Setup 0p5xCO2 configuration

[blimlim]

1. Summary:
This PR sets up a configuration for running the abrupt-0p5xCO2 CMIP7 fast track experiment. It sets the CO2 MMR to 2.15945e-04, i.e. half of the preindustrial value of 4.3189e-04

2. Issues Addressed:

• 2x and 0p5x CO2 experiments #718

3. Dependencies (e.g. on payu, or model)

This change requires changes to (note pull request(s) where relevant):

• workflow manager (payu):
• model deployment (ACCESS-ESM1.6):
• model component or library dependency:
• input workflow:

4. Ad-hoc Testing

What ad-hoc testing was done? How are you convinced this change is correct (plots are good)?

5. CI Testing

• !test repro has been run

6. Reproducibility

Is this reproducible with the previous commit? (If not, why not?)

• Yes
• No - !test repro commit has been run.

7. Performance

Has the model performance (say, throughput of model-years/wall-day) changed?

• Yes
• No
• N/A (if selected, please add a brief explanation why performance testing is not necessary for this PR)

Setting change not expected to affect performance.

If yes, provide the numbers from your testing. Is the performance better or worse?

8. Manifests

Have you changed the executable, the input files and/or the restart files?

• Yes
• No

If yes, have you updated the manifests?

• Yes
• No

To update the manifests, run payu setup (in a cloned copy of your feature branch) with reproducibility tests turned off:

manifest:
  reproduce:
    exe: false
    input: false
    restart: false
runlog:
  enable: false

Then commit the newly created manifest files (under manifests/) only to the branch for this PR.

9. Documentation

Is the documentation updated?

• Yes
• N/A

10. Merge Strategy

• Merge commit
• Rebase and merge
• Squash

[blimlim]

!test repro commit

[github-actions]

❌ The Bitwise Reproducibility Check Failed ❌

When comparing:

• set-0p5-CO2 (checksums created using commit 0609e48), against
• dev-0p5xCO2+concentrations (checksums in commit a80e81a)

🔧 The new checksums will be committed to this PR, if they differ from what is on this branch.

Further information

The experiment can be found on Gadi at /scratch/tm70/repro-ci/experiments/access-esm1.6-configs/pr720/set-0p5-CO2/0609e4864faf4e06da122d6f493679e14ac922b5, and the test results at https://github.com/ACCESS-NRI/access-esm1.6-configs/runs/72825503161.

The checksums generated by this !test command are found in the testing/checksum directory of https://github.com/ACCESS-NRI/access-esm1.6-configs/actions/runs/24873518417/artifacts/6617728642.

The checksums compared against are found here https://github.com/ACCESS-NRI/access-esm1.6-configs/tree/a80e81a266ab7802b7a3a23532e36745abac6af7/testing/checksum

Test summary:
❌ test_repro_historical
✅ test_repro_determinism
✅ test_repro_payu_setup