China’s National Audit Office last counted the country’s local government debt at the end of 2010, when it put the figure at 10.72 trillion yuan, or 27% of GDP. But borrowing has exploded since then as local governments have sought to keep growth going as Beijing scaled back the huge stimulus it launched to offset the global financial crisis. U.S. state and local government debt, by comparison, stands at $3 trillion, according to the St. Louis Fed, which equals about 18% of GDP.

“Local government debt has been growing at a speed of nearly 20% a year in the last couple of years. If this trend continues, it will definitely bring about systemic risks for China’s economy,” said Nomura economist Zhiwei Zhang.

The numbers matter a lot to Beijing. Analysts say it could be on the hook for a significant portion of that debt if it goes bad, and yields on many of the bonds reflect the expectation that the central government would back the bonds. If the debt is backed by Beijing, they argue, it should be considered part of China’s national debt, pushing that total to a worrisome 200% of GDP, up from 129% at the end of 2008, according to Fitch Ratings. The U.S. debt-to-GDP ratio is 82%, according to the World Bank, not including state and local debt.

Another worry is who would lose if some of that debt went bad. According to Standard & Poor’s, 80% of local government bonds are owned by Chinese investment firms, which manage money for individuals and insurance companies, and some of the bonds have ended up in wealth-management products sold by banks to individuals. Foreign investors have negligible access to the bond market at the moment.