Independently, selloffs drive a debt crisis and spillover to other assets.

Should the U.S. fail to pay its debts, in addition to creating havoc in global stock markets and sending the American economy into recession, it would trigger a sell-off in U.S. Treasury bonds, weakening the dollar and raising interest rates. This would affect foreign currency reserves held by other countries and make the costs of borrowing more expensive, potentially leading countries with already high levels of borrowing into a debt crisis.

"If interest rates in the United States go up, it's going to take all other interest rates up with it. It's going to make all other risk assets look very shaky," said Desmond Lachman, former deputy director at the International Monetary Fund and now a fellow at the American Enterprise Institute.

Lachman agreed with Yellen that a U.S. debt default would be an economic catastrophe that must be avoided.