“Because public blockchain networks lack a central authority, consensus mechanisms enable trust-less oversight. These autonomous rules are specific to the blockchain network on which they operate. For any transaction to occur on a decentralized network, all network participants (nodes) must reach consensus (agree).
Although novel in concept, reaching consensus can sometimes require the use of a lot of electricity. Further, because blockchains rely on a vast network of decentralized computers, they naturally draw more energy than centralized solutions.
In combination, these factors drive an eco-conscious narrative that questions the sustainability of specific consensus mechanisms — namely, Proof of Work (PoW).”
Source: Lunie HQ, Blog Dec 5, 2019 why-is-proof-of-stake-better-for-the-environment