The outlook for private single employer pension plans has brightened. But don’t pop the cork just yet.

The Pension Benefit Guaranty Corporation, which guarantees these plans, reported Friday that last year’s projections for its single employer defined benefit insurance fund have morphed from a $4.9 billion deficit by 2025 into a $2.6 billion surplus.

But it’s not cause for celebration, says a PBGC official, because most of that $7.5 billion turnaround ($6.3 billion) stems from actuaries predicting people will die sooner than they thought they would.

Meanwhile, forecasts for union multiemployer pension plans continue to be as gloomy as they have been since the financial crisis.

The agency said funding shortfalls for the multiemployer insurance fund could threaten the retirement security of 1.5 million workers and their families.

The continuing deterioration of union plans means that employer premium increases of up to 552 percent would be needed to keep the multiemployer fund solvent for the next 20 years, the PBGC warned.

Employer premium hikes of 59 percent to 85 percent are needed to pay projected benefits fully for 10 years and hikes from 363 percent to 552 percent are needed to pay for 20 years.

The current annual premium for an employer per participant is $27.

Congress would have to approve the increase.

Under the PBGC’s projections, the agency’s multiemployer fund will use up all of its assets by 2025.

The benefits PBGC pays to beneficiaries of failed multiemployer plans are based on a scale that calculates years of employee service. The payouts are less than they are for single-employer plans.

The maximum annual guarantees for multiemployer plans range from $1,320 for a worker with 10 years of service to $12,870 for a worker with 30 years on the job to $17,160 for an employee with 40 years.

By contrast, the maximum guaranteed benefit for a single-employer plan participant is $60,136 per year.

How the increase is designed is important, as well as the amount, the report says.

“A well designed increase may encourage additional contributions, encourage continued participation in plans, and strengthen the multiemployer system,” said the report. “A poorly designed premium increase may encourage employer withdrawals and accelerate plan insolvency with a resulting cost to plan participants and a need for even larger premiums.”

The PBGC insures 1,400 multiemployer defined benefit union pension plans covering more than 10 million participants. These account for one out of every four private sector workers in defined benefit plans in the nation.

For single employers, the agency covers 22,000 plans with more than 30 million participants.