Former Federal Reserve chairman Alan Greenspan said the recent rise in Treasury yields represents a “canary in the mine” that may signal further gains in interest rates. Higher yields reflect investor concerns over “this huge overhang of federal debt which we have never seen before,” Greenspan said in an interview today. “I’m very much concerned about the fiscal situation,” said Greenspan, 84, who headed the central bank from 1987 to 2006. An increase in long-term interest rates “will make the housing recovery very difficult to implement and put a dampening on capital investment as well.” The yield on 10-year Treasury notes was 3.86% at 11:02 a.m. in New York, little changed from late yesterday. That’s up from 3.69% at the end of last week. US interest-rate swap spreads declined to the lowest levels on record this week, reflecting investor concerns about the ability of nations to finance rising fiscal deficits.