All cases use modern atmospheric CH4 even for pre-industrial or transient cases

### Brief summary of bug

For I cases atmospheric concentration of CH4 comes from the paramfile which has a modern value in it. So hence it's wrong for pre-industrial and transient cases (or for that matter paleo cases).

### General bug information

**CTSM version you are using:** applies to all versions of ctsm

**Does this bug cause significantly incorrect results in the model's science?** Yes

At least for methane, it needs to be evaluated how big of a problem this is.

**Configurations affected:**

This value is only used when NOT coupled to CAM. When coupled to CAM it uses the values that CAM uses.

### Important output or errors that show the problem

bld/namelist_files> ncdump -v atmch4 $CSMDATA/lnd/clm2/paramdata/clm_params.c170913.nc | tail -2
atmch4 = 1.7e-06 ;
}
bld/namelist_files> ncdump -v atmch4 $CSMDATA/lnd/clm2/paramdata/clm5_params.c171117.nc | tail -2
atmch4 = 1.7e-06 ;
}

This problem was pointed out by Esther Brady...

```
Erik and Keith,

Thanks for all your help so far with getting our paleo CMIP runs going.

Bob and I have noticed that in all the current CESM2 runs, the atmospheric methane concentration in the land is set to 1700.e-9, which seems to be over twice the value prescribed in CAM for the preindustrial simulation and more like a 1990 value.

This seems to be prescribed in the paramfile used with variable atmch4.

We were thinking that we should change this for the paleo runs, but if all other cases have it constant at 1700e-9, then we will leave it as is.

Is this setting for the offline methane model not impactful in any way? It comes out in the history h0 file.

Thanks for any insight!
Esther

```