Variability in wetland methane emissions simulated by CLM4Me¢ and its contribution to atmospheric methane concentration in CAM-chem

Abstract:

Understanding the temporal and spatial variation of wetland methane emissions is essential to the estimation of the global methane budget. Our goal for this study is three-fold: (i) to evaluate the wetland methane fluxes simulated in two versions of the Community Land Model, the Carbon-Nitrogen (CN, i.e. CLM4.0) and the Biogeochemistry (BGC, i.e. CLM4.5) versions using the methane emission model CLM4Me¢ so as to determine the sensitivity of the emissions to the underlying carbon model; (ii) to compare the simulated atmospheric methane concentrations to observations, including latitudinal gradients and interannual variability so as to determine the extent to which the atmospheric observations constrain the emissions; (iii) to understand the drivers of seasonal and interannual variability in atmospheric methane concentration. Simulations of the transport and removal of methane use the Community Atmosphere Model (CAM-chem) model in conjunction with CLM4Me¢ methane emissions from both CN and BGC simulations and other methane emission sources from literature. In each case we compare model simulated atmospheric methane concentration with observations. In addition, we simulate the atmospheric concentrations based on the TransCom wetland and rice paddy emissions derived from a different terrestrial ecosystem model VISIT. Our analysis indicates CN wetland methane emissions are higher in tropics and lower in high latitudes than emissions from BGC. CAM-chem model simulations with CLM4Me¢ methane emissions suggest that both prescribed anthropogenic and predicted wetlands methane emissions contribute substantially to seasonal and inter-annual variability in atmospheric methane concentration. Our results suggest that different spatial patterns of wetland emissions can have significant impacts on N-S atmospheric CH₄ concentration gradients and growth rates. Our analysis also indicate the existence of large uncertainties in terms of spatial patterns and magnitude of global wetland methane budgets, and that substantial uncertainty comes from the carbon model underlying the methane flux modules.