[cesarb]

> It seems this problem should be addressed by more energy-efficient Bitcoin mining rigs

That doesn't work. Suppose, as a thought experiment, that every miner is instantly replaced by one twice as efficient. That means only that their owners can now run twice as many miners before the cost of energy makes it unprofitable. The Bitcoin protocol adjusts the difficulty so that, no matter how many miners or how efficient they are, on average a block is found every 10 minutes; it's the reward for these blocks that pays for the electricity, so the total amount of energy used by the Bitcoin network will tend to converge towards the block rewards divided by the electricity costs.