There's a firesale on raw materials like copper, aluminum, gold and oil, raising concerns about the health of the global economy.

The CRB raw industrials spot price index is now at its lowest level since November 2009. The Bloomberg Commodities Index touched levels unseen since June 2002.

Just this week, crude oil prices retreated to $50 a barrel, while gold tumbled below $1,100 an ounce to five-year lows.

And mining stocks like Newmont Mining (NEM), Barrick Gold (ABX) and Coeur Mining (CDE) are down 20% to 25% this month alone.

What's going on?

"It's a yellow flag for the global economy. I don't think it's a heads up that we're headed for disaster, but I do think it tells us something," said David Kelly, chief global strategist at JPMorgan Funds.

Related: Gold prices plunge to five-year low

Why are these commodities tanking? First, there simply isn't enough demand.

Soft global economic growth has hurt demand for some of the most closely-watched industrial metals like copper and iron ore, as well as oil.

While China is growing faster than many countries, it has slowed drastically in recent years. That's a critical factor, because China's previously-insatiable demand for natural resources made it the world's "swing consumer."

China's slowdown is playing a huge role in the demand picture. Growth in the first half of 2015 slowed to the weakest level since 2009 -- and there's growing rumblings among investors that Beijing may be fudging the numbers.

Many other emerging markets like Brazil and Russia are growing at a sluggish pace, too. So are developed economies like Europe and to a lesser extent the U.S. The International Monetary Fund recently downgraded its global growth forecast to 3.3%, the weakest pace since the financial crisis.

"We're not going into a global recession, but there isn't a lot of growth out there either," said Michael Block, chief strategist at Rhino Trading Partners.

Related: Is China cooking its books?

Supply glut, dollar strength: The poor demand is being exacerbated by too much supply. The commodities boom, fueled by China's explosive growth in previous years, caused energy and metals companies to ramp up production to levels that today's market just can't support. Look at how North American oil producers pumped so much oil out of the ground that now there is a huge supply glut.

And then there's the U.S. dollar. Because the American economy looks better than its peers and the Federal Reserve has stopped its stimulus program, the greenback has raced ahead of rival currencies like the euro and the yen.

That's bad news for commodities because most of them are priced in dollars. A stronger greenback depresses demand by making oil and other natural resources more expensive to buy in other currencies.

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Mining stocks tank: No wonder the S&P 500 energy and materials sectors are down this year 10% and 3%, respectively. And specific mining stocks like Newmont Mining (NEM), Barrick Gold (ABX) and Coeur Mining (CDE) are down 20% to 25% this month.

"The whole commodities complex is under fire here," said Block.

The commodities rout is also creating big headaches for countries in Latin America like Brazil that rely on natural resource exports to keep their economies going. The same is true for mining-centric Australia, which is trying to avoid its first recession in 24 years.

Related: The next domino to fall: Latin America

When will commodities bottom? At some point these prices will stop tanking. Eventually, enough production will get taken off line to balance the lack of demand, which should stabilize prices.

While no one knows exactly when that will happen though, Peter Boockvar thinks it's coming soon.

"This is the last throes of the commodity bear market," said Boockvar, chief market analyst at The Lindsey Group.

Will Janet Yellen delay rate hike? Until that happens, the commodities selloff may make Janet Yellen's job tougher. The Federal Reserve chief has signaled an interest rate is coming later this year, but lower commodity prices will only put inflation further behind the central bank's targets.

"If commodity prices are going to hell, that's going to hurt inflation. How does the Fed raise rates?" said Block.

Kelly disagrees, pointing out that U.S. inflation readings are looking better and jobs growth remains solid.

"I think it makes it more complicated, but it shouldn't change the Fed's decision -- if it's thinking clearly," said Kelly.