‘The whole aim of practical politics,’ wrote H.L. Mencken, ‘is to keep the populace alarmed (and hence clamorous to be led to safety) by menacing it with an endless series of hobgoblins, all of them imaginary.’ Newspapers, politicians and pressure groups have been moving smoothly for decades from one forecast apocalypse to another (nuclear power, acid rain, the ozone layer, mad cow disease, nanotechnology, genetically modified crops, the millennium bug…) without waiting to be proved right or wrong.