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This is precisely what ETS (Emissions Trading System) already is - you can buy the permissions, and you can trade them. They already *are* a commodity, so a new coin is not necessary. The problem is what you can actually buy for the coin, because this is where it gets political - for example, Germany "avoids" massive amounts of CO2 emissions by burning wood which of course emits CO2 but in ETS it's declared as zero-emissions.