A kilogramme of beef protein reared on a British hill farm can generate the equivalent of 643kg of carbon dioxide. A kilogramme of lamb protein produced in the same place can generate 749kg. One kilo of protein from either source, in other words, causes more greenhouse gas emissions than a passenger flying from London to New York. This is the worst case, and the figure comes from a farm whose soils have a high carbon content. But the numbers uncovered by a wider study are hardly reassuring: you could exchange your flight to New York for an average of 3kg of lamb protein from hill farms in England and Wales. You’d have to eat 300kg of soy protein to create the same impact.