Defined-benefit pension plans were already barely treading water heading into 2020. In the years ahead, the risk is as great as ever that a large swath of them will drown.

As the name implies, defined-benefit pensions promise to pay a set amount to retirees. While corporate America has largely moved away from this structure in favor of 401(k) defined-contribution plans, virtually all state and local governments still offer pensions' reliable retirement payouts. And they've been falling behind in a big way: In the 2019 fiscal year, states had $1.48 trillion in unfunded pension liabilities, while the 50 largest local governments faced $478 billion in adjusted net pension liabilities, according to calculations from Moody's Investors Service. The 100 largest corporate defined-benefit plans had a deficit of $285 billion in November, according to Milliman data.

That $2 trillion hole is only going to get deeper as the Federal Reserve pledges to keep interest rates near record-low levels for years to come as the United States emerges from the Covid-19 pandemic. Moody's, unlike many states and cities, uses a market-based discount rate to determine the present value of a pension's future liabilities. The lower the rate, the larger the current value. Analysts expect to apply a 2.7 percent rate to local governments' fiscal 2021 reporting, down from 4.14 percent in fiscal 2018 and about the same as Milliman's current discount rate for corporate pensions. It will likely cause pension shortfalls "to increase by double-digit percentages" in the next two years, Moody's says.