You fail to understand just how big a large container ship is. If a shirt comes on a ship from China, the truck taking it from the warehouse to the shop emits more CO₂ for that shirt than the ship did bringing it from to the port. In fact, if you walk for ten minutes to the shop and back, you have emitted more CO₂ than the ship did to carry that shirt. A container ship emits a large total amount of CO₂ because it’s very big, carrying a massive amount of cargo. But overall, shipping is one of the most efficient modes of transport we’ve invented.