> Environmental externalities from cryptomining may be large, but have not been linked causally to mining incentives. We exploit daily variation in Bitcoin price as a natural experiment for an 86 megawatt waste coal-fired power plant with on-site cryptomining. We find that carbon emissions respond swiftly to mining incentives, with price elasticities of 0.69–0.71 in the short-run and 0.33–0.40 in the longer run. A $1 increase in Bitcoin price leads to $3.11–$6.79 in external damages from carbon emissions alone, well exceeding cryptomining’s value added (using a $190 social cost of carbon, but ignoring increased local air pollution). As cryptomining requires ever more computing power to mine a given number of blocks, our study highlights both the revitalization of US fossil assets and the need for financial industry accounting to incorporate cryptomining externalities.