The "Methane" experiment was proposed during the TransCom 2008
meeting in Utrecht. The first protocol was discussed during the
post-ICDC8 TransCom meeting in Jena, followed by the final
protocol in 2010. Since then 16 models or model variants have
performed the simulations. Previous TransCom experiments focused
on chemically non-reactive species (SF6, CO2, 222Rn). A CH4
intercomparison requires introduction of atmospheric chemistry,
which means a significant new model development for the
traditional TransCom participants. However, to focus on model
transport properties, the CH4 chemistry is reduced to offline
radical (OH, O1D, Cl) only, which means the full-chemistry
modellers have to scale down chemistry. During discussion at
Jena, methyl chloroform (CH3CCl3) was included for tracking
tropospheric OH abundance in the models, as well as SF6 and
222Rn for model transport evaluations. Prescribed fluxes are
input to a transport model and 20 years of simulation is run
with meteorological forcing appropriate for 1