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

Terrestrial net CH₄ surface fluxes often represent the difference between much larger gross production and consumption fluxes and depend on multiple physical, biological, and chemical mechanisms that are poorly understood and represented in regional- and global-scale biogeochemical models. To characterize uncertainties, study feedbacks between CH₄ fluxes and climate, and to guide future model development and experimentation, we developed and tested a new CH₄ biogeochemistry model (CLM4Me) integrated in the land component (Community Land Model; CLM4) of the Community Earth System Model (CESM1). CLM4Me includes representations of CH₄ production, oxidation, aerenchyma transport, ebullition, aqueous and gaseous diffusion, and fractional inundation. As with most global models, CLM4 lacks important features for predicting current and future CH₄ fluxes, including: vertical representation of soil organic matter, accurate subgrid scale hydrology, realistic representation of inundated system vegetation, anaerobic decomposition, thermokarst dynamics, and aqueous chemistry. 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.