Copper ETFs are rallying, with copper prices hitting their highest in almost four years, after China halted activities at its top producer in a bid to diminish winter pollution.

On Wednesday, the iPath Bloomberg Copper Subindex Total Return ETN (NYSEArca: JJC) was 0.1% higher, United States Copper Index Fund (NYSEARCA: CPER) was up 0.2% and copper miner-related Global X Copper Miners ETF (NYSEARCA: COPX) gained 2.1%.

Copper for delivery in three months rose $7,236.50 a metric ton or $3.28 per pound on the London Metal Exchange as trade resumed after the Christmas hiatus. Earlier pricing touched $7,259, its highest since January 2014. The metal has now rallied for nine days, the longest streak since 2004, and increased 31 percent this year, the most among LME metals after aluminum.

Copper prices and related ETPs have all rallied this year, especially since the lows earlier this month. Year-to-date, JJC rose 29.7%, CPER increased 27.8% and COPX advanced 34.0%.

Fueling the recent gains, Jiangxi Copper Co. was ordered to cut production for at least a week before a new assessment is made on local pollution levels, Bloomberg reports. This follows Tongling Nonferrous Metals Group’s instructions to make similar cuts earlier this month.

China has been cracking down on heavy pollution levels and has taken on a more hands-on approach to large polluters, especially during the winter months when heating is still reliant on coal.

The supply disruptions have bolstered copper prices just as the outlook for demand rises on a strengthening global economy and potential new avenues of demand, such as electric vehicles.

Looking ahead, Chile’s Codelco, the world’s largest producer, projected prices could test records above $10,000 while UBS Group AG’s wealth management unit predicted even more gains.

“The China shutdown news is certainly propping up copper further than it would be potentially without it,” David Meger, a director of metals trading at High Ridge Futures, told Bloomberg. “All of that is tied into this whole thread of optimism in U.S. and global growth.”

For more information on the copper market, visit our copper category.