GDP is a number without lot of meaning. Better thing to compare is the percentage of tax revenue. Total tax revenue of the world is $14.8T. So you need more than 30% relative increase in taxation.

Keep in mind that most countries subsidise fossil fuels so that energy and transportation remains cheap and drives the economy — or at least that's the logic I hear, idk if it actually works that way

By not doing that, you free up quite a bit of tax money. I can't imagine it's the whole 30% but it would bring it down. Emissions tax would be another way to fund this figure at the same time as having corporations find ways to reduce emissions