The world's central banks have spent the past several years propping up the global economy and rescuing the world from crisis. Now, argues the head of the International Monetary Fund, it is time to grapple with the consequences.

The Federal Reserve, European Central Bank, Bank of Japan and other central banks have used a range of tools to try to spur growth and fight crises in their nations; the Fed, for example, is on track to soon own about $4 trillion in assets, up from the $800 billion it held before the crisis.

Christine Lagarde, managing director of the IMF, has been broadly supportive of these efforts, and she stresses that they should be unwound only as economic conditions improve. But in a speech to leading central bankers and economists Friday at the annual economic symposium organized by the Kansas City Fed, she urged more reckoning with some of the less pleasant side effects for global finance, particularly the risk of bubbles and instability.