America’s economy is steaming along. Fourth-quarter GDP figures, released today, are expected to show that it grew by about 3.6% on an annualised basis. The world as a whole is doing less well: the IMF recently downgraded its global forecast for 2015. The euro zone is barely growing; Japan’s economy may well be shrinking; China’s, though still expanding fast, is decelerating. American strength may yet push the global economy forward. Yet it is also partly responsible for woes elsewhere. Recently the Federal Reserve has talked about raising interest rates from near zero. That has led to a swift rise in the dollar: a year ago a euro would buy you $1.40, but now it fetches just $1.10. That is a problem for the many economies with big dollar-denominated debts. The strong greenback could knock the global economy off course.