It wasn’t very long ago that the dread hovering over global financial markets was that things were getting too calm. Just this summer, Federal Reserve officials were fretting over markets being so stable that it might create complacency, and we were writing about a global boom in asset prices.

Even if many Americans don’t fully realize it yet—though an unnerving drop in a wide range of global markets Wednesday may have gotten our collective attention—the autumn has brought a rather darker set of worries with a series of dives in financial markets across the globe.

On Wednesday alone, the Standard & Poor's 500 briefly fell into negative territory for the year and the interest rate investors were willing to accept on 10 year U.S. Treasury bonds edged below 2 percent for the first time since June 2013. (As of late morning, the S&P was down 1.4 percent for the day and narrowly up for the year, and the 10 year Treasury bond was back up to 2.05 percent).

But those moves underlie a bigger story: Many crucial indicators in markets for international bonds, currency and commodities are pointing toward a heightened risk of a worldwide economic slowdown that may be beyond the ability of policy makers to halt. It would inevitably have ripple effects even on the relatively strong American economy.