Sure, but it's also the biggest single source of calories in the world (wikipedia says 1/5 of globally consumed calories, citing a book), making it a good point

Sure, but it's also the biggest single source of calories in the world (wikipedia says 1/5 of globally consumed calories, citing a book), making it a good point of comparison. And on top of that, if you look at the linked data, rice isn't even the most efficient (in terms of CO2/calorie) food source. I'm not suggesting everyone switch from beef to rice, I'm just saying that beef is egregiously inefficient. For comparison, even rice is 17 as bad as nuts, and beef is over 500x as bad as nuts. Also you basically can live on potatoes, and the chart shows root vegetables as basically on-par with rice for CO2/kcal. Beef is just so bad that any other diet looks fantastic in comparison.