Sure, the "CO2 budget" is measured in gigatons so not a huge dent. But add up a lot of these rounding errors and it might actually make a slight difference. And since the climate interactions are non-linear, slight differences can have a large effect over time.
But that's only 30,000 tonnes.
10,000 tonnes of co2 for 30,000 tonnes of cargo seems... Wrong.
This seems to back me up https://timeforchange.org/co2-emissions-shipping-goods
10 - 40g co2 per tonne. So about an order of magnitude less.
Edit: Doh see below. This is per km
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].
Airfreight is 500g /tonne/km so 10 tonnes of co2 / tonne of freight.
Guess that's why co2 calculators always ask about flying.
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...