Imagine owing over $100,000—not for a home or college, but simply for being an American.

That’s each citizen’s share of the nearly $40 trillion U.S. national debt—and it’s climbing fast. The government now spends more on interest than on Medicare or defense. Former Council of Economic Advisers Chair Greg Mankiw explains why this path is unsustainable and what it will take to fix it.

He lays out five possible outcomes: some painful, some unlikely, and all politically explosive.

One thing is clear: The bill is coming due.