A surge in growth in emerging markets led to an excess of savings that pushed global long-term interest rates down between early 2000 and 2005, Greenspan wrote in an article. That caused mortgage rates and the benchmark Fed-funds rate to diverge after moving "in lockstep" from 1971 to 2002, he said.

The article is part of the former Fed chief's defense against charges in books such as Greenspan's Bubbles by William A. Fleckenstein that his policy of keeping rates too low for too long inflated the housing bubble. The collapse in the US subprime-mortgage market led to about $1.2 trillion in writedowns and the bankruptcy of Lehman Brothers Holdings Inc.

"Given the decoupling of monetary policy from long-term mortgage rates, accelerating the path of monetary tightening that the Fed pursued in 2004-2005 could not have prevented the housing bubble," Greenspan said.

The Fed cut its target rate for overnight lending between banks to 1 percent in June 2003 from 6.5 percent in December 2000, and left it unchanged for the next year. Between June 2004 and June 2006, it raised the rate in quarter-point moves to 5.25 percent.

Foreign demand for US Treasuries helped keep long-term debt yields from rising as the Fed started to raise rates in 2004, leading Greenspan in 2005 to call the anomaly a "conundrum". Foreign ownership of US government debt doubled between 2000 and 2005 to $2.03 trillion, Treasury data show.