[Cthulhu_]

There is an argument that could be made that if that power was from a renewable source, it could instead have been used to replace non-renewable power output - all of bitcoin's power usage combined is several coal burning power plants, I'm sure.

Second there's the fact that most of that power is changed into heat. I don't think the heat emitted from anything using electricity is significant enough to have an impact on climate (outside of the cities at least), but still.