Copper just crashed to a 6-year low

Copper just tanked to a new six-year low.

On Thursday morning in New York, copper was down about 1.5%, near $2.183 a pound, the lowest level since 2009.

Copper is a key commodity because it is used in a whole lot of stuff that's manufactured, from cars to bathroom sinks.

And so, weaker demand for copper is sometimes seen as a symptom of underlying weakness in the world's biggest economies, particularly China.

Copper's decline is part of the broad sell-off in commodities we've seen this year that's also taken iron ore and crude oil to multi-year lows.

In their latest global economic outlook, Barclays economists wrote Thursday that the current cycle of low commodity prices would likely continue for longer.

"Although we do not expect any further significant declines in oil, copper or gold prices, the prospect of the type of v-shaped rebound that has characterized recent recoveries looks slim," they wrote to clients. "The bottom has probably been reached, but the stay in a markedly lower range is likely to prove a long one."

Here's a chart of the commodity in the last four years:

Here's a longer-term chart:

And according to Barclays, the current downturn in commodity prices matches or exceeds those experienced before or during economic crises: