I wrote this paper on this exact topic a few days ago:

https://docs.google.com/document/d/1Lb_iLMmH5sIiYfGeHr9tCTnaoWjK25zDlvO0nXXgEng/edit?usp=sharing

The TL,DR is that we're fine now, but as Bitcoin grows in value we may have a problem with electricity usage.  We (I) must reanalyze later!