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The MPGs do not contain an explicit, unconditional "shall" requirement for a country to recalculate or update the specific BAU scenario that was submitted with its NDC 2.0 solely for the purpose of its first BTR if the NDC itself has not been updated.

However, continuing to use an old BAU scenario without re-evaluation is often not conducive to the transparency, accuracy, and reliability that the ETF aims to achieve. The primary purpose of the BTR concerning mitigation is to track progress in implementing and achieving NDCs. An outdated BAU scenario - one that no longer accurately reflects current national circumstances, economic development, technological advancements, or the impacts of unforeseen events (excluding new climate policies not yet implemented) can lead to a misleading assessment of a country's mitigation efforts.

The ETF is built on the principles of Transparency, Accuracy, Completeness, Comparability, and Consistency (TACCC). If significant changes have occurred since the NDC 2.0 BAU was formulated, or if new data and improved methodologies are available, updating the BAU scenario would enhance the accuracy and transparency of the information presented in the BTR.

BTRs are an opportunity to provide the most current information. Updating projections, including the BAU scenario, reflects a commitment to robust reporting. Many countries update their BAU baselines when they submit new or updated NDCs (e.g., to incorporate more recent economic data or methodological improvements). While a BTR is not a new NDC, it is a key mechanism for accountability. A BTR will undergo a technical expert review. Using an up-to-date and well-justified BAU scenario will facilitate a smoother and more credible review process. It is important to note that many developing countries have utilized previous GWP values from the second to fourth IPCC Assessment Reports for the BAU scenario of 2.0, which will not align with the fifth assessment report according to the MPG.

BTRs are required to include projections of GHG emissions and removals (Decision 18/CMA.1, Annex, para 92). A BAU scenario is typically a core component of these projections ('without measures' scenario). If new projections are being developed for the BTR, it is logical to also update or revalidate the BAU.

If a country decides to continue using the BAU scenario from its NDC 2.0 submission for tracking in BTR1, it should clearly explain in the BTR why the existing BAU remains valid and appropriate for tracking progress. This would involve demonstrating that its key assumptions and underlying data are still representative of the country's current national circumstances and future outlook, absent new policies.

While not strictly mandated by a specific clause for an unchanged NDC, updating the BAU scenario for BTR1 is strongly encouraged if the previous BAU is outdated to ensure accurate and transparent tracking of NDC progress, in line with TACCC principles. If not updated, its continued validity must be justified.

If the BAU scenario is updated for the BTR, what should be the starting year for projections? This is clearly addressed by paragraph 95 of the MPGs. "Each Party’s projections shall begin from the most recent year in the Party’s national inventory report and extend at least 15 years beyond the next year ending in zero or five (e.g. 2030, 2035, 2040)."

According to paragraph 58 of the MPGs, the GHG inventory in a BTR must cover years up to "two years prior to the submission of the report" (e.g., if BTR1 is submitted in December 2024, the inventory should generally cover data up to 2022). Developing countries that need flexibility in light of their capacities may provide inventory data up to three years prior (e.g., up to 2021 for a BTR submitted in 2024).

Therefore, if a country updates its BAU scenario for BTR1, and the latest GHG inventory year reported in that BTR is, for instance, 2022 (for a BTR submitted in late 2024), then the projections for the updated BAU scenario must start from 2022, not from an earlier year like 2018 that might have been used for the NDC 2.0 submission. If the latest inventory year reported is 2023 (as in your example, perhaps for a BTR submitted later in 2025 with 2023 inventory data), then projections would start from 2023.

Paragraph 95 of the MPGs does offer flexibility to "those developing country Parties that need it in the light of their capacities." However, this flexibility, as generally interpreted and applied in guidance materials, pertains to the extension period of the projections ("instead extend their projections at least to the end point of their NDC under Article 4 of the Paris Agreement") rather than altering the requirement for the projection to begin from the most recent inventory year available in that BTR.

If a country needs to update its BAU scenario (or provide any new projections) for BTR1, the starting year for these projections must be the latest year for which GHG inventory data is reported in that BTR, in accordance with paragraph 95 of Decision 18/CMA.1. This ensures that projections are anchored to the most recent available historical emissions data.

The ETF encourages continuous improvement in reporting. For BTR1, while you might not be strictly forced to recalculate an NDC2.0 BAU if the NDC itself is unchanged, doing so if it's outdated is best practice. If you do provide updated projections (including a BAU) in the BTR, these projections must start from the latest reported inventory year in that BTR. This ensures that progress tracking is based on the most current and accurate information available, upholding the integrity of the Paris Agreement's transparency system.

From the perspective of international reporting and transparency, I fully agree with Jaypal’s explanation.

In practice, many non-Annex I Parties may not have clearly defined their accounting approach. For instance, they may not have specified whether their indicator for tracking progress is:

absolute GHG emissions in CO₂eq,
the difference between BAU and inventory in CO₂eq,
or the percentage reduction from BAU.
Often, NDCs are framed as “X% reduction from BAU (YY GgCO₂eq),” but without a firm definition of accounting modalities, ambiguity persists. If a Party has clearly defined these modalities in its NDC, then that definition should guide the tracking and reporting process.

However, if the BAU scenario is revised during the NDC implementation period, and the target is defined relative to BAU, this can complicate planning—especially in relation to the use or transfer of ITMOs. In this context, I personally advocate for maintaining the original BAU scenario throughout the NDC period for accounting purposes. This ensures consistency and comparability.

That said, updated BAU scenarios can still be useful for analytical purposes—such as comparing projections or improving future scenario development.