Chemical oxidation of CH 4
The H 2 production rate from the chemical oxidation of CH 4 was estimated based on its temperature-dependent reaction rates with OH (ref. 60), which produces HCHO that subsequently forms H 2 through further photolysis (Supplementary Note 3). We ignored H 2 produced from the CH 4 + O one-dimensional reaction pathway61,62, which accounts for less than 5% of CH 4 loss in the atmosphere63.
The overall fraction of H 2 produced in the global CH 4 reaction chain can be calculated for each time step as
$${P}_{{{\rm{CH}}}_{4}:{{\rm{H}}}_{2}}=\int {k}_{1}(T)\times [{\rm{OH}}]\times [{{\rm{CH}}}_{4}]\times {F}_{{{\rm{H}}}_{2}}$$ (1)
where k 1 (T) is the temperature-dependent reaction rate between CH 4 and OH, \({F}_{{{\rm{H}}}_{2}}\) is the total yield factor of H 2 within the whole chain, and [OH] and [CH 4 ] are the tropospheric OH and CH 4 concentrations, respectively. \({P}_{{{\rm{CH}}}_{4}:{{\rm{H}}}_{2}}\) is usually simplified by applying a global mean of [OH], k 1 (T), [CH 4 ] and \({F}_{{{\rm{H}}}_{2}}\) in previous studies17,21, which can lead to large uncertainty and bias because a large variation of the global mean must be included, considering the inhomogeneous distribution of the chemical species and the reaction rates. Here, we improved the estimation by considering spatial distribution and temporal variations of H 2 production from CH 4 oxidation using available data from data assimilation and atmospheric chemistry model simulations. The \({P}_{{{\rm{CH}}}_{4}:{{\rm{H}}}_{2}}\) was estimated by integrating global grids with a spatial resolution at 3.75° (longitude) × 1.875°(latitude) and a temporal resolution of 3 h (3-D), which substantially reduced the uncertainty of our estimates.
The 3-D distribution of \({k}_{1}(T)\) was estimated using the temperature field from ERA-Interim reanalysis meteorology data64. We used eight [OH] fields and three [CH 4 ] fields to produce 24-member estimates to obtain an ensemble-based mean estimate. The eight [OH] fields were obtained from the INVAST model and seven CMIP6 (refs. 65,66) models: CESM2-WACCM (ref. 67), EC-Earth3-AerChem (ref. 68), GISS-E2.1-G (ref. 69), GISS-E2.1-H (ref. 70), GISS-E2.2-G (ref. 71), MPI-ESM-1.2-HAM (ref. 72) and MRI-ESM2.0 (ref. 73). For the CMIP6 runs, we used the historical simulations that cover 1990–2014 supplemented with the SSP3-70 scenario simulation for the 2015–2020 period. The SSP3-70 scenario simulation did not consider the impact of COVID-19 on OH changes 2020, which was corrected using change ratios from INVAST. The three 3D distributions of tropospheric CH 4 were produced by atmospheric transport models after surface measurement assimilation, which are CIF-LMDz (ref. 74), MIROC4-ACTM (ref. 63) and NISMON (ref. 75), respectively. The 3D \({F}_{{{\rm{H}}}_{2}}\) was computed based on the reaction chain, in which grid level values range from 0.25 to 0.7 and global mean ranges from 0.41 to 0.43 (Supplementary Note 3).
We found that our ensemble of OH fields overestimates CH 4 oxidation, yielding a global mean oxidation of 517 Tg CH 4 yr−1 over 2007–2018, compared with the IPCC AR6 top-down estimate of 472 Tg CH 4 yr−1. Furthermore, our ensemble underestimates the uncertainty associated with OH-driven CH 4 oxidation (about 8%) and H 2 production (about 10%) when compared with the approximately 11% uncertainty in CH 4 lifetime reported in AR6. Notably, our estimate is bottom-up, and the tendency of bottom-up models to overestimate oxidation fluxes is well-documented51, which is one reason why IPCC AR6 prioritizes top-down values for CH 4 budget assessments. To reconcile these differences, we corrected the H 2 production from CH 4 oxidation by scaling our bottom-up H 2 fluxes to match the AR6 CH 4 oxidation value. Specifically, we applied a constant scaling factor of 472/519 ≈ 0.913 to the H 2 production from CH 4 oxidation.
To propagate the uncertainty in CH 4 oxidation into the H 2 production estimate, we estimated the new standard deviation (SD) as
$${{\rm{SD}}}_{{{\rm{H}}}_{2}-{\rm{production}}}^{{\prime} }={\rm{sqrt}}({\left(\frac{{{\rm{SD}}}_{{{\rm{H}}}_{2}-{\rm{production}}}}{{{\rm{SD}}}_{{{\rm{CH}}}_{4}{\rm{oxidation}}}}\right)}^{2}+{\left(\frac{{{\rm{SD}}}_{{{\rm{IPCC\; CH}}}_{4}{\rm{lifetime}}}}{{{\rm{Mean}}}_{{{\rm{IPCC\; CH}}}_{4}{\rm{liftime}}}}\right)}^{2})$$ (2)
where \({{\rm{SD}}}_{{{\rm{H}}}_{2}-{\rm{production}}}^{{\prime} }\) is the new SD for H 2 production from CH 4 –OH oxidation, \({{\rm{SD}}}_{{{\rm{H}}}_{2}-{\rm{production}}}\) is the old SD for H 2 production from CH 4 –OH oxidation and \({{\rm{SD}}}_{{{\rm{CH}}}_{4}{\rm{oxidation}}}\) is the SD for oxidized CH 4 through reacting with OH estimated using our ensemble of OH and CH 4 fields. \({{\rm{SD}}}_{{{\rm{IPCC\; CH}}}_{4}{\rm{lifetime}}}\) and \({{\rm{Mean}}}_{{{\rm{IPCC\; CH}}}_{4}{\rm{liftime}}}\) are taken from IPCC AR6, which are 1.1 years and 9.7 years, respectively.
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