Visiting America last week to talk to audiences across the country about "global warming", I was struck by television commercials for the two presidential candidates. Senators McCain and Obama were each shown in front of film of the same giant wind farm, to lay claim to virtually identical "green" credentials. Since America has already built five times as many wind turbines as Britain, covering thousands of square miles, I checked out how much electricity all those 10,000 turbines actually produce. The answer is around 4.5 gigawatts - not much more than a single large coal-fired power station.