Monday’s report from the Congressional Budget Office on the country’s long-term economic outlook underscores what budget experts have long known: The rising cost of health care is the single largest driver of the gloomy long-term fiscal outlook for the U.S. Yes, the pace of increase has moderated in recent years. But many of the factors reducing cost increases are likely to prove transient.

In a balanced analysis of the evidence issued in September, the Center for American Progress (CAP) gives the Affordable Care Act some credit for slower growth—for example, by providing incentives that lower the rate at which patients are readmitted to hospitals after being discharged. But CAP also finds that between 37% and 70% of the slowdown is attributable to the Great Recession, which reduced the demand for health-care services by consumers with private insurance.

One recent study suggests that rising copayments and deductibles account for another 20%; other studies find that the number could be even higher. The use of new technologies has also moderated costs, and cheaper generic-drug prescriptions constitute a higher share of the total spent on medicines. The difficulty, the CAP report concludes, is that several of these factors had a “one-time effect” on health-care spending and “cannot be expected to moderate the growth of spending over the long term.”

A simple calculation underscores the significance of this finding. Holding the rate of increase in health-care costs to the growth rate of the economy would be an exceptional achievement. Even if that happened, federal health-care outlays as a share of GDP would rise by 37% over the next two decades. And this, says CAP, would eventually force either much higher taxes or steep reductions in discretionary spending, including vital public investments in infrastructure and basic research. The more likely projection—health-care costs rising between 1.15% and 1.65% a year faster than economic growth—makes the problem even more threatening.

There are grounds for hope, however. As often happens in the federal system, state experimentation may be blazing a trail for national policy.