Agriculture produces 10% of the greenhouse gases emitted by humans.

Let's say we can make a wish, and snap our fingers and have all the necessary factories in place to replace *ALL* food farming for the globe.

I doubt the entirety of the 10% of green houses gases will be eliminated, probably cut in half, however then we'll need to take into account the electricity needed to run all the new food factories, which will probably add back in another 3%, so once the dust clears we'll see a reduction of about %2 of green house gases, that's assuming all food agriculture is automatically replaced, via a wish.