There are no roads or railroads between China and US so the bulk efficiency by ship can not be directly compared. The first rule in decreasing emissions are red

There are no roads or railroads between China and US so the bulk efficiency by ship can not be directly compared. The first rule in decreasing emissions are reduce, so reducing the need to transport goods half way around the world are an effective measure against emissions. Ship fuel (also called bunker fuel) is the dirties form of fuel there is and goes way beyond CO2 emissions. It is heavily contaminated and because of incomplete combustion it contribute heavily to pollution in water, atmosphere, and coast cities. The environmental impacts of heavy fuel oil spills also converts the ships into mobile nature disasters. The costs from oil spills generally spills over to society and people who live near the cost, rather than being carried by the shipping company. Collecting the real cost of that risk should be part of the product price when importing goods. Tariffs are fungible, so the same tariffs could be used for the purpose of protectionism, geo-political stability, CO2 emissions in transportation, CO2 emissions in production, oil spill insurance costs, air/water pollution, regulative differences in worker rights, or differences in product safety. It can make the market competition more fair or unfair, and depending on what goals one have the definition of fair and unfair will likely change. People who lean both left and right can find arguments both in favor and against, likely at the same time given the fungible nature of tariffs. Thus if one want to look at it on the bright side, pick the benefits of the political goals that talks to you.