(Bloomberg) — The 25 largest U.S. public pensions face about $2 trillion in unfunded liabilities, showing that investment returns can't keep up with ballooning obligations, according to Moody's Investors Service.

The 25 biggest systems by assets averaged a 7.45 percent return from 2004 to 2013, close to the expected 7.65 percent rate, Moody's said in a report released this week. Yet the New York-based credit rater's calculation of liabilities tripled in the eight years through 2012, according to the report.

"Despite the robust investment returns since 2004, annual growth in unfunded pension liabilities has outstripped these returns," Moody's said. "This growth is due to inadequate pension contributions, stemming from a variety of actuarial and funding practices, as well as the sheer growth of pension liabilities as benefit accruals accelerate with the passage of time, salary increases and additional years of service."