Janet Yellen is just 152 cm tall and looks like a sweet old lady. How looks can be deceptive. She has just taken over as the head of the U.S. Federal Reserve System, making her the most powerful woman in the world.

Time magazine called her the "16 trillion dollar woman" referring to the size of the U.S. economy, but as the events of the last few weeks have shown, she might more correctly be called the "88 trillion dollar woman," that being a rough calculation of the value of total gross world product, the annual value of goods and services produced worldwide, on a purchasing power parity basis. (In current dollars, the figure is lower, about $75 trillion.)

The question of the hour for Yellen, who took over this month from Ben Bernanke as the first woman head of the U.S. Federal Reserve System in its 100-year history, is whether she will accept that she has a responsibility to the global economy as well as to the United States.