A Honey of an Anklet

In response to “Burgernomics, indeed,” Leta asked me a good question: What’s the difference between eating chicken from a farm in Delaware and fresh broccoli from California’s Central Valley? (We live on the East Coast.) Isn’t trucking all that foliage cross-country less environmentally-friendly? Recent research by Christopher L. Weber and H. Scott Matthews attempts to answer that question. Their results are also discussed in a post by Jane Liaw. In short, Weber and Matthews’ findings are that it comes out the same, but for different reasons.

The Carnegie Mellon researchers looked at the life-cycle impact, from production to retail, in equivalent greenhouse gas (GHG) emissions, for the production of food for consumption in the United States, where food is analyzed as 50 commodities grouped into seven USDA-style categories. They use a methodology, informed by the work of Wassily Leontief, termed input−output life-cycle assessment (IO-LCA). Input-output analysis accounts for the fact that some goods are produced and shipped around only in order to make other goods for final consumption: chickens have to be fed corn that was grown somewhere else, broccoli has to be irrigated with water that has to be piped from somewhere else, and so forth. The approach aggregates across the country, so it’s not going to account for regional differences in production or consumption (compare the work of Colman and Päaster on wine production). Beyond that, I am limited in my ability to critique the methods of the paper.

The first figure that stands out from the paper is 12,000. That’s the number of equivalent ton-kilometers of freight, per household, required to meet food-demand in the U.S. in 1997. You could think of this as a monthly truckload of 1 metric ton of food (and products that went into making the food) travelling 1,000 km (600 miles) around the country, ending up at the supermarket, to feed a “typical family of four.” (The paper omits the “last mile” of transportation from store to home.) But only 25% of that freight mileage is part of the “direct” tier, from farm to retail. The remaining three-fourths is used in intermediate production.

When the numbers are crunched by food category, things get more interesting.

Final delivery (direct t-km) as a proportion of total transportation requirements varied from a low of 9% for red meat to a high of around 50% for fruits/vegetables, reflecting the more extensive supply chains of meat production (i.e., moving feed to animals) compared to human consumption of basic foods such as fruits/vegetables and grains.

But we’ve still got to work out the GHG impact. The researchers assign CO₂-equivalences for ten modes of transport, including rail, truck, ocean (by container or bulk), air, and oil and gas pipeline (fertilizer feedstocks gotta get there somehow). Due to transmission losses, natural gas pipelines are only as efficient as trucks.

Once this calculation is made, the relative unimportance of local transport in the total picture begins to emerge.

Total GHG emissions are 8.1 t CO₂e/household-yr, meaning delivery accounts for only 4% of total GHG emissions, and transportation as a whole accounts for 11%. Wholesaling and retailing of food account for another 5%, with production of food accounting for the vast majority (83%) of total emissions.

Within food production, which totaled 6.8 t CO₂e/household-yr, 3.0 t CO₂e (44%) were due to CO₂ emissions, with 1.6 t (23%) due to methane, 2.1 t (32%) due to nitrous oxide, and 0.1 t (1%) due to HFCs and other industrial gases. Thus, a majority of food’s climate impact is due to non-CO₂ greenhouse gases.

Okay, so what about the chicken-and-broccoli question? The paper presents the relative GHG effect by the seven commodity categories, scaled by weight, retail expenditure, and (most importantly, I believe) calorie content. By any of these measures, red meat comes out with the largest carbon footprint, followed by the milk and cheese category. Scaled by food energy content, the chicken/fish/eggs group matches the fruit and veg group.

The authors’ take-away message is that even a small change in diet can have a significant impact, given some additional reasonable assumptions. Just switching your calories for one day a week out of red meat and dairy and into veggies has the equivalent effect of a completely “localized” consumption habit.

… [but] this is conversely true for households which already exhibit low-GHG eating habits. For these households, freight emissions may be a much higher percentage of the total impacts of food, and especially will be important for fresh produce purchased out of season.

They also consider briefly the upswing in food imports into the U.S. Since ocean transport is relatively efficient (more than ten-to-one better than trucking), they infer that globalization has less of a deleterious effect than some fear.

It’s also worth noting that Weber and Matthews’ work is only concerned with GHG emissions. Other differential impacts on the environment by food category—for instance, land use, water quality, acid rain, noise pollution, and smog—are not part of their analysis.