Gold is about 0.8 tons / troy oz. Given the current exchange rate of ~30 troy oz / BTC, that's 24 tons of CO2 / one-bitcoin-worth-of-gold.

At current mining rewards, 329,000 BTC are mined each year, producing 37 megatons of CO2, or about 110 tons of CO2 / one-bitcoin-worth-of-bitcoin.