The U.S. economy is likely facing a future of slower growth that will make it more vulnerable to economic shocks, says Moody’s Investors Service.

The rating agency is anticipating relatively strong GDP growth for this year, and again in 2016. Notwithstanding a weak first quarter, Moody’s says that it expects 2.8% annual growth for the next couple of years. But, it says that after that, it expects the U.S. economy “will likely move into a long-term period of growth that is lower than the average prior to the financial crisis”.

Its baseline scenario is for output growth to average 2.3% annually, over the long term; which, it notes, is significantly lower than the 3.7% growth that was experienced from 1992 to 2007. Under a gloomier forecast, Moody’s envisions a scenario where annual output growth averages just 1.5% over the next 15 years; and, its optimistic scenario forecasts growth of 3.2% annually.

The causes of the expected slowdown include an aging population, which will lead to a lower percentage of the population being active in the labour force, and a possible deceleration in overall productivity growth, Moody’s says.

This slower growth rate can support the U.S.’s current Aaa credit rating, Moody’s says, although it says the rating will be more vulnerable to economic shocks and fiscal policy changes.

“While the debt metrics could deteriorate due to slower growth, it would not necessarily render the government’s credit profile incompatible with its current rating in the next few years. Higher debt levels, however, would increase the vulnerability of the U.S. to potential shocks in the longer term, putting pressure on the country’s credit profile in the decade of the 2020s,” says Moody’s senior vice president, Steven Hess.

In its baseline scenario, Moody’s calls for the U.S. debt-to-GDP ratio to climb during the decade of the 2020s, to reach 87% by 2030, after remaining stable for the remainder of the current decade.