The Chancellor believes that it will cost a trillion pounds, which could otherwise be spent on welfare programmes, health and education; the  climate committee believes it will cost at least twice what the Treasury estimates; and the New Zealand Government have estimated that it will cost five times as much relative to their economy, as has been suggested. Given that, is it not irrational to enter this, as any, policy programme without first estimating the costs and calculating the benefits? Why are we doing it?