Once you've cut your greenhouse gas emissions by ditching the car and have stepped off the coal-fired electricity grid, how much does the transportation of food contribute to greenhouse gas emissions?

Eleven per cent.

At least, that is how much transportation contributes for the average American, according to Christopher L Weber and H Scott Matthews in their research into food miles. If Americans ate everything locally, and became the most committed of hardcore locavores, without changing their current diets they would only reduce the amount of emissions by a relatively small amount. While food is transported long distances in the US, averaging 1640 kilometres for delivery and 6760 kilometres in the entire life cycle, the emissions caused by transporting food are outweighed by the emissions from the production of food itself. For the consumer looking to reduce the amount of greenhouse gases produced on their behalf, this has gigantic implications. Weber and Matthews suggest that:

...only relatively small shifts in the average household diet could achieve greenhouse gas reductions similar to that of localization. For instance, only 21%-24% reduction in red meat consumption, shifted to chicken, fish or an average vegetarian diet lacking dairy, would achieve the same reduction as total localization.

For anybody that prides themselves eating locally where possible as a way to mitigate their personal carbon footprint (or ruminant meat junkies), it is fairly incendiary research. Eating locally has a relatively small impact upon greenhouse gas emissions, and in some cases, may be worse in terms of emissions than shipping a food a great distance. Research from Lincoln University in New Zealand offers an extreme example. In the New York Times, James E McWilliams writes:

lamb raised on New Zealand’s clover-choked pastures and shipped 11,000 miles by boat to Britain produced 1,520 pounds of carbon dioxide emissions per ton while British lamb produced 6,280 pounds of carbon dioxide per ton, in part because poorer British pastures force farmers to use feed. In other words, it is four times more energy-efficient for Londoners to buy lamb imported from the other side of the world than to buy it from a producer in their backyard. Similar figures were found for dairy products and fruit.

The Lincoln University research suggests that the discrepancy between the two countries is not only due to increased use of fuel in the UK, but also larger amounts of fertilizer used on pastures (which is also shipped from great distances). So why buy local or even pay attention to a concept like food miles when they don’t necessarily help in reducing greenhouse gas emissions?

Even marginal gains are worth making. Food miles acts as a handy measure to concatenate a handful of other, non-greenhouse gas related indicators for your food. Eating food from closer to you helps you stay in touch with seasonal food. Seasonal foods, shipped shorter distances, tend to be riper and better tasting than a variety that has been grown elsewhere with the sole purpose of being good for shipping – a prime example is styrofoamy and floury tasting tomatoes available year round in any supermarket. They’re tough because they ship well, rather than being tough because consumers have a preference for a tasteless tomato.