Discovering reasons for global atmospheric methane growth using deuterium isotopes

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This proposal is to measure and model deuterium/hydrogen (D/H) isotope ratios in methane, to constrain the uncertainties in the global methane budget. Measurement will include 1) Field campaigns to determine isotopic source signatures; 2) time series from remote stations in both hemispheres; and 3) modelling to extract global budgets and causes of change. Atmospheric methane is growing rapidly. Its mixing ratio has risen 80 ppb (over 4% of total burden) since 2007. Growth accelerated in 2014 (13 ppb/yr) and has continued to be high since (7 to 10 ppb/yr). This high methane growth was unexpected and presents one of the greatest immediate challenges to the Paris Agreement. The reasons behind renewed methane growth since 2007 and acceleration in 2014 are not understood. Was it caused by increased emissions, and if so from which sources, or by declining OH, the main sink of methane? Is growth a feedback from climate change, the warming feeding warming? Or is it a direct consequence of human activities? Mixing ratio measurements alone are inadequate to solve the methane budget, though geographic foci indicate the main driving factors are in the tropics and low northern latitudes. Isotopologues (variations in the relative amounts of 12CH4, 13CH4 and 12CH3D) identify and discriminate between source and sink changes. After two centuries of becoming more 13C-rich, methane has shifted 'light' (more 12C-rich) since 2007. The C-isotope change gives insight into the main driving factors behind growth, but more information is needed to fully understand the reasons for interannual variability and continued methane growth. The greatest need is to measure H-isotopes, which provide extremely powerful discriminants of methane sources and sinks.A new technical advance in measuring H-isotopes in methane in ambient air permits this project. A new rapid multiple-sample high-precision mass spectrometric system, which radically cuts the per-sample cost of measurement was installed in late 2019 and was a major goal of NERC's MOYA highlight project. It will allow thousands of ambient air samples per year to be analysed for H-isotopes.Currently only very few labs worldwide make this challenging measurement and source isotopic signatures and time series of ambient air measurements are sparse. The new work will reinstate a global two-hemisphere network, measuring time series in the Arctic, northern mid-latitudes, tropics, southern mid-latitudes, and Antarctica. D/H isotopic signatures of the major sources will be characterised: wetlands, waste, biomass burning, fossil fuel, ruminants and rice agriculture. Field campaigns will focus on tropical Africa, East Asia and S America, with high emissions of methane, but very few measurements of methane isotopic signatures. Results will give regional source signatures for the source types.Modelling will use the new measurements and source signatures to constrain the global methane budget. Combining time series measurements of methane mole fraction and 13C/12C and D/H in methane with improved source signatures will determine latitudinal gradients and temporal trends, Numerical modelling using the UM-UKCA chemical transport model will use D/H as a key discriminant, to test the various hypotheses and identify the causes of methane's rise.The new rapid multi-sample system, which permits us to go from studying methane in 2D (mixing ratio + C-isotopes) to 3D (adding H-isotopes), is a very radical advance in solving the methane budget problem. Understanding why methane is rising is critical to driving mitigation policy to attain the Paris Agreement's goals. This project will lead to a major improvement in understanding the global methane budget, and help shape decisions on strategies needed to stabilise and reduce methane.