Let's look at organic milk. Organic milk comes from dairy cows that aren't pumped full of hormones, and therefore produce 8 percent less milk than conventionally raise cattle. An organically raised cow will also personally emit 16 percent more emissions than their counterpart. In addition, the organic milk requires 80 percent more land to produce. The effect is threefold; more cows are required that individually emit more emissions over more land. The energy require to produce some organic vegetables is almost double the requirements for non-organic ones. Potentially found next to each other in supermarket, an organic tomato grown in a heated greenhouse in Britain generate 100 times more carbon dioxide than tomatoes grown non-organically in Spain.
There is one thing organic agriculture has in it's favor: better soil. Quite simply from the more restricted use of fertilizers, high levels of carbon is captured in the soil. According to the Rodale Institute, organic agriculture can remove 7,000 pounds of carbon dioxide from the atmosphere per year.
One of the biggest contributing factors to global warming from organic agriculture hasn't even been mentioned yet: transportation. When you buy an organic product, that product likely has to travel thousands of miles to reach you in a refrigerated truck belching greenhouse gasses all along the way. So forget organic, and buy local.