The shipping index has dropped 50%. What does it mean?

Yesterday, the Baltic Dry Index saw its 27th consecutive day of losses. That's the longest continuous slump for five years. The Baltic Dry measures the cost of shipping dry bulk goods – commodities such as iron ore and coal, for example. The index fell off a cliff during the credit crunch in 2008, as credit to finance global trade froze. So what is the current sell-off telling us about the state of the global economy?