OCN_TRACER_MODULES (found in env_build.xml): POP2, the active ocean component, always computes temperature and salinity. There are additional tracers that can also be computed, and they are broken into different modules that can be mixed and matched. Including iage in this variable will turn on the ideal age tracer; including cfc will turn on the two cfc tracers (cfc11 and cfc12, I believe). ecosys turns on the ocean BGC tracers, the exact count of which has changed over time so it will depend on which version of CESM you run. Somewhere in the neighborhood of 25 or 30 tracers, though.CCSM_BGC (found in env_run.xml): controls whether co2 fluxes from one component are applied to another (e.g. the land model can receive co2 flux from the atmosphere instead of reading it from a file or using a constant value)ecosys tracer module compute chlorophyll; this flag determines whether that computed surface chlorophyll concentration is used to determine shortwave penetration, or if the model should use chlorophyll from a dataset. If the latter is selected, then turning on the BGC ocean tracers does not change the dynamics of the model (choosing the former means the BGC tracers feedback to the rest of the model)ecosys tracer module. I am not familiar with the atmosphere model, so I don't know the details.