Disabling CO2 fertilization

[idunkl]

Dear community,
I have set up an attribution framework with CESM2, and have two questions about my setup.
So far I have simulated two runs:

My factual simulation with historical emissions:

COMP=FHIST_BGC
./xmlchange CCSM_BGC=CO2A

A counterfactual simulation with historical land-use and pre-industrial co2 and aerosol forcing otherwise

COMP=FHIST_BGC
./xmlchange CAM_NML_USE_CASE=1850_cam6
./xmlchange CAM_NAMELIST_OPTS=co2_cycle_rad_passive=.true.
./xmlchange CCSM_BGC=CO2A

Q1: I have copied the co2_cycle_rad_passive=.true. from a different setup from this forum, but I don't fully understand its explanation.

Q2: I want to start a third simulation with the radiative effects of CO2 in the atmosphere, but no CO2 fertilization in CLM. The vegetation should see constant pre-industrial CO2 concentration. According to the description in env_run.xml I can set CLM_CO2_TYPE to "constant", which means that clm sees CCSM_CO2_PPMV. Would this work if "CCSM_CO2_PPMV" value="284.7" but the atmosphere still used transient co2 due to "CAM_NML_USE_CASE" value="hist_cam6"? Also, if CLM gets a constant 284.7, does it mean that CO2 is constant across all latitudes and seasons? Is there an alternative way to disable CO2 fertilization?

Thanks a lot for the help!
Cheers,
Istvan

 

[slevis]

Q2: @idunkl it has been a long time since I tried what you're asking. If you're correct and it's as simple as setting CLM_CO2_TYPE to constant, then YES the land should see the 284.7 value globally and all the time.

If this doesn't work, then you could figure out where the co2 gets set in the land model code and hardwire it directly in the code to the value that you want.