Guardian: The decision to allow a third runway at Heathrow was a mistake, according to one of the government’s most senior advisers on the economics of climate change. Lord Stern said if the decision had been properly considered in light of the UK’s climate targets, he thought it would not have been granted. Speaking at the Guardian Hay festival , Stern said: “You cannot have a carbon policy which is quite tough and ambitious and then take a decision like that [Heathrow] which could be …