CO2 doesn't directly poison people in the same way the industrial waste/emissions poison people. This fundamental fact is one of the major reasons it is difficult to coordinate CO2 emissions reductions -- the harm is global but diffuse.

I was just commenting on the aluminium smelter thing being very, very non-environmentally friendly at a local level.

Depends on what the miners are being powered with.

Geothermal energy has very few emissions.

If Iceland shuts down aluminium smelters in favor of bitcoin miners it is a net benefit.

I wonder if it's more complicated that that. Narrowly looking at the CO2 effects of shutting down AL smelters in Iceland, that will make the global supply for the metal lower, raising prices. Perhaps that would make it uneconomical for usage in vehicles, meaning that cars would again use heavy steel instead of aluminum. Perhaps that would cause the global car fleet to burn more fuel and net pollute more CO2.