Indicator: The Baltic Dry Index ($BDI)

The Baltic Dry Index ($BDI) tracks international shipping prices cargoes on dry bulk carriers, like building materials, coal, metallic ores, and grains

Because the supply of cargo ships is so inelastic, and because the cargoes are raw material inputs, the index is very sensitive to economic changes and is a leading indicator.

Here is a chart of the BDI. You can see that it has been falling since late October, which mirrors the decline in Chinese stock indexes.