It is in effect totally from fossil.
50% is self admitted. The other 50% - if no bitcoin mining then that 50% would go to replace fossil fuels in other places. Saving that amount of fossil fuel
Thus bitcoin mining increase total fossil fuel usage (and it is the total that matters not what each thing does) by the full amount of the enrgy minig uses.
This is a myth.
Energy, in a useable-to-humans form, does not generally "go to waste". Harvesting any kind of energy requires resources (windmills, solar panels, gas turbines, heat pumps, ...), many of them non-renewable themselves, as well as labor, that could always be deployed elsewhere.
https://www.argusmedia.com/en/news/2261931-bitcoin-miners-he...
Some will argue it becomes a crutch incentivizing oil to keep producing, but it's not like the plant was uneconomical or likely to close down before the recapture outfit came along.
TeraWulf is another interesting one - they plug directly into a nuclear plant and help smooth the demand curve off-peak (reactors can't quickly ramp up and down so the energy produced would otherwise go unused / unpaid for). Discussed in this article along with a few others:
https://www.forbes.com/sites/digital-assets/2023/07/04/4-bit...
In fairness, by some accounts (I can't personally attest to their accuracy) more than 50% of that is already from renewables, eg. https://www.nasdaq.com/articles/renewable-energy-sources-tha...
I do know from experience there are mining outfits that have located themselves where the energy was going to waste anyway.
But it doesn't excuse the huge use of power, and I wouldn't be surprised if over the long term a more efficient algorithm like proof-of-stake displaces proof-of-work. It's basic physics once you can do the same thing for less energy, and technologies more efficient than their predecessors tend to win out.
If by that time there's a popular proof-of-whatever coin with a similar focus, lean feature set and works-as-expected usability as Bitcoin I would definitely consider switching to it!