It's worth comparing Bitcoin's proof of work to other schemes of maintaining currency value to understand this in a relative sense. The Military Industrial Complex which arguably is the mechanism by which the dollar maintains its position as the global reserve currency puts out 152MtCO2/yr [1].

Without making any judgement on if this ratio is reasonable: this is 3-5x more than BTC POW but arguably also contains other negative externalities like loss of life, etc.

[1] https://earther.gizmodo.com/groundbreaking-report-gives-us-a...