It will not be long before the spread of negative interest rates reaches the U.S., former Federal Reserve Chairman Alan Greenspan said.

"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's "Squawk on the Street" on Wednesday, adding investors should watch the 30-year Treasury yield.

The 30-year U.S. rate traded at 1.95% midday Wednesday. It reached an all-time low last week.

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.