It's possible to estimate an upper bound based on the dollar value of the BTC miners receive as revenue from inflation (subsidy) and transaction fees. The infla

It's possible to estimate an upper bound based on the dollar value of the BTC miners receive as revenue from inflation (subsidy) and transaction fees. The inflation rate in Bitcoin drops over time; transaction fees will likely increase, although not in proportion to the total economic value of transactions (and other uses) secured by Bitcoin (bitcoin is not "backed by energy", it is secured by energy expenditure). Currently miner revenue is less than $7 million/day. It's a bit tricky to convert that into, say, CO2 emitted as miners tend to use the cheapest possible sources of energy, which are stranded renewable resources like hydroelectricity dams built in places distant from consumers (apparently China among others have built a lot of uneconomic dams for various political reasons). The PoW security model doesn't require the amount of energy destroyed to increase indefinitely as a percentage of total value secured. Rather, the amount of energy destroyed by PoW simply has to be more than the highest funded attacker can afford. That's easy to do, as attackers who would attack via PoW are few and far between - as your attacker gets more powerful (governments) there are much cheaper attacks that can be done than naively attacking PoW directly. In any case, I wouldn't be at all surprised if the amount of CO2 emitted by Bitcoin mining operations is dwarfed by the amount of CO2 emitted by plane travel associated with people attending Bitcoin conferences...