When metals warehouses in top consumer China are so full that workers start stockpiling iron ore in granaries and copper in car parks, you know the global economy could be in trouble.

At Qingdao Port, home to one of China’s largest iron ore terminals, hundreds of mounds of iron ore, each as tall as a three-storey building, spill over into an area signposted “grains storage” and almost to the street.

Further south, some bonded warehouses in Shanghai are using carparks to store swollen copper stockpiles – another unusual phenomenon that bodes ill for global metal prices and raises questions about China’s ability to sustain its economic growth as the rest of the world falters.

Commodity markets are used to seeing China’s inventories swell in the first quarter, when manufacturing slows down due to the Lunar New Year holidays, and then gradually decline during the second quarter when industrial activity…