"tendency_of_X" means derivative of X with respect to time. The construction "atmosphere_moles_of_X" means the total number of moles of X in the entire atmosphere,i.e. summed over the atmospheric column and over the entire globe. The chemical formula for methane is CH4. Methane is a member of the group of hydrocarbons known as alkanes. Thereare standard names for the alkane group as well as for some of the individual species.