I believe that the more local a product can be produced, the less was is spent in transforming it. Also, if people are buying a local product from a local company, people see the behavior of said company,and will hold them to a higher standard. We never see first hand manufacturers in china or how they pollute. If we did, we would not blindly buy their products like we do.
International shipping is responsible for nearly 30% of nitrogen oxide pollution and nearly 10% of sulphur oxide pollution, caused by the emissions of the cargo ships. There is always the risk that cargo ships can leak fuel oil (either from a fractured hull or more slowly by seeping into the bilge water). There is also the problem of sewage. Each crew member on a cargo ship generates about 30 gallons of sewage water each day, which is simply released into the ocean.
International Trade has gone on since humans learned to feel comfortable leaving their homelands. But in this sped up world we live in where the non-entities of the corporations meddle without regard to the future of humanity we have allowed international trade to destroy the environment in locales close and far.
International trade is not bad for the environment. Other than goods we trade services and ideas and evolve ourselves with information not readily available in a localized form. Granted there are environmental impacts caused by industries that utilize international trade That is however, a responsibility of the industry and supported entities. Not international trade itself.