Gün Sirer was quoted in a Vice magazine article on Bitcoin's sustainability, 
which pointed out that Bitcoin consumes 5000 times more energy per transaction 
than Visa. He noted that while keeping power consumption high has security 
benefits --- attackers would need to spend millions to launch a major attack --- 
the same security could still be achieved while allowing for more transactions, 
thus shrinking the cost per transaction.

There is much work going on at Cornell, by Ittay Eyal, Ari Juels, Elaine Shi, 
and Sirer, to address this issue.