Irrational exuberance in China’s commodities markets used to be a curiosity. Every few years the price of mung beans or garlic would spike by a few hundred percent, but few outside the country paid much attention. Now that Chinese speculators are moving the global price of key commodities, it’s no joking matter.

The price of iron ore has increased by 50% since the beginning of the year, driven largely by trading on the Dalian Commodity Exchange. On March 7 it increased by 20%, the largest one-day move ever. Yet analysts believe...