable under some conditions. They go on to present the latest trends and developments of atmospheric CH₄ concentrations measured on Svalbard and discuss the need for further investigations as well as their plans to improve estimates of the Arctic and global CH₄ budget with new oceanographic measurements and surveys, and with the inclusion of methane isotope and satellite data into atmospheric models.

Arctic wetlands and surrounding ecosystems are understood to be both a significant source of methane (CH₄) and a sink of carbon dioxide (CO₂) during summer months. Arctic wetlands contribute an estimated 7 – 16 Tg CH₄ year^-1 to the global methane budget, whereas the terrestrial Arctic represents an average CO₂ sink of -0.13 Pg CO₂ year^-1. Surface exchange of methane and CO₂ in the Arctic is also highly sensitive to climate feedbacks due to the presence of Arctic amplification of global warming. Arctic mean air temperatures have increased at more than twice the rate of the global average, with current arctic temperature growth over 1.5 °C higher than the 1971–2000 global average temperature growth with further warming predicted for the future. Higher temperature may result in enhanced microbial methanogenesis in wetland ecosystems. Furthermore, thawing of permafrost as a result of increasing temperature may result in an increase in arctic wetland extent as well as enabling the release of organic carbon from the estimated ~1700 Pg of stored soil organic carbon in arctic permafrost. It is therefore clear that methane emissions from high-latitude wetlands may become increasingly important over time due to their high sensitivity to climate change. However, precise quantification of the Arctic methane source and CO₂ sink remains poorly characterised.

The Methane Observations and Yearly Assessment (MOYA)-Arctic field campaign was conducted from 29 July 2019 to 2 August 2019 based in Kiruna, Sweden. This field campaign used in situ aircraft measurements to quantify emissions of CH₄ and other trace gases from northern Swedish and Finnish (Fennoscandian) wetlands (66-69°N, 22-28°E) during the summer period. This work presents aircraft mass balance flux estimates for wetland CH₄ emission and biospheric carbon dioxide uptake during one of the survey flights carried out during the MOYA-Arctic campaign and compares with previous similar aircraft studies in the region. Additionally, this study compares the fluxes obtained via aircraft mass balance with fluxes from Global Carbon Project (GCP) wetland process models, where both the magnitude and spatial distribution of CH₄ fluxes are compared with the aircraft results.

Permafrost thaw increases active layer thickness, changes landscape hydrology, and influences vegetation composition. These changes alter belowground microbial and geochemical processes, affecting production, consumption and net emission rates of carbon dioxide and methane fluxes, determining the radiative forcing contribution from these climate-sensitive ecosystems. Permafrost peatlands may be a mosaic of dry frozen hummocks, semi-thawed or perched sphagnum dominated areas, wet permafrost-free sedge dominated sites, and open water ponds. Scaling emissions requires a combination of field measurements of above- and below-ground biology and geochemistry, remote sensing and biogeochemical modelling. 

For more than a decade, the authors have monitored trace gas emissions, microbial communities and vegetation composition at Stordalen Mire, a permafrost peatland in northern Sweden. They have linked above-ground vegetation to below-ground microbial communities responsible for CH₄ production giving confidence in the use of remote sensing and modelling to scale emissions. They estimate that the Mire continued to transition from dry permafrost to sedge and open water areas causing the net radiative forcing of Stordalen Mire to shift from negative to positive from 1970–2014. While still remaining a net overall sink of C, these climate sensitive ecosystems can become a positive feedback to climate change on decadal timescales.

The carbon isotopic signature (δ¹³C) of CH₄ fugitive emissions to atmosphere has been measured in downwind plumes from 300+ sources in England and Wales. This distinguishes the main source categories, separating combustion, fossil fuels, waste and agriculture. Further isotopic subdivisions are possible between solid and gaseous fuels, and between different types and methods of waste processing, such that signatures can be assigned to the categories used for the UK reporting to UNFCCC and in production of mapped products.

This has allowed production of isotopic prediction maps and an assessment of the changing source mix isotopic signature emitted by the UK, which contributes to long-range transport and eventually the changing signatures recorded at background measurement sites. The predicted δ¹³C for averaged UK emission of CH₄ changed from -50.5‰ in 1990 to -57.8‰ in the 2016–18 period based on the 2018 NAEI inventory. Approximately 67% of this ¹³C depletion is explained by changing proportions of the different sources, particularly emissions reduction from coal mining, landfill sites and gas leaks, but 33% is the result of changing signatures within source categories, particularly in gas distribution with the switch from southern North Sea fields to sources further north, and from changes in landfill practice.

It has been recommended that anthropogenic methane emissions should be reduced 45% by 2030 to slow increasing rates of atmospheric temperature, and to avoid adverse feedbacks of climate warming. Urban methane studies show significant methane emissions attributed to city infrastructure, eg the gas distribution network, unveiling opportunities for mitigating emissions.

Extensive mobile surveys conducted in Greater London and Bucharest, Romania between 2018 and 2019 measured street-level methane mole fractions to locate areas where levels were above atmospheric background, known as a leak indicator (LI). LIs were analyzed for C₂:C₁ ratios and δ¹³C_CH4 for emission source apportionment. 

London’s leak density was 0.3 LI km^-1 and emission factor (EF) was 0.42 L min^-1km^-1, whereas Bucharest had a leak density of 0.7 LI km^-1 and an EF of 1.60 L min^-1km^-1. Both cities had larger leak densities and an EF larger than other recently surveyed European cities. London and Bucharest had different dominating emission sources, with London emissions mostly from gas pipeline leaks, while Bucharest had a higher proportion of emissions from wastewater. This research will help local governments prioritize methane mitigation strategies to fall in line with the 45% methane reduction target.

Methane from waste is primarily of biogenic origin and varies with temperature and production process, resulting in emissions that vary with time of day and year. Furthermore, waste sites now commonly extract and either burn biogas to electricity, or use it directly, and emissions from each component activity producing CH₄ can be characterised by their distinct carbon-13 and deuterium isotopic signatures. This study aimed to characterize the carbon isotopic signatures (δ¹³C-CH₄) of several waste categories in the UK, Netherlands and Turkey, and assess CH₄ emission rates from UK biogas plants. 

Both Landfills and Biogas plants are important sources of fugitive methane emissions in the UK and Europe. Emissions from 10 UK biogas plants were estimated using Gaussian plume modelling to be in the range of 0.1 to 58.7 kg CH₄ hr^-1. Where there are multiple sources, emission plumes can be identified by their isotopic signatures, helping to identify specific sources. The emissions from 26 different biogas plants showed that feedstock differences affect the isotopic signature.  For landfills, active sites (open cells) have a more depleted isotopic signature with a lower oxidation proportion than closed sites in the UK. Overall landfill ¹³C-CH₄ results were similar in the UK, Netherlands, and Turkey.

This talk will review the current state-of-play in unmanned aerial vehicle (drone) measurements of methane and other greenhouse gases. It will discuss the platforms and instrumentation currently available and how drone-based sampling of gas concentrations can be used to derive emissions fluxes at local spatial scales and how to design surveys to capture this. This will be demonstrated by showing results from emission case studies of landfill sites, fracking sites and dairy herds conducted in recent years and how such results may be useful for policy and regulation.

A decade ago, publicly available empirical data were too scarce to guide industry stakeholders and policy-makers in terms of prioritizing and effectively mitigating fossil fuel related methane emissions. Since then, a vast number of studies have developed and refined methods to provide more robust emissions data across the supply chain and gain insights into the emission mechanisms to explain the atmospheric data. These recent studies have highlighted inconsistencies and low biases in many emission inventories, and they emphasized the contribution of measurements to drive emission reductions. However, more work is needed to (i) fill major data gaps in many world regions, (ii) address supply chain sectors which received little attention so far, and (iii) translate and adopt the scientific methods of empirical data collection at scale in the fossil fuel industries. This presentation will summarize ongoing work to address the three challenges above. It will include the current status and plans of the International Methane Emissions Observatory (IMEO) to conduct measurement studies internationally. Further, it will focus on the currently understudied offshore oil and gas sector. It will close with ongoing work to improve transparent emissions reporting from the fossil fuel industry by incorporating empirically-based estimates and integrating them with independent datasets (satellite data, field studies).

Atmospheric methane is the second anthropogenic greenhouse gas after carbon dioxide. With a lifetime around 10 years in the atmosphere and a diversity of emission types, methane is an important target for climate change mitigation. Different approaches are used to estimate methane sources and sinks. Top-down approaches assimilate atmospheric data from a large variety of in-situ and remote-sensed observations from the surface or from space to estimate methane sources and sinks at different scale (from the facility scale to global scale). Bottom-up approaches include inventories of anthropogenic emissions and numerical models able to simulate methane emissions at the surface from freshwater systems for example or the methane sinks through atmospheric chemistry schemes. A large international effort gathers the current knowledge on methane sources and sinks to release regularly updates of the methane budget. However, large uncertainties still remain in the spatio-temporal quantification of methane sources and sinks. An overview of the last synthesis is presented, along with a discussion on the sectors contributing to the recent atmospheric methane increase, and on the limitations and challenges remaining to close the methane budget.

Atmospheric methane removal (eg in situ methane oxidation to carbon dioxide) may be needed to offset continued methane release and limit the global warming contribution of this potent greenhouse gas. Because mitigating most anthropogenic emissions of methane is uncertain this century, and sudden methane releases from the Arctic or elsewhere cannot be excluded, technologies for methane removal or oxidation may be required. Carbon dioxide removal (CDR) has an increasingly well-established research agenda and technological foundation. No similar framework exists for methane removal. The authors believe that a research agenda for negative methane emissions – 'removal' or atmospheric methane oxidation – is needed. The authors outline some considerations for such an agenda here, including a proposed Methane Removal Model Intercomparison Project (MR-MIP). They also define a novel metric, the effective cumulative removal, and use it to show that each effective petagram of methane removed causes a mean global surface temperature reduction of 0.21ű0.04°C and a mean global surface ozone reduction of 1.0ű0.2 parts per billion. These results demonstrate the effectiveness of methane removal in delaying warming thresholds and reducing peak temperatures, and also allow for direct comparisons between the impacts of methane and carbon dioxide removal that could guide future research and climate policy.

Agriculture is the largest anthropogenic source of methane (CH₄), emitting 145 Tg CH₄ y⁻¹ to the atmosphere in 2017. The main sources are enteric fermentation, manure management, rice cultivation and residue burning. There is significant potential to reduce CH₄ from these sources, with bottom-up mitigation potentials of ~10.6, 10, 2 and 1 Tg CH₄ y⁻¹ from rice management, enteric fermentation, manure management and residue burning. Other system-wide studies have assumed even higher potentials of 4.8 to 47.2 Tg CH₄ y^-1 from reduced enteric fermentation, and 4 to 36 Tg CH₄ y^-1 from improved rice management. Biogas (a methane-rich gas mixture generated from anaerobic decomposition of organic matter and used for energy) also has potential to reduce unabated CH⁴ emissions from animal manures and human waste. In addition to these supply-side measures, interventions on the demand-side (shift to a plant-based diet and a reduction in total food loss and waste by 2050) would also significantly reduce methane emissions, perhaps in the order of >50 Tg CH₄ y^-1. Despite CH₄ being a short-lived greenhouse gas, the urgency of reducing warming means we must reduce any GHG emissions we can as soon as possible.

Meeting the Paris Agreement temperature goal necessitates limiting methane-induced warming, in addition to achieving net-zero or (net-negative) carbon dioxide (CO₂) emissions. In the median 1.5°C scenario (based on IAM pathways contributing to the IPCC Special Report on Warming of 1.5°C) assessed between 2020 and 2050, methane mitigation lowers temperatures by 0.1°C; CO₂ increases it by 0.2°C. CO₂ emissions continue increasing global mean temperature until net-zero emissions are reached, with some warming reversal once negative emissions are achieved. In contrast, reducing methane emissions starts to reverse methane-induced warming within a few decades. These differences are hidden when framing climate mitigation using annual ‘CO₂-equivalent’ emissions, including targets based on aggregated annual emission rates. Dr Cain shows how this insight can be accurately incorporated to estimate warming by using ‘warming-equivalent emissions’, which provide a transparent and convenient method which can inform policies and measures for mitigation, or demonstrate progress towards a temperature goal. The method presented (GWP*) uses well-established climate science concepts to relate GWP100 to temperature, as a simple proxy for a climate model.