That depends on the shipped goods, i think? One calculation was that of a 15 inch notebook at 2kg weight, they wrote: 28 days per ship, at 2,50 € and 184 kg co². By air 3 to 4 days at 23 € and 4950 kg co² emission. By train 17 to 18 days at 5,50 € and 270 kg co². But that is for a laptop, packaged in a specific volume with that weight. They also ship tires, automobile parts, shoes, clothing, textiles, whatnot.... in both directions btw. BMW ships parts from .de to .cn