That forecast is close to $7 million under the $26 million initially estimated for 2016.

Young said the first forecast was indeed based on worst-case assumptions. He qualified his numbers to the council in 2011 by saying the predictions were unlikely to come to fruition but were indeed possible.

Both the 2011 and recent data sets are based on assumptions made about CalPERS, which pools and invests contributions made by public employees in a variety of financial vehicles, such as stocks and bonds.

Because it is impossible to predict the behavior of financial markets, government agencies rely on forecasts made about CalPERS' likely rate of return.

Costa Mesa's

2011 chart assumed that CalPERS would lower its rate of return from 7.75% to 7.5% that March — a significant drop in a $236-billion portfolio. When the fund doesn't perform at expectations, cities and other agencies that pay into it must increase their contributions to make their contractually required pension payments.

Though 7.5% may seem like a healthy return to private investors, CalPERS has outperformed that figure over the long haul. From 1983 to the present, it returned 9.2%, according to agency spokeswoman Amy Norris.

The rolling average for the last 10 years has been 6.1% — which is below the present-day forecast — but is healthy considering the recession and volatile worldwide market.

There were other factors that influenced Young's worst-case scenario forecast in 2011: The chart assumed Costa Mesa's employees would stop paying into their own pensions once their current agreements ended, significantly increasing the city's obligation, or unfunded liability.

However, most city employees, save for the firefighters, had been contributing to their retirement plans since 2007. Whether they will continue to contribute to their plans in the future is something that will be collectively bargained, but several Southern California cities are reaching agreements that require employees to pay into their plans.

Righeimer, however, is not so sure that the employee associations would agree to concessions.

He said he told Young when he was preparing the budget data not to include forecasts for employee contributions.

"You never budget based on what somebody may or may not give you," he said recently.

Young stands by the numbers and denied receiving any input from the council, outside of a request in October or November 2010 to estimate future CalPERS payments.

"These were coming from me," he said.

He said he kept the contributions out to try and "not to influence future negotiations."

"Any forecast is based on assumptions, either good or bad," Young said. "People did look at one graph and say 'ah-ha!' It's all about how people receive it."

Had the quarter-point decrease taken effect, as Young calculated, it would have added 1% to 2% to the payroll contribution percentages Costa Mesa would owe nonpublic safety employee pensions, and 2% to 3% for police officers and firefighters.

Ultimately, CalPERS chose not to lower the rate that year.