If recession strikes, central banks might be out of ammo

"At the outset, we think the BOJ and ECB will have a very hard time repeating the big currency moves of the last few years," he said.

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"All three have adequate tools to deal with a moderate shock, but not an outright recession," said Ethan Harris, global economist at Bank of America Merrill Lynch, in a note.

Harris and his team looked at current 10-year bond yields for a "rough gauge" of how much firepower the central banks have at their disposal, on the theory that the basic aim of easing monetary policy is to push down the entire yield curve.