As the federal debt has gone from astounding to unbelievable to incomprehensible, a new problem has emerged: The US government is actually running out of places to borrow.

How Many Zeros Are in a Trillion?

The $20 trillion debt is already twice the annual revenues collected by all the world’s governments combined. Counting unfunded liabilities, which include promised Social Security, Medicare, and government pension payments that Washington will not have the money to pay, the federal government actually owes somewhere between $100 trillion and $200 trillion. The numbers are so ridiculously large that even the uncertainty in the figures exceeds the annual economic output of the entire planet.

Since 2000, the federal debt has grown at an average annual rate of 8.2%, doubling from $10 trillion to $20 trillion in the past eight years alone. Who loaned the government this money? Four groups: foreigners, Americans, the Federal Reserve, and government trust funds. But over the past decade, three of these groups have cut back significantly on their lending.

Foreign investors have slowed the growth in their lending from over 20% per year in the early 2000s to less than 3% per year today. Excluding the Great Recession years, American investors have been cutting back on how much they lend the federal government by an average of 2% each year.

The Fed is the only game left in town.

Social Security, though, presents an even bigger problem. The federal government borrowed all the Social Security surpluses of the past 80 years. But starting this year, and continuing either forever or until Congress overhauls the program (which may be the same thing), Social Security will only generate deficits. Not only is the government no longer able to borrow from Social Security, it will have to start paying back what it owes – assuming the government plans on making good on its obligations.