Mining rigs already require tremendous amounts of power, and there are already laws on the books that allow the government to know what you're doing with that power (i.e. for safety and environmental reasons). You can easily determine which power sinks are mining rigs, and require them to use only excess renewable power, or off-grid renewable power (it's pretty easy to tell if you're using off-grid fossil fuels with satellite imagery -- you can't hide the CO2).

Governments can easily enforce this -- simply require all blocks to be signed by a government-issued keypair to attests to the green-ness of the power used to produce them, and require exchanges under its jurisdiction to deny, limit, or tax the sale of non-green coins.