NASA - National Aeronautics and Space Administration

A chemistry-transport model (CTM) intercomparison experiment (TransCom-CH4 ) has been designed to investigate the roles of surface emissions, transport and chemical loss in simulating the global methane distribution. Model simulations were conducted using twelve models and four model variants and results were archived for the period of 1990–2007. All but one model transports were driven by reanalysis products from 3 different meteorological agencies. The transport and removal of CH4 in six different emission scenarios were simulated, with net global emissions of 513 ± 9 and 514 ± 14 Tg CH4 yr−1 for the 1990s and 2000s, respectively. Additionally, sulfur hexafluoride (SF6 ) was simulated to check the interhemispheric transport, radon (222 Rn) to check the subgrid scale transport, and methyl chloroform (CH3 CCl3 ) to check the chemical removal by the tropospheric hydroxyl radical (OH). The results are compared to monthly or annual mean time series of CH4 , SF6 and CH3 CCl3 measurements from 8 selected background sites, and to satellite observations of CH4 in the upper troposphere and stratosphere. Most models adequately capture the vertical gradients in the stratosphere, the average long-term trends, seasonal cycles, interannual variations (IAVs) and interhemispheric (IH) gradients at the surface sites for SF6 , CH3 CCl3 and CH4 . The vertical gradients of all tracers between the surface and the upper troposphere