NEW YORK — Copper inventories in COMEX-registered warehouses are nearing full capacity, with another roughly 100,000 tonnes already bound for the United States in coming weeks, market sources said. U.S. exchange-monitored copper stocks have reached about 696,000 tonnes — roughly 69% of all copper held in warehouses tracked by the world's major metals exchanges — leaving buyers elsewhere scrambling for spot metal.

The buildup is a direct consequence of U.S. tariff policy. Since Washington signaled possible duties on refined copper imports as part of its critical-minerals strategy, American buyers have pulled purchases forward to avoid higher costs. Cargoes have converged on ports from New York to Baltimore to New Orleans, filling certified storage faster than the industry anticipated.

In Shanghai, the squeeze is unmistakable. Copper premiums — the amount buyers pay above the benchmark for immediate physical delivery — have climbed to a four-year high. Stocks at the Shanghai Futures Exchange have fallen to about 43,900 tonnes, the lowest since 2023, after shedding roughly 70% since early June. China consumes more than half of the world's copper across construction, electric vehicles, power-grid buildout and AI data centers. The drain of metal toward the U.S. has left Asian physical markets unusually tight.

The LME cash-to-three-month copper spread has flipped back into backwardation this week — prompt metal now trades at a premium to forward contracts, reversing weeks of contango — just as COMEX warehouses approach their ceiling. In effect, copper has accumulated on the wrong side of the Atlantic from where demand is strongest.

The dislocation is largely policy-driven. On paper, global refined copper supplies appear adequate. But tariff expectations have rerouted metal that would normally flow to Shanghai and Rotterdam toward U.S. warehouses, and once that storage reaches its limit, the arbitrage incentive that drew it there will fade. Traders expect the roughly 100,000 tonnes already in transit to be absorbed over the next four to eight weeks, after which the COMEX–Shanghai price gap should begin redirecting flows back toward Asia.

Copper has historically moved to the highest premium within days. This time, American warehouses are nearly full while Chinese buyers pay four-year-high premiums for metal that exists somewhere in the global system. How quickly that gap narrows — and when tariff policy itself becomes clearer — will likely set the tone for copper trading through year-end.