Former Federal Reserve Chairman Alan Greenspan warned that it won’t be much time before the spread of negative interest rates hits the U.S.

“You’re seeing it pretty much throughout the world. It’s only a matter of time before it’s more in the United States,” Greenspan told CNBC.

There are currently more than $16 trillion in negative-yielding debt instruments around the world as central banks try to ease monetary conditions to sustain the global economy. The 10-year sovereign bonds in Belgium, Germany, France and Japan — among others — are trading with a negative rate, CNBC.com explained.

An aging population is driving demand for bonds, pushing their yields lower, Greenspan said.

“We’re so used to the idea that we don’t have negative interest rates, but if you get a significant change in the attitude of the population, they look for coupon,” Greenspan said. “As a result of that, there’s a tendency to disregard the fact that that has an effect in the net interest rate that they receive,” said Greenspan, who led the central bank from 1987 to 2006.

He added that gold prices have been surging recently because people are looking for “hard” assets they know are going to have value down the road as the population ages. Gold futures are up more than 21% in 2019 and are trading around levels not seen since 2013.

For his part, U.S. President Donald Trump, who has decried the relative loftiness of U.S. interest rates compared to rates in nations that compete with the United States in global markets, recently said he did not want to see negative rates in the United States.

Trump, who has repeatedly noted that rates are negative in Germany - Europe’s trading powerhouse - made the comment in response to a question from reporters as he departed the White House for the presidential retreat of Camp David in Maryland, Reuters said.

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