Tag name (required for release branches):
Originator(s): @jimmielin
AI tools used (if applicable; please also add the "AI-generated code" label to the PR):
What: claude-fable:5
How: wrote the companion PR

Description (include the issue title, and the keyword ['closes', 'fixes', 'resolves'] followed by the issue number):

• Companion PR to Export CO2 value to the land model via coupler atmospheric_physics#427
• Adds standard name of co2diag previously co2diag_tbd

Describe any changes made to build system:

Describe any changes made to the namelist:

List any changes to the defaults for the input datasets (e.g. boundary datasets):

List all files eliminated and why:

List all files added and what they do:

List all existing files that have been modified, and describe the changes:
(Helpful git command: git diff --name-status development...<your_branch_name>)
M src/data/registry.xml
add standard name for co2diag

If there are new failures (compared to the test/existing-test-failures.txt file),
have them OK'd by the gatekeeper, note them here, and add them to the file.
If there are baseline differences, include the test and the reason for the
diff. What is the nature of the change? Roundoff?

derecho/intel/aux_sima:

derecho/gnu/aux_sima:

derecho/nvhpc/aux_sima (test is run via Github workflow. Only run the test manually if we need to save new baselines):

  SMS_Ln2.ne3pg3_ne3pg3_mg37.FPHYStest.derecho_nvhpc.cam-outfrq_rrtmgp_derecho_gpu (Overall: NLFAIL) details:
    FAIL SMS_Ln2.ne3pg3_ne3pg3_mg37.FPHYStest.derecho_nvhpc.cam-outfrq_rrtmgp_derecho_gpu NLCOMP
  - extra NL variable: scale_dry_air_mass (appears to be intro'd in sima0_17_000 but nvhpc baseline missing for that tag.)

If this changes climate describe any run(s) done to evaluate the new
climate in enough detail that it(they) could be reproduced:

CAM-SIMA date used for the baseline comparison tests if different than latest: