Let’s forget the moralizing, and focus on the numbers
The New Yorker magazine has an interesting article by Michael Specter on the science and ethics of measuring carbon footprints.
For some reason, many environmentalists have a bias in favour of local consumption, and against the consumption of goods that have traveled a long distance between their place of production and their place of consumption. The reasons usally given for this preference are that local consumption involves fewer carbon emissions. But this is not necessarily the case, since trans-shipment is not the only source of emissions in the production, distribution or consumption of goods.
“It is a logical and widely held assumption that the ecological impacts of transporting food—particularly on airplanes over great distances—are far more significant than if that food were grown locally. There are countless books, articles, Web sites, and organizations that promote the idea. There is even a “100-Mile Diet,” which encourages participants to think about “local eating for global change.” Eating locally produced food has become such a phenomenon, in fact, that the word “locavore” was just named the 2007 word of the year by the New Oxford American Dictionary.
. . .
Yet the relationship between food miles and their carbon footprint is not nearly as clear as it might seem. That is often true even when the environmental impact of shipping goods by air is taken into consideration. “People should stop talking about food miles,” Adrian Williams told me. “It’s a foolish concept: provincial, damaging, and simplistic.” Williams is an agricultural researcher in the Natural Resources Department of Cranfield University, in England. He has been commissioned by the British government to analyze the relative environmental impacts of a number of foods. “The idea that a product travels a certain distance and is therefore worse than one you raised nearby—well, it’s just idiotic,” he said. “It doesn’t take into consideration the land use, the type of transportation, the weather, or even the season. Potatoes you buy in winter, of course, have a far higher environmental ticket than if you were to buy them in August.” Williams pointed out that when people talk about global warming they usually speak only about carbon dioxide. Making milk or meat contributes less CO2 to the atmosphere than building a house or making a washing machine. But the animals produce methane and nitrous oxide, and those are greenhouse gases, too. “This is not an equation like the number of calories or even the cost of a product,’’ he said. “There is no one number that works.”
Many factors influence the carbon footprint of a product: water use, cultivation and harvesting methods, quantity and type of fertilizer, even the type of fuel used to make the package. Sea-freight emissions are less than a sixtieth of those associated with airplanes, and you don’t have to build highways to berth a ship. Last year, a study of the carbon cost of the global wine trade found that it is actually more “green” for New Yorkers to drink wine from Bordeaux, which is shipped by sea, than wine from California, sent by truck. That is largely because shipping wine is mostly shipping glass. The study found that “the efficiencies of shipping drive a ‘green line’ all the way to Columbus, Ohio, the point where a wine from Bordeaux and Napa has the same carbon intensity.”
The environmental burden imposed by importing apples from New Zealand to Northern Europe or New York can be lower than if the apples were raised fifty miles away. “In New Zealand, they have more sunshine than in the U.K., which helps productivity,” Williams explained. That means the yield of New Zealand apples far exceeds the yield of those grown in northern climates, so the energy required for farmers to grow the crop is correspondingly lower. It also helps that the electricity in New Zealand is mostly generated by renewable sources, none of which emit large amounts of CO2. Researchers at Lincoln University, in Christchurch, found that lamb raised in New Zealand and shipped eleven thousand miles by boat to England produced six hundred and eighty-eight kilograms of carbon-dioxide emissions per ton, about a fourth the amount produced by British lamb. In part, that is because pastures in New Zealand need far less fertilizer than most grazing land in Britain (or in many parts of the United States). Similarly, importing beans from Uganda or Kenya—where the farms are small, tractor use is limited, and the fertilizer is almost always manure—tends to be more efficient than growing beans in Europe, with its reliance on energy-dependent irrigation systems.”
Reference: Michael Specter: “Big Foot: In measuring carbon emissions, it’s easy to confuse morality and science” (The New Yorker, 25 February 2008, pp. 44 -53).