It's probably quite small. Press journalists don't realize that Bitcoin has created an arbitrage market for power, such that miners travel the globe to find the cheapest sources of available power, which are renewable sources like overprovisioned hydroelectric where there is no demand for excess output (think: dams in rural China, far away from a city that the power could be economically sent to), and the mining reward lowers to compensate for that power cost. Today, the reward is now lower than the
cost of mining using fossil fuel power.
So, Bitcoin's contribution to fossil fuel power use is small because it has become unprofitable to mine Bitcoins using fossil fuels. The electricity would cost you more than the reward. It would be irrational to do it.
