Ghosts and goblins are so old-school. Here in the 21st century, we've got some new Halloween demons that will haunt your dreams -- and your net worth -- in true nightmarish style.

Take, for instance, the Baltic Dry Index. No, it's not a new kind of extra-cold ginger ale. The BDI is an index that tracks shipping rates -- in the cargo-ship sense -- around the world. The supply of shipping isn't very elastic -- ships are expensive, so the total number doesn't change a lot from year to year. That means rates are very responsive to demand. When demand is high, and all the ships are full, shippers can and do charge a significant premium. That's a sign that global trade is healthy.

And when demand is next to nothing, shipping gets very cheap. Like now. The BDI peaked at 11,793 this past May. On Thursday, it was at 885. That's not a typo. Now, to be fair, prices were way overheated earlier this year, and the current lows aren't unprecedented -- though there are ships out there that are being chartered for less than their operating costs.

That's got to hurt
Yes, the shippers aren't having much fun these days. If you thought your 401(k) had taken a hit, pull up a one-year chart on a major global shipper like Excel Maritime Carriers (NYSE:EXM) or DryShips (NASDAQ:DRYS). DryShips' 52-week high was $121.44. It's under $17 as I write this. The P/E ratio on that stock is 1. Ouch. And no, that's not an automatic buy signal -- earnings will likely catch up with the stock price soon enough.

Why has demand dropped so sharply? Well, you may have noticed that commodities prices have fallen sharply across the board. That's largely attributed to "demand destruction" -- the expectation that people will buy less of everything during a recession.

But some are arguing that part of that demand destruction may be due to the inability to finance these commodity cargoes. They point to the big drop in the BDI after Lehman Brothers went bust. So is the global credit crunch playing a role in lower shipping rates?

One more credit crisis
The credit crunch affects commodities and shipping in lots of ways, but one underappreciated factor is the issuing of letters of credit. These instruments, another form of short-term commercial credit, are key to international trade.

Here's how they work: Before Company X ships goods to Company Y in another country, Company X (and the shippers) will want some assurance that Company Y will pay for the goods when they arrive. That assurance is given via a letter of credit, a note from Company Y's bank to Company X's bank saying, in essence, that Company Y pays its bills, and if it doesn't, in this case the bank will cover it. (If Company Y pays in advance, the letter goes the other way, giving assurances about X. There can be many other complexities, but that's the basic process.)

And just as the commercial paper market has all but grinded to a screeching halt in recent weeks, so has the issuing of letters of credit, for the same reason -- banks don't want to be exposed to risks from other banks.

If you're a commodities exporter like iron ore producer Vale (NYSE:RIO) or diversified commodities giant BHP Billiton (NYSE:BHP), that's a problem. The argument goes like this: If I can't get credit to get iron ore shipped to me today, then I'm not buying iron ore -- and "demand" has dropped.

In danger, there is opportunity
So is this a disaster in the making? Probably not. While theoretically, global trade could be cut back sharply -- Smoot-Hawley revisited, in effect if not in intent -- it doesn't seem likely. Just as government intervention is finally starting to unstick other corners of the credit markets, I expect that this problem will either unstick along with the rest -- or draw a bailout effort of its own, sooner rather than later.

And when that happens, commodity prices could see a bounce as local supplies get replenished and stalled projects get underway again. While a return to the bubble levels of earlier this year seems unlikely in the near term, commodities exchange-traded funds such as Market Vectors Global Agribusiness (AMEX:MOO), PowerShares DB Base Metals (AMEX: DBB), or a more broad-based fund like iShares S&P GSCI Commodity-Indexed Trust (AMEX:GSG) could see a nice gain from present levels as this problem subsides. 

To read more about opportunities in the global credit crisis: 

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Fool contributor John Rosevear has no position in the companies mentioned. Try any of our Foolish newsletters today, free for 30 days. The Motley Fool has a disclosure policy.