The earth's atmosphere and environment is more changeable than we realise - we've only been looking at it for a blink of an eye in the scale of things so we can't claim to understand it. Not only that, they've worked out based on various factors that the CO2 levels were higher in the Cambrian, Ordovician and Jurassic periods. It's like trying to work out who farted in a lift full of people.
As for the locavore thing, it's only better because it's not usually mass produced crap so it has health benefits. Chickens, goats etc are a counterpoint to the OP's "theory" as they live off waste and are highly efficient meat generators. Not only that, anyone can manage them.
I grow huge amounts of edible things myself (because it tastes better and is cost, health and psycologically beneficial). I'm not labelling myself with Carbon Crusader, Green warrior, Locavore or whatever terms they think up next etc though - that's just silly.
You're ultimately trading your health for carbon guilt.