Jerome Powell says the $39 trillion national debt is ‘not unsustainable,’ but warns the trajectory ‘will not end well’.

Federal Reserve Chair Jerome Powell offered a sobering assessment of America’s fiscal health on Monday, telling a Harvard economics class that while the nation’s $39 trillion debt load is not immediately dangerous, the path the country is on demands urgent attention from lawmakers.

“The level of the debt is not unsustainable,” Powell said during a wide-ranging conversation before roughly 400 students, “but the path is not sustainable. It will not end well if we don’t do something fairly soon.”

The remarks came in response to a student asking at what point the size of the U.S. debt breaks “the point of natural systems of repayment.” Powell acknowledged that no one knows exactly where that breaking point lies—pointing to Japan as a country carrying a far higher debt-to-GDP ratio than the U.S.—but said the direction of travel was unambiguous.

“What’s clear is that our debt is growing much faster; the federal government debt is growing substantially faster than our economy,” Powell said. “And that ratio is going up. And in the long run, that’s kind of the definition of unsustainable.”