Modeling modern methane emissions from natural wetlands 2. Interannual variations 1982-1993

A global run of a process-based methane model [Walter et al., this issue] is performed using high-frequency atmospheric forcing fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses of the period from 1982 to 1993. Modeled methane emissions show high regional, seasonal, and interannual variability. Seasonal cycles of methane emissions are dominated by temperature in high-latitude wetlands, and by changes in the water table in tropical wetlands. Sensitivity tests show that globally, +/-1degreesC changes in temperature lead to 20% changes in methane emissions from wetlands. Uniform changes of 20% in precipitation alter methane emissions by about +/-8%. Limitations in the model are analyzed and the effects of sub-grid-scale variations in model parameters and errors in the input data are examined. Simulated interannual variations in methane emissions from wetlands are compared to observed atmospheric growth rate anomalies. Our model simulation results suggest that contributions from sources other than wetlands and/or the sinks are more important in the tropics than north of 30degreesN. In high northern latitudes it seems that a large part of the observed interannual variations can be explained by variations in wetland emissions. Our results also suggest that reduced wetland emissions played an important role in the observed negative methane growth rate anomaly in 1992.