There is a more positive effect if we eat organic, locally grown food and lower on the food chain. Last year in a major report the Union of Concerned Scientists said that the two most environmentally damaging things people do are driving and eating meat. The energy use in the production of meat is extraordinary. And cattle are one of the primary sources of methane. Also important is the waste they produce.
Cattle alone produce many times more waste than humans.
Feedlots and factory farms concentrate large numbers of animals. It's not cost effective for them to roam free. As a result there are concentrated piles of manure. Water use is also an issue. Half the water used in the U.S. is for meat production.