Exchange traded funds that track copper prices, a leading indicator for the global economy, slipped Friday as China’s growth prospects come into question.

The iPath AIG Copper Total Return Sub-Index ETN (NYSEArca: JJC) fell 1.8% Friday. JJC is has declined 13.3% year-to-date.

Copper futures were down 1.7% Friday, trading around $316.2 and moving into bear market territory.

“The copper market has been an accurate leading indicator, and it’s not pointing to a good bottom for China’s economy,” Jeffrey Sica, president of SICA Wealth Management, said in Bloomberg article. “That’s ominous, and it continues to be a major concern in markets.”

The Chinese economy experienced growth of under 8% in the past four quarters, the longest streak below 8% in at least 20 years.

Copper prices are seen as a gauge for the health of the Chinese economy as the country is a big consumer of the commodity. Investors are worried about a potential slowdown in China, the world’s second-largest economy. [Should You Listen to Copper ETF’s Warning?]

Moreover, on the supply side, copper production exceeded demand by 341,000 so far this year, compared to a total surplus of 238,000 tons for all of 2012.