It's not the carbon content of the good or service, but the amount of carbon-equivalent of the greenhouse gases released by the production process. But this would make it tricky to tax automotive fuel for private consumption. But, in the case of final products the consumer can be charged for the expected environmental impact at the time of purchase, not from actual environmental impact at a later date.

Note that generally the 'carbon content' of the inputs is higher than the carbon content of the outputs of a production process, and the difference is released.

Can the last politician to go out the revolving door please turn the lights off?