The price of copper surged as much as 17% Monday on the Chicago Mercantile Exchange after President Donald Trump called for 50% tariffs on the red metal. By the end of the day, copper was still trading about 9% higher than before the announcement.

But the price jump happened only in the United States. In London and Shanghai, copper prices remained relatively stable. The reason? Tariffs affect U.S. importers, and the policy is meant to encourage domestic production.

“The U.S. has been sucking in lots of copper that it didn’t really need from around the world,” Albert Mackenzie is a copper analyst at Benchmark Mineral Intelligence. 

President Trump had signaled that the tariffs were coming months ago, prompting many U.S. buyers to stockpile copper ahead of time.

“One year’s worth of copper has now been imported in a six-month time frame,” said Ole Hansen, head of commodity strategy at Saxo Bank in Denmark.

Even in normal times, the United States imports a lot more than is actually used, Hansen said.

“The U.S. imports more than half the copper they need on an annual basis,” he said.

Meanwhile, the U.S. actually exports a significant amount of raw copper material — such as concentrate and scrap — according to Mackenzie.

“In theory, if it could process all of those raw materials, it wouldn’t need to import,” Mackenzie said. “So, it is a processing deficit rather than a raw material deficit.”

What the U.S. lacks is the infrastructure to process that raw copper into high-purity metal. The country currently has only two operational smelters.

“We’d have to restart a smelter that is on care and maintenance, or expand existing operations, or potentially increase the amount of recycling that is done,” said Mike Moats, a professor of metallurgical engineering at Missouri University of Science and Technology.

There is one issue though, Moats added.

“None of those are an easy thing to do and will be done in a time frame of months,” he said.

Restarting an idle smelter could take years. Building a new one from scratch could take a decade. The White House has floated the idea of constructing smelters on military bases to cut down on permitting delays. But for whoever would build a smelter, there’s still the same question around most of the president’s tariffs: 

“How long is this tariff gonna last?” Moats asked.

Maybe not long enough to justify a multi-billion-dollar investment. That uncertainty has buyers wary — and in the meantime, Americans may be paying significantly more for copper.