Former Federal Reserve Chairman Alan Greenspan said the U.S. may soon face higher borrowing costs on its swelling debt and called for a “tectonic shift” in fiscal policy to contain borrowing.

“Perceptions of a large U.S. borrowing capacity are misleading,” and current long-term bond yields are masking America’s debt challenge, Greenspan wrote in an opinion piece posted on the Wall Street Journal’s website. “Long-term rate increases can emerge with unexpected suddenness,” such as the 4 percentage point surge over four months in 1979-80, he said.

Greenspan rebutted “misplaced” concern that reducing the deficit would put the economic recovery in danger, entering a debate among global policy makers about how quickly to exit from stimulus measures adopted during the financial crisis. U.S. Treasury Secretary Timothy F. Geithner said this month that while fiscal tightening is needed over the “medium term,”   More from Bloomberg, here.