It has been called the only metal with a PhD in economics, and for good reason. Copper’s record as a leading indicator of economic slowdown or recovery is impressive. Used mainly for wiring, plumbing or electronics, an uptick in demand often precedes a recovery in building and manufacturing. A recent rise in prices to nearly $5,000 a tonne in London trading, up almost 75 per cent from the December low, seems to underpin the “green shoots” thesis and the recovery in animal spirits. But Dr Copper’s diagnosis may be misleading this time.

While it is far simpler to take copper’s price rather than physical shipments as a proxy for global growth, this holds pitfalls. The most obvious is that commodity prices are set by supply as well as demand, so a miners’ strike in South America will send prices soaring in the same way as a sabotaged pipeline in Nigeria will for oil. Neither says anything about growth. With capacity utilisation tightening in recent years, copper price volatility has increased the odds of false signals. Other problems are speculation or stockpiling.