Barriers to predicting changes in global terrestrial methane fluxes: Analyses using CLM4Me, a methane biogeochemistry model integrated in CESM

We compared the seasonality and magnitude of predicted CH₄ emissions to observations from 18 sites and three global atmospheric inversions. Simulated net CH₄ emissions using our baseline parameter set were 270, 160, 50, and 70 Tg CH₄ yr⁻¹ globally, in the tropics, in the temperate zone, and north of 45° N, respectively; these values are within the range of previous estimates. We then used the model to characterize the sensitivity of regional and global CH₄ emission estimates to uncertainties in model parameterizations.

Of the parameters we tested, the temperature sensitivity of CH₄ production, oxidation parameters, and aerenchyma properties had the largest impacts on net CH₄ emissions, up to a factor of 4 and 10 at the regional and gridcell scales, respectively. In spite of these uncertainties, we were able to demonstrate that emissions from dissolved CH₄ in the transpiration stream are small (<1 Tg CH₄ yr⁻¹) and that uncertainty in CH₄ emissions from anoxic microsite production is significant.

In a 21st century scenario, we found that predicted declines in high-latitude inundation may limit increases in high-latitude CH₄ emissions. Due to the high level of remaining uncertainty, we outline observations and experiments that would facilitate improvement of regional and global CH₄ biogeochemical models.