Planes, trains or automobiles?

Faced with a San Diego avocado and one from Chile, which do you buy? And of a wine from Australia and one from Washington state: Which cost the most carbon to deliver? For the shopper keen on being green, it’s a no-brainer: Local foods and wine trump anything from overseas.

Or do they really?

For those willing to apply some brainpower to daily shopping excursions, the answer is interesting, if not simple. Because how foods move from farm to finish is as important, if not more so, than the distance they travel. If you sign up for personal doorstep deliveries via 3-ton truck from “local” community supported agriculture maybe 150 miles distant, is that “greener” than buying bananas, pineapples and coffee delivered by cargo ship from Central America? Answers will vary case by case, but experts agree that loaded cargo ships are about 10 times as fuel efficient as trucks and twice as efficient as trains in moving freight.

To put this into practical terms, a banana shipped 3,000 miles from Costa Rica to San Francisco and driven by truck 100 miles to Sacramento may be equally carbon costly as a California avocado that travels 400 miles to your favorite produce stall in a truck.

We can also calculate that a Chilean wine that goes by ship 6,000 miles from Santiago to San Francisco and goes the short haul over the highway leaves a carbon footprint roughly equal to a bottle of wine transported 700 miles by truck only from Washington (Napa would be a better choice).

This isn’t to say that a ship burns less fuel than a truck. In fact, ships incinerate so much tarlike bunker fuel that they measure it in tons—often hundreds or thousands. But those tons of fuel move such huge quantities of cargo that the efficiency rate of a ship far exceeds that of a loaded vehicle. It might be “greener” not to have ships at all, but ships are here, and they’re not going anywhere. Actually, they’re going everywhere, and if a few pallets of wine hitchhike on a loaded freighter, that boat won’t know the difference.

Dale Bergeron, a maritime transport expert with the University of Minnesota’s Sea Grant program, said that a cargo ship can carry the cargo equivalent of 15,000 semi trucks.

“So once you’re rolling, the cost efficiency per mile on a ship is staggeringly efficient,” Bergeron said.

Overland, the most efficient system for moving freight is the railroad. According to Jacob Park, vice president of the San Francisco Bay Railroad, the average train carries the cargo equivalent of 400 semi trucks. And while a truck can move a ton of cargo on a gallon of fuel 155 miles, a train can go almost four times the distance.

But America depends on its trucks and freeways (which cost taxpayers $20 million per mile to build, according to Bergeron). In 2006, according to industry numbers, trains moved 2 billion tons of freight across the United States, while trucks moved more than 10 billion tons.

Most California-grown produce sold locally never touched a train, and, yes, CSA home deliveries are about the poorest example of fuel efficiency in moving goods. Yet, buying locally grown foods remains a wise option on principle. Sure, they were delivered by truck and truck alone—but the imported tropical fruits we all love come by boat and by truck.

So what’s the final answer? Best, probably, to buy San Diego avocados and West Coast wine. But if you opt for Chilean, it may not be local, but it’ll hardly make a difference.