Do you have any reason to assume that the infrastructure of banks is frendlier to the environment than Bitcoin mining is? The required logistics for thousands of buildings to be safely supplied with new fiat, the manual labour, enforcing security etc are probably way more demanding already.

Now add the whole high frequency trading thing and all the efforts that surround it on top of that and we are probably good for another few years before Bitcoin mining is even worthy of being mentioned. Until then, I don't see we wouldn't have worked out a better proof of work system that's more friendly to the environment, should this really become a concern.