According to an article I read, the total lifecycle greenhouse emissions is 10% of fossil jet fuel.

Here’s how it works: A farmer grows crops like, say, beans, which take energy from the sun and carbon from the air, and use it to make tasty sugars and proteins. You eat the beans, and your body absorbs the easy nutrients to get. But the stuff that’s hard to get out is left in the food mass and turned into poo. You go to the toilet and your waste is collected by the sewage system. Then this company takes your poo, and uses energy from the grid to subject it to a process that makes crude oil. Then they distill jet fuel from the crude.

All of the carbon that is in the jet fuel came from those beans you ate, which got it from the air. So the jet fuel isn’t adding any new carbon to the air. There are still emissions associated with putting energy into the poo to refine it into oil, though, because it’s using energy from the grid.