armchair academics take note in the event that bitcoin scales to a million times its current size and market cap over the next 30 years, its environmental impact will still be insignificant compared to existing systems."
When considering Moores Law, we can expect $/GH to continue to half every 18 months until at least 2020.
Dollars per hash is not any kind of a limit. If you can create more efficient hardware in the Watt per hash sense, it does not mean there will be less electricity used. It means that there will be more hashes calculated.
Electricity will always be the limiting factor.
Simple economics dictate that almost always, most of the block rewards will be spent on electricity. How can we be sure of this?
Armchair academics, take another note: Bitcoin mining market is one of the most perfect free markets ever created. Anybody can mine in their bedroom. If there are profits to be made, there will be miners. Rarely, if ever do you find a market more efficient than this.
Bitcoin miner is a perfect money printing machine. Electricity goes in, money goes out. This simple fact overwhelms everything else, and ensures that Bitcoin will always use a lot of electricity.