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Another option would be to reduce the network's 12. 5 bitcoin-per-block reward sooner than the scheduled 2020 reduction. But that's easier said than done. Bitcoin mining companies are not going to go along with this willingly, and Bitcoin fans are likely to oppose such a move as well.
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Reducing bitcoin reward for newly mined blocks (it happened twice in the past, 50->25->12.5) never had the effect on mining hashrate whatsoever. So why do you expect it would have its effect now?
To make my post more constructive, let me suggest something:
Maybe we as humanity should work on getting to better sources of energy, instead? One such option is to produce the energy with solar panels on Earth's orbit and the energy would be beamed using microwave technology to Earth's surface. This work is well advanced, you know, and it would really be green energy.