US national debt passed $40tn (£29.4tn) this week, a milestone economists say marks a significant warning sign for the world's largest economy, even as they stop short of calling it a crisis. The debt has doubled in the decade since 2016, driven by rising spending on social programmes, crisis responses such as the 2008 financial crash and the Covid pandemic, and tax cuts that have eroded revenue, all compounded more recently by higher interest rates that make servicing the debt increasingly costly.

The debt is growing by roughly $90,000 a second, or $7.8bn a day, and interest payments are now 15% higher than a year ago, consuming almost 20% of tax revenue — more than defence spending. The US is nearing its $41.1tn debt ceiling, with the Congressional Budget Office forecasting debt could reach $64tn by 2036. Economists note the US retains a "longer runway" than other nations thanks to the dollar's reserve currency status and comparatively lower debt-to-GDP ratio than Japan or Italy, but warn that diminishing investor appetite for US bonds is pushing borrowing costs higher for America and other countries alike.