The 30-year Treasury yield has climbed back above 5%, its 2007 level, as investors confront inflation, deficits and a surge in AI-related borrowing.

U.S. Treasury bond sell-off coincided with the nonpartisan Congressional Budget Office's release of statistics that the national debt is about to cross $40 trillion.

Normally quiet, the bond market can occasionally send warning signals loud enough to hit stock markets worldwide and even grab the attention of U.S. presidents and other world…