The UV part of sunlight breaks it down into CO2 and water. And before you get too worried about the CO2, that's from "current" carbon in the environment (the grass that the cows ate) not fossilized carbon. So it's not a net addition of carbon to the atmosphere.

The carbon is coming from the methane, though. I don't have a degree in chemistry, but the carbon from the grass doesn't end up in the atmosphere naturally, does it?