There goes copper ...

Copper is at a six-year low.

On Thursday, copper fell nearly 2% to as low as $2.385 a pound, the weakest level since 2009.

This is part of a big sell off in commodities. Iron-ore prices have plunged, crude oil is trading below $50 per barrel, and gold is far from recovering after Sunday's flash crash and this week's continued weakness.

Each market has its own set of factors motivating traders to sell. However, concern about China's slowing economy is something that is working its way across a few markets.

In a note to clients on Wednesday, Goldman Sachs analysts downgraded their forecasts for copper prices.

They wrote:

"Though we have been bearish on copper on a 12-mo forward basis for the past two and a half years, we have maintained a more bullish medium to long-term stance on the assumption of Chinese copper demand growth of 4% per annum and a major slowing in supply growth around 2017/2018 ... we substantially lower our short, medium, and long-term copper price forecasts, on the back of lower Chinese copper demand growth forecasts (we have been highlighting that the risk has been skewed to the downside for some time), increased conviction in copper supply growth over the next three years, and increased conviction in the outlook for mining cost deflation in dollar terms."

They see the spot price of copper falling to as low as $4,500/t by the end of 2016, 20% below spot prices, and 30% below the consensus estimate.

Here's a chart showing the slide in copper prices on Thursday:

FInviz

And here's how the commodity has traded in the last five years:

Finviz