Assuming that miners are rational actors, the hash rate should settle at a point where the expenses to mine a block (energy, hardware depreciation, etc) equal the rewards (block reward and fees), so, if the price of BTC and the fees stay constant, halving the block reward should reduce energy waste.
Conversely, energy waste is proportional to, the BTC price.
Energy waste could also be reduced by instituting a global tax on Bitcoin mining. The cost to miners would remain the same (because it's still equal to the rewards), but a part of what would be spent on wasted energy is now spent on taxes instead. Unfortunately this is highly dependent on avoiding tax cheating.
Just BTC mining? Not all uses of energy? It makes more sense to do a fossil fuel tax, which penalizes all activity that doesn't justify its environmental cost, as opposed to playing whack-a-mole with whatever uses are least popular right now.
Crypto mining is unique in that the absolute consumption provides no benefit at all. The "work" consists purely in demonstrably having spent a certain amount of value, and the energy is purely an unit of accounting. So if you could manage to make this unit more expensive for EVERY miner, the accounting system would work as before, the safety properties of the system would be unchanged, and less energy would be used.
I can think of no other use of energy where this situation holds.
Conversely, energy waste is proportional to, the BTC price.
Energy waste could also be reduced by instituting a global tax on Bitcoin mining. The cost to miners would remain the same (because it's still equal to the rewards), but a part of what would be spent on wasted energy is now spent on taxes instead. Unfortunately this is highly dependent on avoiding tax cheating.