Currency and equity markets from Hong Kong to Johannesburg and London were already faltering. Then US stocks joined in, Bloomberg said. The stresses that have been building in global markets roared ashore in America on Thursday as the Standard & Poor’s 500 Index tumbled the most in 18 months and helped send the MSCI All-Country World Index to the lowest since January. Until now, the S&P 500 had traded in a narrow range even as China’s slowdown, Greece’s debt crisis and a plunge in emerging-market currencies roiled other markets. The Chicago Board Options Exchange Volatility Index soared 49% this week with one trading day remaining. Losses earlier in the week in American shares were limited as the rout in emerging-market assets deepened, with developing-nation equities sinking to the lowest level since 2009 and currencies from Malaysia to Kazakhstan tumbling. That changed Thursday as the S&P 500 closed below its average level of the past 200 days, wiping out gains for 2015 as investors sought the safety of gold and Treasuries.
