Air freight is not clean indeed. Which leads to a kind of ironic situation here in Norway. Salmon export is hailed as one of the primary income sources once the oil is gone, yet significant portion of the salmon is exported via air to Asian markets. Not exactly a very green change...
Coudn't that be mitigated with factory ships that produce the product en route and drop off at the end location? Ship to ship transfer is easily possible. I'm honestly asking as I don't know the true logistics behind this, but I thought factory ships already existed and did this.
The key is it's per ton per km transported.
For reference, Shanghai to Rotterdam route (a bit of a worst case I guess) is just shy of 20000 km[1].
[1]: https://sea-distances.org/