What I'd like to add to the environmental argument is the fact that it really depends on the energy sources. We in Canada have are fortunate to have more clean energy sources than anywhere else in North America. When doing the calculations, the energy source has to be taken into consideration before examining the energy consumption aspects if we are to get an accurate calculation for the environmental impact.
Moreover, Bitcoin and other non-inflationary cryptocurrencies also provide a solid foundation for lowering energy use. If we are going to discuss the environmental footprint, then it's important to ask whether the economic system within which we operate, the debt-based monetary system that constantly goads us to consume more, is necessarily the right foundation to build on.
I'll end by pointing out that paper money has not been backed by gold since 1971, and is now tied to a country's sovereignty, and hence its status, along with its military and geopolitical alliances. That means it can go all the way to nuclear weapons. That's been included in the arguments. What's really needed, then, is an attempt to put it all in perspective when we ask whether or not Bitcoin mining is a pollutant.