Economist: ONE of the gestures that saved the UN climate conference in Copenhagen last December from complete meltdown was American support for the idea that, by 2020, $100 billion should be flowing from the north to the south every year to pay for emissions reduction and climate adaptation. In March Ban Ki-moon, the UN secretary-general, set up an “advisory group on finance” made up of financiers and politicians from developed and developing countries to report on how this rather arbitrary target might be…