The Global Methane Budget

Lead Research Organisation: NERC CEH (Up to 30.11.2019)
Department Name: Hydro-climate Risks


Methane is the second most important greenhouse gas contributing to human-induced global warming. Atmospheric methane concentrations have increased sharply since 2007, for reasons that are not fully understood, resulting in ever-increasing uncertainty for future climate projections. The overall increase since 2007 is comparable to the largest growth events over the past 1000 years. The recent rises have occurred in the tropics and southern hemisphere, with the sharpest year-on-year increase thus far occurring in 2014. Strong growth continues in 2015. Carbon isotopic evidence suggests that the increase is due to sources that are predominantly biogenic in origin, with changes in the anthropogenic sources from fossil carbon and burning (e.g., natural gas leakage, fracking and so on) playing a minor role. This, taken with the tropical locus on growth, suggests that the increase has primarily been driven by meteorological change (e.g., temperature, rainfall). Moreover, the global methane budget is not well described. "Bottom-up" estimates, made by aggregating inventories of emissions (e.g. from gas leaks, fires, landfills, cows, etc) or from process models (e.g., wetlands) balanced with known loss processes, are significantly different from '"top-down" budgets assessed by direct measurement of methane in the atmosphere. Why this discrepancy occurs is not understood.

The project has four components:
1. Better Observations are needed to derive estimates of emissions. The project will support a UK observation network for methane and its isotopes. Continuous stations will be at Kjolnes (Norway), Weybourne, Jersey, NERC ship RRS JC Ross, Cape Verde, Ascension, Falklands, Halley Bay, Hong Kong, with partner stations in Canada, Spitsbergen, Bolivia, S. Africa, India, Rwanda and Malaysia. Flask or bag sampling (for methane, 13C and D/H isotopes) will also be undertaken at these stations and at a number of continental stations in S. America, Africa and S, SE and E Asia, with offline analysis in the UK. A D/H measurement facility will be set up. The UK FAAM aircraft will carry out flights across the Atlantic tropics, from Azores to Cape Verde to Ascension.

2. Process Studies will address the largest information gaps in the global budget. Tropical emission fluxes and isotopic signatures are not well constrained. Field campaigns will be undertaken in tropical wetlands in Amazonia, Africa, India and SE Asia, and C4 savanna biomass burn regions. Poorly understood anthropogenic sources will be studied in Kuwait and S, SE and E Asia. Characteristic isotopic signatures of regional emissions will be determined, to support global and regional modelling. Land surface modelling and satellite studies will study emissions and responses to change in temperature and precipitation. Major sink processes will be investigated in the tropical atmosphere, with vertically and latitudinally resolved OH and Cl budget studies by the FAAM aircraft, and quantification of tropical uptake by soils.

3. Atmospheric modelling will be used to derive regional and global fluxes, apportioned by source type and geography using integrated in situ and remote sensing observing systems. We will carry out regional trajectory studies using models like NAME to assess regional emissions. Global modelling using 3D models will test synthetic estimates of the methane mole fraction and isotopic record. Global inverse modelling for mole fraction, 13C and D/H will be used to estimate fluxes by geographic source and source type, including a comprehensive assessment of the uncertainties that remain once all available observations have been used.

4. Integrative studies will use the results from the project to test top-down and bottom-up emission estimates, and evaluate the responses of the global methane budget to projections of climate change.

The project will deliver a state of the art UK greenhouse gas monitoring network and reduced uncertainty of the global methane budget.

Planned Impact

This project will produce a much better understanding of the global methane budget, and the role of climate feedbacks in driving emissions. Atmospheric methane has increased sharply since 2007. Simultaneously, there has been a shift in its carbon isotopes implying the increase is primarily biogenic, not driven by fossil fuel emissions. A better knowledge of the global methane budget is vital if we are to understand what is driving climate change and predict future emissions. This work will have impact on a very wide range of beneficiaries, from scientists to policy-makers.

Measurement: The project will create a UK Observation network available as a long-term legacy from the work. The observation network will sustain and improve global methane mole fraction and isotopic measurement, especially in the tropics where data gathering is presently very weak. In particular, the project will sustain the greenhouse gas measurement on UK Ascension Is. station, one of the very few tropical background stations globally and currently unfunded from 2017.

Policymakers: With the Paris Climate Conference later this year, policy makers and governmental bodies are strongly focussed on climate change. 195 nations participate in the United Nations Framework Convention on Climate Change (UNFCCC). This commits signatory countries to assess their greenhouse gas emissions. For methane, there is a major discrepancy between global total emissions as assessed by atmospheric measurement ('top-down' measurement) and the sum of national emissions declared under UNFCCC (the 'bottom-up' inventory). This project will make major advances towards resolving this problem.

Space: When Sentinel 5P, GOSAT-2 and MERLIN satellites are launched, this project's in-situ observation of equatorial and Southern Hemisphere methane will make a significant contribution. Specifically, the work will help validate the TCCON station at Ascension Island, a key equatorial site for satellite ground-truthing.

Interpretation of the observations by the modelling component of the project will help ecologists and geographers to understand the impact of climate change globally and especially in less developed nations. There will be strong impact on those carrying out global security studies, benefitting from the significant improvement the work will bring to greenhouse gas emissions inventories in tropical nations, in which methane is very poorly constrained at present. The work will support marked improvements in emissions estimate for less developed nations.

In the private sector, a direct beneficiary will be Isoprime Ltd. (Cheshire: Queen's Award 2013), who will partner the development of the D/H analysis system at Royal Holloway. Wider beneficiaries include the gas, coal and oil industries, as the strong improvement of isotopic work, especially in D/H, will facilitate leak identification and location. Cutting leaks will improve efficiency and productivity as well as help compliance with regulatory frameworks.

Education: The project will support a number of younger staff, who will sustain the UK's key skills in greenhouse gas measurement and modelling, especially in the use of isotopes to characterise emissions. Career development will come through skills learned, publications, conferences, and training opportunities.

Public/Media: The results of this work will also be of interest to the wider public. Greenhouse gas, global warming and climate change are high on the political and media agenda, especially with the Paris climate conference later this year. Decisions made there will have implications for all.

Methane and its feedbacks rank among the most important and the most poorly understood problems in the global climate system. In wide global constituencies, from specialist scientists to policy makers, there is great need for better knowledge. Thus this project will have unusually strong and very wide impacts worldwide.


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Ganesan A (2018) Spatially Resolved Isotopic Source Signatures of Wetland Methane Emissions in Geophysical Research Letters

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Gondwe MJ (2021) Methane flux measurements along a floodplain soil moisture gradient in the Okavango Delta, Botswana. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

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Helfter C (2021) From sink to source: high inter-annual variability in the carbon budget of a Southern African wetland in Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences

Description In collaboration with the Okavango Research Institute, UKCEH has installed and operated eddy covariance flux towers at two sites in the Okavango Inland Delta (Nxaraga and Guma, Botswana), since October 2017. The measurements have been very successful despite the remoteness of the study sites. We now have over 2 years of continuous flux measurements, making our dataset possibly the longest one for an African wetland.

The dominant control of methane emissions at the seasonal wetland is soil moisture and we are now working with remote sensing experts at the universities of Botswana and Edinburgh on mapping the spatial extent of the seasonal wetlands in the Okavango Delta in order to upscale the methane fluxes. In the permanent wetland, we have found a statistically-significant correlation between methane fluxes and the enhanced vegetation index (EVI, which is NDVI with blue-band correction) at the seasonal timescale; in turn, EVI is correlated to water level in the lagoon which explains the twice-removed correlation initially noted between methane fluxes and water level. Additionally, mean measured monthly methane fluxes are significantly correlated with GPPmax, the maximum potential gross primary production, obtained from the non-linear relationship between carbon dioxide fluxes and PAR (photosynthetically active radiation). These findings indicate that plant-mediated transport is the dominant exchange pathway of methane between water and atmosphere in the permanent wetland. The next step consists in mapping the extent of papyrus beds in the Okavango and upscale fluxes. The University of Edinburgh are currently working on deriving estimates of annual methane fluxes for the Okavango Delta using satellite observations, and these top-down estimates will be compared with the bottom-up estimates from our experimental work.

In order to understand the reasons behind the increase in atmospheric concentrations of methane, carbon isotopes can be used to help identify changes in emission sources and sinks. In collaboration with project partners at Bristol and also with the Met Office, we produced a new global map of the 13-carbon isotope signature associated with wetland methane emissions, the largest global source of methane to the atmosphere. We show how this newly synthesized information can lead to more accurate understanding of the causes of variations in the amount and rate of increase of methane in the atmosphere. The work has been published (Ganesan et al., 2018; see Publications section).

In another wetland application with project partners from Leicester, Leeds and Edinburgh, we used atmospheric methane observations from the GOSAT satellite to evaluate methane wetland emission estimates from JULES. We found that the JULES wetland emissions failed to capture the extent of the tropical wetland seasonal cycle. We focus further analysis on the major natural wetlands in South America: the seasonally flooded savannah of the Pantanal (Brazil) and Llanos de Moxos (Bolivia) regions; and the riverine wetlands formed by the Paraná River (Argentina). We see large discrepancies between simulation and observation over the Pantanal and Llanos de Moxos region in 2010, 2011 and 2014 and over the Paraná River region in 2010 and 2014. We find highly consistent behaviour between the time and location of these methane anomalies and the change in wetland extent, driven by precipitation related to El Niño Southern Oscillation activity. We conclude that this is primarily caused by the current lack of land surface models to increase wetland extent through overbank inundation. This work has been published (Parker et al., 2018; see Publications section).
Exploitation Route Research community: to improve modelling of wetlands and to understand the changes in the Global Methane cycle
Sectors Environment

Title Flux data for the GUMA site in Botswana (BW-Gum) 
Description Eddy-covariance fluxes of sensible and latent heat, carbon dioxide, methane and water vapour from 2017 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact The data are being used in a global synthesis of methane flux measurements. 
Title Flux data for the NXARAGA site in Botswana (BW-Nxr) 
Description Eddy-covariance fluxes of sensible and latent heat, carbon dioxide, methane and water vapour from 2017 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact The data are being used in a global synthesis of methane flux measurements. 
Title Monthly global methane emissions from natural wetlands modelled by JULES with dynamic vegetation (1980-2014) v1.0 
Description This dataset is a model output from the JULES land surface model driven with the Watch Forcing Data methodology applied to Era-Interim (WFDEI) data. It provides monthly global methane emissions from natural wetlands on 0.5 x 0.5 degree grid between 1980-2014. It includes the following variables: - fch4_wetl: modelled methane flux from natural wetland, in mg CH4 m-2 day-1 - fwetl: fraction of wetland - cs: soil carbon in each of these four soil carbon pools: decomposable plant material, resistant plant material, microbial biomass and humus), in kg m-2 - t_soil: sub-surface temperature of the four modelled soil layers (0-0.1 m, 0.1-0.35 m, 0.35-1.0 m and 1.0-2.0 m), in K 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Description Wetland flux measurements in collaboration with the Okavango Research Institute 
Organisation University of Botswana
Department Okavango Research Institute
Country Botswana 
Sector Academic/University 
PI Contribution CEH will measure methane fluxes from a permanent and seasonal wetland in the Okavango delta, one of the largest global wetlands. This project will provide the first field scale methane flux measurements from this delta, using the eddy covariance (EC) method over a one year period.
Collaborator Contribution The Okavango Research Institute will be responsible for site selection and logistical support for the eddy covariance measurements. The Okavango Research Institute will also make plot scale chamber measurements in the footprint of the EC tower, thus providing information on the spatial heterogeneity. The combined data will improve our understanding of the magnitude and seasonality of methane emissions from this inland delta and will feed into global methane models.
Impact None at present
Start Year 2016
Description Wetlands modelling with the Met Office Hadley Centre 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Academic/University 
PI Contribution Wetlands are the largest natural source of methane to the atmosphere and projected to increase significantly with climate change. As part of the JULES land surface modelling effort and in collaboration with the Met Office Hadley Centre, the CEH research team contribute to improvements in the modelling of wetlands and the associated emissions of methane.
Collaborator Contribution With CEH, the Met Office provided expertise to produce a new global map of the 13-carbon isotope signature associated with wetland methane emissions, the largest global source of methane to the atmosphere. We show how this newly synthesized information can lead to more accurate understanding of the causes of variations in the amount and rate of increase of methane in the atmosphere. Tropical South America is thought to emit a significant proportion of the total CH4 emissions from inundated areas. However there is considerable uncertainty in the regional total. Part of this uncertainty is likely to be related to the recently discovered, but as yet unmodelled, process by which tropical trees can act as a significant conduit for CH4 emissions over this region. Met Office researchers are utilising field data from other MOYA partners and combining these with other available measurements over this regionto develop an explicit representation of the transport of methane through trees over Amazonia in the JULES land surface model.
Impact The work on the wetland isotope map has been published; see Publications section: Ganesan et al., 2018.
Start Year 2017