A cow cannot add a single atom of carbon to the sky, and the people who say she can have never followed one atom round. Grass is made out of air. The plant pulls carbon dioxide down and builds a leaf from it. The cow eats the leaf. The carbon is now in the cow. Some of it comes back out of her as methane. One carbon atom, four hydrogens, belched. Methane lasts about 12 years up there. Then it breaks back down into carbon dioxide. The grass pulls it down again. The cow eats the grass again. The same atom, round the same loop, since the first grazing animal walked onto the first grassland 50 million years ago. Nothing is added. The cow is a carbon atom's way of getting from one blade of grass to the next. A car works differently. The petrol was carbon that spent 300 million years locked under rock. Burn it and that carbon is in the sky, new, for a thousand years, and no cow on earth can bring it back down. A herd that stays the same size adds no warming at all. The methane going up equals the methane breaking down. Her grandmother's belch is already in this year's hay. The whole case against the cow rests on counting a loop as a leak. It is an error you can only make with the answer already decided.