There’s this organic carrot, and it’s doing my head in. It’s a nice carrot, as carrots go: fat, orange, with feathery green tufts on top. It has lived a blameless life in a field of joy, innocent of pesticides and artificial fertilisers. And now, here it is in the supermarket, rooting me on to take it home. Only, here’s the thing: the carrot is from Israel. That’s nearly 2,500 miles away. If I buy it, I will take on its carbon footprint, garnishing every mouthful with the greenhouse gas that it has splurged into the atmosphere to be here today....