When a country’s economy grows too slowly, the standard short-term remedies are to increase government spending, cut taxes or reduce interest rates. When none of those options is available, governments often resort to pushing down their currencies to make their exports more attractive to foreigners (and, these days, to push up import prices and thus bring inflation back up to desired levels).

When the world economy is sputtering, and every big country increases spending, cuts taxes and reduces interest rates, the global economy benefits from the increase in demand. That's the story of 2009.