Many popular cryptocurrencies use "proof of work" to secure transactions. Proof of work is incredibly wasteful: current estimates suggest that it currently uses around 0.5% of the world's electricity. We cannot afford this waste if we want to reach net zero.
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Many computers used to mine cryptocurrencies are specialised, can't be repurposed and rapidly become obsolete. This generates a lot of electronic waste. Recent estimates have concluded that mining operations consume 4% of specialised semiconductor manufacturing and 20% of all graphics card sales. A ban on proof of work based cryptocurrencies, would not be a full ban on cryptocurrencies but would instead incentivise a move to more environmentally friendly mining algorithms such as proof of stake.