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What about all the CO2 increases caused by cutting down trees in order to store value on paper, and physically mining gold?

However, the energy costs are upfront and non-recurring. The number of bit coins produced every 4 years is halved, capping out at about 21,000,000.

Nope, after 21M BTC are generated the miners keep going; new blocks are still needed to record transactions.

It's a valid concern, but it would be better to weight it against the current carbon costs of existing currencies.