- Chain grocery store (e.g. Safeway)
- Buying directly from a local farm (e.g. Community Supported Agriculture, perhaps)
- Buying from farmer’s market that sells local food
Realizing that my calculations are sensitive to the numbers I choose, I will use an order of magnitude comparison. Essentially, an order of magnitude is the amount of 0’s in a number. For example, 10 and 100 differ by one order of magnitude. If two numbers differ by an order of magnitude or more, we can hopefully assume that difference between them is real and robust to modifications to how they were calculated. Conversely, with numbers of the same order of magnitude, it may not be safe to assume that the differences are real or robust, and a more precise analysis would be required to parse them out.
Scenario 1: Grocery store
Distance from farm to store: 3000 miles
Load: 20,000 lbs.
Food miles: 0.03 gallons/lb.
All of these parameters likely overestimate the food miles, but I will use them for arguments sake.
Distance from house to grocery store: 10 miles (round-trip)
Load: 20 lbs.
Food miles: 0.02 gallons/lb.
Total for Scenario 1: 0.03 + 0.02 = 0.05 gallons/lb
Scenario 2: Buying from local farm
Distance to farm: 50 miles (roundtrip)
Load: 20 lbs. (same as above)
MPG: 25 (same as above)
Food miles: 0.1 gallons/lb.
Total for Scenario 2: 0.1 gallons/lb
I used the number of 50 miles for mathematical simplicity, but I think it is not a bad guess since the average citizen lives in a city or suburb, not in a rural town.
Scenario 3: Farmer’s Market
Distance from farm to market: 50 (roundtrip; same as farm to house as farmer’s markets tend to be centrally located in a community)
Load: 2000 lbs. (1 ton pick-up)
MPG: 10 MPG
Food miles: 0.0025
Distance from house to grocery store: 10 miles (round-trip; same as grocery store since farmer's markets, like grocery stores, are centrally located)
Load: 20 lbs.
Food miles: 0.02 gallons/lb.
Total for Scenario 3: 0.0025 + 0.02 = 0.0225 gallons/lb
I have used some useful approximations to generate order of magnitude comparisons. What have I found? Buying produce from the grocery store or the farmer’s market uses about about half or a fifth, respectively, of the gas used to transport food bought at a local farm directly. The grocery store and farmer’s market comparison is more equivocal, but it appears that food from a farmer’s market may use substantially less fuel. One caveat though. Since people who shop at farmer’s market usually still go to grocery stores for other goods, a more appropriate comparison might remove the food miles from the grocery store to the home, in which case buying from the grocery store or the farmer’s market are practically indistinguishable at this level of refinement.
What does this exercise tell us about the environmental impact of buying food locally? It is likely that under a wide variety of conditions, driving to a local farm to buy food increases the food miles compared the grocery store or farm market. The reason is clear: by driving a car, you substitute an efficient mode of transport (semi-tractor trailer or utility vehicle carrying 1000-20000 pounds) for an inefficient one (your car carrying 20 pounds). If one substitutes going to the grocery store by going to the farmer’s market, then buying food from the farmer’s market likely reduces food miles. However, as farmer's markets carry a limited basket of goods, this is not necessarily true for most consumers. Therefore, food miles for grocery store food and farmer’s market food might be about the same under most conditions.
If local food does not generally reduce food miles, and may increase them, why should one buy local? The typical response is that locally produced food, such as that done by the USDA Community Supported Agriculture (CSA), produces food using less energy. My first response would be that if local food saves energy on the production and not transportation, why purchase it locally in the first place? That said, does local food produced under the CSA model use less energy? I don’t know, but I can provide a couple reasons why it might not.
The local food model saves energy through reduced reliance on pesticides, fertilizer, and irrigation. How might it use more energy? First, locally produced food is more labor intensive, encouranging more people to move into rural communities. While we may cherish rural communities for their aesthetic or cultural value, they are not bastions of energy efficiency. Without public transportation and dense commercial areas, rural citizens must drive more and further. In addition, the densely-packed high rises of cities confer energy savings on heating and cooling compared to rural areas. Locally produced food, even without inputs of fertilizer, pesticides, and irrigation may just take more energy. For example, in northern areas, growing strawberries is often done in greenhouses that operate year-round and must be maintained and heated. Strawberries grown in California can grow without those inputs. It may well turn out that despite these factors, local food is still more energy efficient, but I think they should provoke cause for further investigation rather than wanton dismissal.