Greenhouse gas Abatement Cost Model (GACMO)

Calculating current and future greenhouse gas emissions is a prerequisite for countries to inform political decisions concerning climate action. It is also the foundation for transparent and credible climate action at national and global levels.

The GACMO (Greenhouse Gas Abatement Cost Model) is a greenhouse gas emissions projections tool developed over more than twenty years by the UNEP Copenhagen Climate Centre.

It allows countries to carry out rapid but accurate scenario analysis of how a variety of mitigation options impact emissions, thereby increasing transparency and contributing to informed decision making.

The tool can provide valuable input for preparing and updating NDCs by identifying NDC targets for emissions reductions and by providing assessments of future emissions in different climate action scenarios. It can also be used to prepare biennial transparency reports (BTRs) under the Paris Agreement.

The GACMO tool can be downloaded and used free of charge.

The tool is regularly updated. The last version of the GACMO tool is always made available on this webpage.

A new version of the GACMO tool (version 2.3) is now available as of October 2025. This version maintains the same methodological approach as previous releases. Key updates in version 2.3 include:

1. A brief explanation of the approach to be used for defining units of mitigation options in the “Mitigation Options” sheet:

• Units of penetration for mitigation options in the column I should be defined relative to the start year, excluding those implemented before and during the start year, and including those implemented in the years following the start year (if those options are still operational). For example, the units of penetration of mitigation options in 2030 should include mitigation measures that were not only launched in 2030, but also those implemented earlier (such as in 2025) and still operational in 2030. Users should carefully consider the lifetime of each mitigation measure when specifying units of penetration in future years.”

2.  A revised approach for tracking progress in the “Tracking Progress” sheet:

• Achieved emission reductions from mitigation measures can be calculated annually from 2021 to 2035.
• Percentage achievement of progress is calculated for milestone years: 2025, 2030, and 2035.
• The unit penetration of implemented mitigation options now aligns with that of planned options: units for mitigation options in both the Tracking Progress and Mitigation Options sheets should be defined relative to the start year—excluding those implemented before or during the start year, and including those implemented in subsequent years (e.g., mitigation options for 2030 should include those implemented in 2025 and still operational in 2030).

The GACMO version 2 has been developed with the support of the Initiative for Climate Action Transparency (ICAT).

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