I actually address this point in my post: "Many/most miners use hydroelectric power because competition compels them to use cheap renewable energy instead of other polluting power sources such as coal power plants" examples:

BitFury: https://www.businesswire.com/news/home/20151211005837/en/Bit...

KnC: http://thenodepole.com/2015/06/04/kncminer-to-build-new-20mw...

Therefore your worry about CO₂ emissions is misplaced.