The Methane Border Adjustment Mechanism Act amends the Internal Revenue Code of 1986 to establish a new tax on imported petroleum and natural gas. This mechanism, known as a Methane Border Adjustment Mechanism (MBAM), aims to reduce global methane emissions, which are identified as a significant contributor to climate change and public health issues. The bill's findings emphasize methane's potent warming power and the availability of cost-effective reduction methods.

Under the proposed mechanism, a tax is imposed on "methane adjustment substances" (initially petroleum and natural gas) when sold or used by an importer. The tax amount is determined by comparing the volume of the imported substance to the total volume produced in the exporting country, then multiplying this ratio by a hypothetical "total methane emissions charge" for that country. This charge is calculated as if the foreign facilities were subject to U.S. Clean Air Act methane regulations.

The bill includes provisions for the Secretary to periodically recommend additional substances for inclusion in the MBAM, particularly those reliant on oil and natural gas as feedstock or with significant embodied methane emissions. Furthermore, it directs the Secretary to facilitate the establishment of an international body. This body would be responsible for developing interoperable standards, creating a multi-stage certification process for supply chain emissions, and collecting and reporting methane emissions data globally.

An alternative tax calculation method is available for importers who can provide detailed supply chain emissions data, provided the exporting countries have specific customs agreements with the U.S. This allows for a more precise, supply-chain-specific tax rather than a country-wide average. Finally, the Secretary of the Treasury is tasked with encouraging major oil and gas importing countries to adopt similar methane border adjustment mechanisms and participate in the international data-sharing initiative.