The Federal Reserve's strategy of keeping interest rates low to coax the economy back on track has proved costly for corporate defined-benefit pension plans, whose funding is deteriorating as a result of the low rates. Actuarial consulting company Milliman calculates that the total combined deficit of the 100 biggest corporate pension plans grew by $106 billion in August, with $91 billion, or 84% of that increase, reflecting the expansion in the plans' liabilities caused by lower rates.

The double-A corporate bond rate used to estimate corporate plans' liabilities "is very low right now, and that's probably the number one driver of a low funded status," says Gordon Young, integrated retirement financial management leader at HR consultancy Mercer.

The double-A corporate rate was 4.94% in August, its lowest level in a decade. Young says the equilibrium level is probably around 6%. "It's going to go up eventually," he says. "But we don't have a view of how quickly it will go up, or whether it might go down first, and then go up."