By Sarah Mosko
Special to the OC Voice
“Buying local” has become a mantra of many committed to shrinking their personal climate footprint by limiting the miles their food travels from producer to plate. The increasing globalization of food supplies has served to fan this trend.
However, a new study finds that what you eat has a far greater impact on greenhouse gas (GHG) emissions than where that food was produced. What’s more, saying no to red meat and dairy products even one day a week matters more than buying local all week long.
Number crunchers Christopher Weber and Scott Matthews at Carnegie Mellon University drew on U.S. government statistics from 1997 to expose the entire life-cycle GHG emissions associated with the diet of the average American household.
Emissions fell into one of four categories, starting with upstream supply chain transportation wherein equipment and supplies are supplied to food producers. Then comes the food production phase, followed by final delivery transportation from point of production to retailer. The latter is synonymous with so-called food-miles that are the focus of advocates of buying local. The fourth source of emissions occurs during wholesaling and retailing and includes store heating and air-conditioning and food refrigeration.
Consumer commutes between home and market was not included in the analysis.
On average, our food travels an impressive 1,020 miles just during final delivery, yet this accounts for a mere 4 percent of the total GHG emissions of our diet. The lion’s share, 83 percent, occurs during the production phase. Even when the contribution from upstream supply chain transportation is included, transportation altogether makes up just 11 percent.
Carbon dioxide is the dominant greenhouse gas associated with transportation. The key to understanding how dietary choices pack a bigger wallop than food-miles is appreciating the role in the production of certain food groups of two less-oft mentioned but far more potent greenhouse gases: methane and nitrous oxide.
Cud- chewing ruminants, like cows, sheep and goats, are notorious for copiously spewing methane from both ends during the process of digestion. Modern agriculture, including that directed at growing animal feed, is heavily dependent on synthetic nitrogen fertilizers that soil bacteria convert to nitrous oxide. And both gases are major byproducts of manure management. Pound for pound, methane and nitrous oxide exhibit greenhouse warming potentials 25 and 298 times that of carbon dioxide, respectively.
So when methane and nitrous oxide emissions are expressed as carbon dioxide-equivalents, together they account for over half of the annual 6.8 metric ton carbon dioxide equivalents per household attributed solely to the food production phase. Consequently, the relative climate impact, of different food groups is heavily influenced by their generation of these two non-carbon dioxide GHG gases.
It’s perhaps no surprise then that red meat is hands down the worst climate offender, with dairy products (milk and cheese) trailing not too far behind. This holds true whether food groups are compared for carbon dioxide-equivalent emissions on a per-household, per dollar spent, or per calorie basis.
More surprising perhaps is that other non-red meat protein sources (chicken, fish, eggs, nuts) fare about the same as fruits and vegetables in terms of total GHG emissions, each contributing roughly one-third the yearly GHG emissions per household as red meat. Although chicken, fish, eggs and nuts emit more during production, fruits and vegetables travel farther on average, so it evens out.
To drive home their main points, the researchers calculated how small a shift in food choices would achieve the same result as eliminating all food-miles from the American diet, an unrealistic but instructive comparison.
Two bombshells summarize their finding. Reducing all dietary food-miles to zero would achieve “at maximum, around a 4-5 percent reduction in GHG emissions” because of the far greater emissions associated with the production of food. Furthermore, “shifting less than one day or week’s worth of calories from red meat and dairy products to chicken, fish, eggs, or a vegetable-based diet achieves more GHG reduction than buying all locally sourced food.”
Nonetheless, it’s still a good idea to buy fruits and vegetables grown locally since final delivery accounts for half of all transportation within this food group. For red meat, in contrast, food-miles make up less than a tenth of total transportation and contribute just 1 percent to red meat’s total GHG emissions.
The authors of the study freely admit that care must be taken in interpreting their findings. For example, the study did not explore how figuring in organically grown produce or grass-fed meat would tilt the findings. Or, it might be argued that choosing fish over red meat to protect the climate is like “robbing Peter to pay Paul,” given that overfishing of oceans is another impending environmental crisis.
Moreover, some people seek out locally-grown foods for reasons unrelated to climate change, like maximizing freshness or to support local farmers.
Yet, given that diet sits right up there with the kind of car a person drives and home energy consumption as major spheres wherein consumers can control their contributions to climate change, spotlighting the impact of food choices is obviously of merit. And, as Weber and Matthews point out, focusing on food is attractive to many people in that it doesn’t require a locked-in or single lump financial commitment-as when investing in an energy-efficient car or home solar panels-to lessen one’s climate footprint.
This study appeared in the April 16, 2008 web release issue of “Environmental Science & Technology.”