These are the facts. George W Bush left behind a set of books that were not so much unbalanced as vertiginous. At the end of 2008, US debt was $9.9 trillion, or 69.7 per cent of GDP, and the ballooning deficit was $683 billion. Since then, all the key indicators have worsened markedly. By the end of this year, gross debt is forecast to reach $16.3 trillion (the number to which Letterman was alluding), more than 100 per cent of GDP, or a rise of two thirds under Obama. The annual deficit is close to $1.5 trillion, 10 per cent of GDP. Worse still, according to official forecasts, US debt is on course to hit $20 trillion by 2016.