Former Federal Reserve Chairman Alan Greenspan said nothing is stopping the U.S. from getting sucked into the global trend of negative yielding debt, Bloomberg reported Tuesday.

"There is international arbitrage going on in the bond market that is helping drive long-term Treasury yields lower," Greenspan said in a phone interview. "There is no barrier for U.S. Treasury yields going below zero. Zero has no meaning, beside being a certain level."

With global central banks engaging in unprecedented monetary easing, a record $15 trillion of government bonds worldwide now trade at negative yields. As uncertainty reigns, investors are looking for a safe haven for their money, even if it means getting back less than they gave.

"Why people continue to buy long-term Treasurys at such low yields may be also due to forces having altered people's time preferences," Greenspan said. "But there is hundreds of years of history showing the long-term stability in time preference, so these changes won't be forever."