A 49 percent spike in unfunded pension promises could drive up taxpayer contributions to Sonoma County government pensions by $13.6 million over the next three years.

For the second time in two years, officials overseeing the pension system for Sonoma County government have lowered the fund’s projected rate of return on investments, a move that will increase taxpayer costs in the short term but is intended to reduce long-term market-driven shortfalls.

County pension costs are up more than 400 percent since 2000 and the average annual compensation on which pensions are computed has risen 75 percent during that time to nearly $92,000 for workers retiring in 2011. The Board of Supervisors, in charge of setting benefits for a retirement system they acknowledge is unsustainable, has made no changes despite public outcry that bloated pensions are compromising essential public services. But last week, they indicated add-ons like ones that boost pensions would be high on their list of fixes.

A 42 percent spike in unfunded pension promises could further drive up taxpayer contributions to Sonoma County’s retirement fund, with costs jumping by a third next year and going up by millions more each year through 2017. The projected increases, triggered mostly by investment losses, were contained in a pair of sobering reports accepted Wednesday by the board of the $1.87 billion county government pension system.

California’s pension system lowered a key estimate of future investment returns Wednesday, a move that will drive up pension costs for cities across Sonoma County and further squeeze public services. The decision will mean yet another hit to beleaguered local budgets as CalPERS jacks up pension contributions by public agencies to make up for lower investment returns.

A state appellate court has upheld a lower court ruling that requires the disclosure of pension figures for thousands of Sonoma County government retirees.