I haven't followed this thread closely, but mining is a considerable energy waster. What would be the alternative here that stops making cryptocurrency currently such a lousy deal for the environment?
I will leave out Proof os Stake as it's been already mentioned. Further, I don't think Proof of Stake is coming to most of the coins, not any time soon. The teoretical & engineering foundations for PoS are still being develped, and when it will come for ETH it will be an achievent, and it will make ethereum a pioneer of PoS.
Fundamentally I think it's up to the market (i.e. all of us!) to minimise mining energy consumption. Transfering assets without need of intermediaries, without being able to be censored by anyone certainly has intrinsic value. However, it's not concievable that this is worth say 10% of the planet's power. The market should instead lead the price to the correct value, given time (in fact, that's the job of the market).
Say the price of Ethereum drops to $200 and Bitcoin to $2000. Over night, none of the miners will be profitable now. 90% of them will just have to quit. So with these prices, mining will suddenly just use 10% of the power it consumed previously.
The coins themselves can also (more subtely) influence this, and they are in the case of Ethereum. If they feel miners are too incentivized, reducing block rewards and mining fees are tweeks they can attempt, without waiting for the market.
There are further tweaks possible for proof of work coins. A better control of the mining hardware is something that can have an impact. I'm looking at SIA for this, they create their own ASIC to mine the coin, and they can sell limited quantities of it to interested parties. They are in the position also to favor the distribution of hardware evenly to miners.
So the mining being migrated further towards ASICs would help as well. Being able to switch at will between coins is only helping GPUs and CPUs to mine continously, drawing power continously.
With ASICs there should be a much limited choice of coins, so one would think twice before buying a single - purpose piece of hardware, which depends on unstable prices. Also not being a consumer mass market product, it should be easier to control who purchases it and in what quantities