This is a big question with a lot of parts. One of the biggest contributors to global warming is believed to be methane from the digestive tracts and the manure disposal processes of raising cattle and pigs.
Additionally, large commercial agriculture uses a lot of diesel-powered machinery, a lot of pesticide sprays, and kicks up a lot of dust. Also the food tends to be trucked farther from farm to consumer, involving even more fuel usage and more pollution.
As #1 suggested, clearing forested land for agriculture is a problem too. The existing vegetation is usually burned, releasing particulate and stored carbon dioxide into the atmosphere, and also reducing the land's ability to remove carbon dioxide from the atmosphere, thus increasing the greenhouse effect.
Agricultural residual burning is significant source of carban black in the air. This activity also produces volatile organic compounds (VOCs).