Change NDC target calculation from relative to absolute targets

[fschreyer]

Purpose of this PR

This PR changes the NDC target calculation from relative to absolute emissions targets and complements the mrremind change. Now:

NDC emissions target per region =

sum of emissions targets of all countries with targets + share of emissions of countries without target * NPI emissions in target year.

It also increments the input data revision to use the new absolute NDC targets with target filtering removed. This caused NDC targets to increase quite a bit in multiple REMIND regions (MEA, SSA, OAS etc., see below).

Moreover, the NPI emissions are not loaded from a mif file anymore, but come from the input_bau.gdx. This is a refactoring to ideally remove the dependency on the prepare_NDC.R script. This script can probably be removed, but let's check that in a separate process.

Type of change

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Parts concerned

• ☑️ GAMS Code
• ◻️ R-scripts
• ☑️ Documentation (GAMS incode documentation, comments, tutorials)
• ◻️ Input data / CES parameters
• ◻️ Tests, CI/CD (continuous integration/deployment)
• ◻️ Configuration (switches in main.gms, default.cfg, and scenario_config*.csv files)
• ◻️ Other (please give a description)

Impact

• ◻️ Bug fix
• ☑️ Refactoring
• ◻️ New feature
• ◻️ Change of parameter values or input data (including CES parameters)
• ◻️ Minor change (default scenarios show only small differences)
• ☑️ Fundamental change of results of default scenarios

Checklist

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In the end all checkboxes must be ticked before you can merge.

• I executed the automated model tests (make test) after my final commit and all tests pass (FAIL 0)
• I adjusted the reporting in remind2 if and where it was needed
• I adjusted the madrat packages (mrremind and other packages involved) for input data generation if and where it was needed
• My code follows the coding etiquette
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• I updated the CHANGELOG.md correctly (added, changed, fixed, removed, input data/calibration)

Further information (optional)

• Runs with these changes are here:
• Comparison of results (what changes by this PR?):

This is how the targets changed (circles are targets with old approach, triangles are new absolute targets without filtering out of too unambitious/ambitious targets):

The blue line is the new resulting NDC scenario.

I think, the China 2030 target still needs a check.

/p/tmp/schreyer/Modeling/remind/Dev1/compScen-NDC_Rel_vs_Abs_Targets-2026-04-01_13.47.47-H12-short.pdf

[Copilot]

Pull request overview

This PR updates the 45_carbonprice/NDC realization to use absolute (country-summed) NDC emissions targets at the REMIND-region level, and refactors the BAU/NPI emissions input to come from a reference GDX (via a new internal GAMS emissions aggregate) instead of relying on externally prepared reporting inputs.

Changes:

• Replace relative-to-2015 NDC target scaling with absolute NDC emissions targets (p45_EmiTargetAbs) in the regional target calculation.
• Load BAU/NPI “GHG excl. LULUCF & excl. bunkers” emissions from input_ref using a new core variable/equation (vm_emiGHG_exclLULUCF_exclBunkers).
• Add a new switch (cm_NDC_CO2PriceLimit_continuation) to optionally extend CO2 price limits beyond the target year with a growing cap and a minimum cap level.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
modules/45_carbonprice/NDC/preloop.gms	Switch regional NDC target computation to absolute emissions targets plus BAU share for non-target countries.
modules/45_carbonprice/NDC/datainput.gms	Load absolute NDC targets and overwrite BAU emissions using values loaded from input_ref based on a new core emissions aggregate.
modules/45_carbonprice/NDC/postsolve.gms	Add optional post-target-year continuation logic for CO2 price limits.
modules/45_carbonprice/NDC/input/files	Update listed NDC input artifact from relative-factor to absolute-target file.
core/equations.gms	Introduce equation computing total GHG excl. LULUCF and excl. bunkers for NDC use.
core/declarations.gms	Declare the new variable and equation for the NDC-specific emissions aggregate.
main.gms	Document and define the new continuation switch for NDC CO2 price limits.
config/default.cfg	Bump default input data revision to 7.90.

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[RahelMA]

Thank you, Felix this is a great and fundamental improvement of the NDC scenario! I only have minor comments ;)

[RahelMA]

Thats really nice. Thank you for this switch, I think it would be useful to have the percentage here variable.
(1 + 0.2 * (t2.val - t.val)) * sm_DptCO2_2_TDpGtC, maybe directly the input of the switch as a percentage and default to 0.2?

[fschreyer]

In principle, I agree. Let's see if we really need it once we spot cases in which we want to limit 2035 carbon prices. So far, my impression was that generally we don't want to limit carbon prices after 2030.

[RahelMA]

Could you please explain why you add this variable? in the NDC calculation we don't use it and in the reporting it is already there?!

[fschreyer]

It is used for reading in the emissions trajectory of the NPI run here. This replaces reading data from pm_BAU_reg_emi_wo_LU_wo_bunkers.cs4r. I'd remove these data files and the infrastructure for them later.