Idea: Change BTC protocol to include a new field in coinbase transactions documenting the CO2 emissions of each block. Introduce an uncle block reward like ETH. Miners agree to audits of their reports. Make block reward dependent on carbon emissions. Use rest for offset programs.
What seems possible is to check what miners mined a block, and externally verify their footprint. But I don't think there is a decentralized way to do it. An entity certifying "green miners" would be a centralized authority.