The exchange traded note that tracks copper, known by traders as the metal with a Ph.D. in economics, has been faltering despite an expanding global economy. Many are pointing to troubles in China as the main culprit in copper’s weakness.

The iPath Dow Jones-UBS Copper Total Return Sub-Index ETN (NYSEArca: JJC) has declined 12.8% year-to-date.

Copper prices are off to their worst start to a year since COMEX futures began trading in 1988, Bloomberg reports. In December, the correlation between copper prices and the S&P 500 was the lowest since 2008. [What the Gold/Copper Ratio is Saying]

The base metal, once considered a useful indicator of global economic growth because of its widespread industrial application, has been declining while the world economy is expected to expand the most in three years.

The metal’s status as a global bellwether may have been attributed to China’s robust demand over the past decade – China consumed five times as much as the second largest user, the U.S.

However, stockpiles in China have climbed for nine straight weeks, the longest advance since February 2012. China’s copper inventory is now up 75% since early January.

“The idea that copper is actually a barometer of global economic health is a bit misleading,” Neil Dutta, the head of U.S. economics at Renaissance Macro Research LLC, said in the article. “The U.S. recovery in auto and housing isn’t going to be able to offset the weakening demand of copper in China. That speaks more towards a mix of growth, not necessarily the health of the overall economy.”