But essentially one problem with using the BDI for economic forecasting is that the BDI could feasibly go up in an environment where commodities demand was shrinking, if the supply of ships was shrinking even faster. These would be negative economic factors. This is because the BDI's value is not solely driven from the demand side. To me, it makes far more sense to just look at nominal demand for commodities rather than the BDI since the BDI has the complicating factor of vessel supply growth one needs to consider. The other thing is that the BDI is a measure of spot rates for dry bulk commodities consumers who, generally, are in the near term forced to pay whatever it takes to get their raw materials shipped (A steel plant needs to keep operating despite some higher ore transportation cost). On the flipside, vessel owners are in a similar boat (no pun intended), and in the near term are generally forced to take whatever rate they can get to fill their ships. (A ship sitting around is just a cost, ie. fixed costs are high, thus using a ship at a loss is usually better than not using it at all)
Because of these inelastic characteristics of supply and demand, and since the BDI is a measure of spot rates, the BDI is thus absurdly volatile. I can explain why via the following simplified example, which I used to use frequently at Citi.
Imagine you have 10 loads of iron ore and 9 ships, and that every load of iron ore must be sent no matter what while every ship must be filled no matter what. Imagine the bidding war between those 10 iron ore consumers fighting over just 9 ships. Shipping cost would skyrocket since they all need to ship regardless of cost. Now imagine if a week later two more ships enter the market. Now imagine the bidding process. Suddenly the tables have completely changed. You have 11 ships, that all need to be filled no matter what, and only 10 loads of ore. Shipping rates would plunge, despite a period of just a week passing by. This is, in a simplified nutshell why the BDI is so volatile.