https://bitcointalksearch.org/topic/m.149581
https://bitcointalksearch.org/topic/m.156457
https://bitcointalksearch.org/topic/m.855455
https://bitcointalksearch.org/topic/m.1166103
And here's another one by Stephen Gornick, three weeks ago, pointing out the exact same thing:
https://bitcointalksearch.org/topic/m.1278020
Using the "4 MJ per banknote" figure in the presentation, and stats from the US Treasury, circulating one-dollar bills alone consumes 338 megawatts, or more than 50 times the entire Bitcoin network. And that's even ignoring the fact (pointed out in the presentation but ignored in the figures) that the number of Bitcoin transactions can increase by a factor of 10 before hitting even the artificial transaction limits imposed by the protocol, which can be removed. After that, you can read the wiki to see how easily Bitcoin can again scale by another factor of 300 with minimal increase in energy consumption.
Altogether, that gets Bitcoin to a market cap of around $10 trillion, while still beating paper money in energy consumption with basically zero modification at all.
Beyond that, if by some miracle Bitcoin ever comes remotely close to the stupid amount of waste generated by the traditional banking system (which it won't), proof of work can easily be replaced.
So energy usage is, again, probably the single biggest non-issue in Bitcoin ever.
I agree it's not easy. The only viable solution so far is the mentioned "proof of stake". A migration to that would be quite painful, though, I guess. But if it's tested enough and the pressure to remove PoW is great enough, it could probably be done.