In the specific case of proof-of-work cryptocurrency, I don’t think a carbon tax is directly useful: If it’s unilateral then mining trivially shifts across bord

In the specific case of proof-of-work cryptocurrency, I don’t think a carbon tax is directly useful: If it’s unilateral then mining trivially shifts across borders; if it’s global then it’s an inflationary pressure on the coin price which offsets the reduction in mining you might otherwise expect.