Virtually all electricity has externalized environmental cost. Even electricity used to power a hospital. I agree this externality is immoral, not the hospital or PoW itself.
You don't see any ethical difference between powering a hospital versus thousands of video cards to solve a useless puzzle? None? Not even a little tiny bit?
What about the fact that those video cards use HUNDREDS of times more power than even a relatively busy hospital?
What about the fact that those video cards use HUNDREDS of times more power than even a relatively busy hospital?