A carbon nightmare Daniel Acker/Bloomberg via Getty Images

WANDER around the local supermarket and you will struggle to find any clues to the environmental impact of the food you eat. If you are lucky, some of the seafood might bear the mark of the Marine Stewardship Council, which certifies fish caught in a sustainable way, but that’s about it.

Yet farming is the second largest source of greenhouse gas emissions worldwide, only slightly behind heating and electricity. And while it’s relatively easy to cut emissions from electricity by switching to solar, reducing emissions from farming is a tougher nut to …