New Scientist: At the G20 summit in London last week, Barack Obama took time out from fixing the world’s financial system to promise that the US will “lead by example to reduce our carbon footprint”. But back in Washington, DC, Congress was saying, in effect, “not so fast”. The basis of Obama’s plan to reduce US emissions to 1990 levels by 2020 is a “cap-and-trade” system, in which emitters will be charged for every tonne of CO2 they put into the air. The cash — tens of billions of dollars of it — …