The text of this paper, Batra and Belladi (2012), claims:
transportation is highly pollution intensive, specially relative to production
They provide some more context:
With international commerce, goods are transported over long distances over the seas, land routes and the skies, and in the process a large volume of gases is emitted into the atmosphere. Once goods arrive at ports, they are further transported to other areas within a country. Local production is usually spread all over the country and does not need as much freight, with trade, which inevitably requires transportation, expanding almost twice as fast as world gross domestic product (GDP) since 1950 (US Council of Economic Advisers, 1991, 1992), international trade damages the global environment much faster than economic growth. There is a double-barreled effect here. First, trade has outpaced world GDP, and second, trade-induced transportation is pollution intensive relative to production. [footnote:] Transport costs have fallen steadily over the years to induce a big rise in world trade. However, minimal transport costs does not mean that emissions from transportation are minimal. In fact, falling transport costs have magnified the polluting effects of transportation.
Is it true that in general, relative to production, transportation is highly polluting?