Barack Obama normally likes to talk about hope. Last week at the United Nations he had the audacity to fear. The US president conjured up a vision of a dystopian world: “Extremists sowing terror ... Protracted conflicts that grind on and on. Genocide and mass atrocities. More and more nations with nuclear weapons. Melting ice caps and ravaged populations.” That is our collective future, according to Mr Obama, unless the world’s leaders co-operate to find “global solutions to global problems”.