Atmospheric-methane source and sink sensitivity analysis using Gaussian process emulation

Abstract

Abstract. We present a method to efficiently approximate the response of atmospheric-methane mole fraction and δ13C–CH4 to changes in uncertain emission and loss parameters in a three-dimensional global chemical transport model. Our approach, based on Gaussian process emulation, allows relationships between inputs and outputs in the model to be efficiently explored. The presented emulator successfully reproduces the chemical transport model output with a root-mean-square error of 1.0 ppb and 0.05 ‰ for hemispheric-methane mole fraction and δ13C–CH4, respectively, for 28 uncertain model inputs. The method is shown to outperform multiple linear regression because it captures non-linear relationships between inputs and outputs as well as the interaction between model input parameters. The emulator was used to determine how sensitive methane mole fraction and δ13C–CH4 are to the major source and sink components of the atmospheric budget given current estimates of their uncertainty. We find that our current knowledge of the methane budget, as inferred through hemispheric mole fraction observations, is limited primarily by uncertainty in the global mean hydroxyl radical concentration and freshwater emissions. Our work quantitatively determines the added value of measurements of δ13C–CH4, which are sensitive to some uncertain parameters to which mole fraction observations on their own are not. However, we demonstrate the critical importance of constraining isotopic initial conditions and isotopic source signatures, small uncertainties in which strongly influence long-term δ13C–CH4 trends because of the long timescales over which transient perturbations propagate through the atmosphere. Our results also demonstrate that the magnitude and trend of methane mole fraction and δ13C–CH4 can be strongly influenced by the combined uncertainty in more minor components of the atmospheric budget, which are often fixed and assumed to be well-known in inverse-modelling studies (e.g. emissions from termites, hydrates, and oceans). Overall, our work provides an overview of the sensitivity of atmospheric observations to budget uncertainties and outlines a method which could be employed to account for these uncertainties in future inverse-modelling systems.

Trends in atmospheric methane concentrations since 1990 were driven and modified by anthropogenic emissions

“…7). Here we have proposed an alternative hypothesis that an earlier increase in the oxidation capacity of the atmosphere can decrease the isotopic ratio in later years, as previously illustrated for idealized step changes in OH 52,53 . Using an isotopic box model, the increase in OH from the 1980s to early 2000s using CEDS-2017 emissions can explain part of the decrease in observed δ 13 C CH4 from 2007, due to the time dependence of the δ 13 C CH4 response (Fig.…”

“…Here we propose an alternative hypothesis that an earlier increase in the oxidation capacity of the atmosphere can decrease the isotopic ratio in later years. Both Haghnegahdar, et al 52 and Stell, et al 53 (in their Fig. S9) illustrated that the isotopic effect of a step change in OH exhibited a sign change in δ 13 C CH4 trend after around one decade.…”

Untangling variations in the global methane budget

“…A proportion of the observed decrease in this ratio since 2007 may have been due to its slow response to the earlier increase in OH concentrations. Previous studies have highlighted this slow response by simulating idealised stepchanges in OH 11,12 , but it is demonstrated here to what extent it affected the observed CH 4 growth rates post-2007.…”