The funded status of pension plans has been on a wild ride since 2008, experiencing major declines in 2008, 2010 and 2011. The drops were due to the "double whammy of declining equity markets and lower interest rates," said Jonathan Barry, Mercer's U.S. Retirement Risk & Finance DB Risk Leader, during a recent webinar.

How plan sponsors manage that pension risk volatility was the subject of a joint Mercer/CFO Publishing study, "Redefining Pension Risk Management in a Volatile Economy."

Mercer conducts a monthly analysis of the S&P 1500. At the end of November, it found that aggregate S&P 1500 defined benefit pension plans were underfunded by $391 billion, which means they were funded at 75 percent. This was down from a funding ratio of 88 percent in April 2011, Barry said.