Simulation of methane emissions from tropical wetlands and rice paddies in the Community Land Model (CLM4)-CN: Introduction and preliminary results

Methane (CH4) is the second most abundant greenhouse gas in the troposphere next to carbon dioxides. Although wetlands are thought to be the single largest source of atmospheric CH4, estimates vary from 80 to 260 Tg annually. Similarly, rice paddies are one of the largest anthropogenic sources, but methane emission rates have been estimated to be 20 to 120 Tg/year. The large uncertainty in the global methane budget is associated with poor understanding of environmental controls on methane production, oxidation, and transport through aerenchyma. Very few methane models have included all of these environmental controls, including climatic and hydrologic conditions, soil temperature, water table positions, soil pH, redox potential, and plant functional type, in predicting methane emissions. The goal of this paper is to help resolve the large uncertainty in methane emissions from wetlands and rice paddies by developing a process-based methane model that includes several environmental factors. This process-based model is coupled to the Community Land Model 4.0 (CLM4)-CN. A new pH function and a new redox potential function have been incorporated into the model to calculate methane production. The model is also forced with satellite fractional inundation and rice paddy fraction in order to further constrain the current global methane budget. Model simulations will be compared with both field measurements and other models’ estimates of global methane budgets. Sensitivity analysis will be conducted and several most important controls on methane emissions will be discussed.