Central banks are essentially out of ammunition, with zero and negative interest rate policies spurring greater savings, not growth, said Michael Heise, chief economist at Allianz Group.
Moves by central banks from Japan to the euro zone to slash interest rates below zero have upended financial markets: investors are now paying some governments for the privilege of parking their funds while commercial lenders are mulling storing their cash in costly vaults instead of keeping them with central banks.
Despite the stimulus, economic growth remains feeble.
"Monetary policy has basically run its course in stimulating the economies," said Heise in an exclusive interview with CNBC in Singapore. "The Japanese example is very telling."
In late January, the Bank of Japan blindsided global financial markets by adopting negative interest rates for the first time ever - a move that should have spurred outflows of the local currency. But instead, the yen surged and signs of the intended effects, such as increased bank loans, have been scarce.
"The impact of monetary policy is actually counter intuitive and that shows that there's a lot of uncertainty that policy makers are facing," Heise said. "They can't even be sure how their instruments are going to affect the economy."