Houston is fighting a losing battle with its pension system: The unfunded liability between Houston’s three plans totals at least $3.9 billion, up from $212 million in 1992. Meanwhile, pension costs as a percentage of the city’s revenue have doubled since 2000 and were one of the reasons behind a recent credit rating downgrade.

A new report from Rice University’s Kinder Institute identifies two main culprits for the funding crisis: Even though the city is now paying its full pension bill, it’s still not enough to chip away at the unfunded liability, and the three plans have assumed investment returns of between 8 and 8.5 percent -- that's higher than the national average and even higher than their own recent experience.

The report's authors looked at examples of pension changes in other major cities and highlighted potential solutions, including raising the cap on the city’s revenues so it can generate more money for pensions; increasing employee contributions; and reducing cost-of-living payments to retirees. “All of these options would generate different amounts of funding in different time frames,” the report said. "[But] none would likely solve the problem alone.”