The transport of food around the world is an important part of the industrial food system, but what impact does food transport have on global emissions? Does eating local food really make much difference? To help answer some of these questions, let's look at the concept of "food miles." Food miles refer to the number of miles a food product has traveled to reach your plate. The idea is that the farther away a product is grown from the point of consumption, the greater the number of food miles and transportation-associated emissions. Although the connection between food miles and emissions makes sense, the relationship is actually more complex.
First, the method of transportation (truck, ship, rail, or air) has a large impact on emissions. The most efficient method of transport is by ship, followed by rail, truck, and air.1 For example, studies have shown that in Sweden, apples shipped from overseas require significantly more energy to get onto the market shelf compared with locally grown apples, especially if they are air freighted.2
The other complication regarding food miles is that if reducing emissions is your target, then knowledge of how the food is produced can be as important as how far it's traveled. The methods of food production (for example, farming intensity and the use of fertilizers) can have a large impact on total emissions. A study by the U.K. Department for Environment, Food and Rural Affairs found that it actually took less energy (and produced fewer CO2 emissions) to grow and ship tomatoes from Spain to the United Kingdom than to grow local tomatoes in the United Kingdom using greenhouses (greenhouses use significant amounts of electricity for light and heat).3 A New Zealand study found that importing New Zealand lamb to the United Kingdom was more energy efficient than growing local U.K. lamb due to the U.K. sheep farms' dependence on farm-grown feed (it takes significant energy to grow the feed), whereas New Zealand sheep graze primarily on local grasses.4 Although the concept of food miles is not always an accurate assessment of emissions and environmental impact, we should not neglect altogether the idea that locally grown foods generally are associated with fewer emissions compared to imported foods. As an example, let's consider a pound of cherries (See Figure 1). Based on previously published estimates, growing one pound (0.45 kg) of cherries emits about 0.3 pounds (0.13 kg) CO2e.5 The emissions due to transportation are as follows:
Local transportation by truck emits 0.06 pounds (26 g) of CO2e - 17 percent of the emissions associated with growing cherries.
National transport by truck emits 0.59 pounds (262 g) of CO2e - 175 percent of the emissions associated with growing cherries.
National transport by rail emits 0.14 pounds (62 g) of CO2e - 41 percent of the emissions associated with growing cherries
International transport for Chilean cherries by boat emits only 0.15 pounds (67 g) of CO2e - 44 percent of the emissions associated with growing cherries, illustrating that transport by boat is quite energy efficient.
Figure 1. Emissions of CO2 e associated with growing and transport of one pound (0.45 kg) of cherries using four different transportation scenarios that differ by the method (truck, ship, rail, or air freight) and distance (local, national, and international). In all scenarios, the emission associated with growing the cherries (production) is assumed to be constant.
However, cherries would probably not survive the two-week trip from farm to market using a boat, so overseas cherries are normally freighted by air, thus emitting 16 pounds (7,410 g) of CO2e, or about fifty times the emissions associated with growing cherries. In this case, transportation emissions due to air cargo are 280 times larger than local transportation by truck and even one hundred times larger than transportation by boat. So purchasing a pound of air-freighted cherries is like letting your car idle in the driveway for two and a half hours.
Labeling air-freighted food products according to their place of origin would help consumers make informed decisions about their purchases. If we carry out the food-miles calculation a bit further, we find that if an individual ends up driving twenty miles (32 km) roundtrip to a farmers market solely to buy these cherries, the resulting emissions would be similar to purchasing the air freighted cherries.
1. IPCC, Aviation and the Global Atmosphere: A Special Report of IPCC Working Groups I and III in Collaboration with the Scientific Assessment Panel to the Montreal Protocol on Substancesthat Deplete the Ozone Layer, J. E. Penner, et al., Cambridge: Cambridge University Press, 1999).
2. A. Carlsson-Kanyama, et al., "Food and Life Cycle Energy Inputs: Consequences of Diet and Ways to Increase Efficiency,"Ecological Economics (2003), 293-307.
3. Great Britain Department for Environment, Food and Rural Affairs (DEFRA), The Validity of Food Miles as an Indicator of Sustainable Development: Final Report Produced for DEFRA (Didcot, UK: AEA Technology Environment, 2005), http://statistics.defra.gov.uk/esg/reports/foodmiles/default.asp, accessed Jan. 25, 2008.
4. C. Saunders, et al., "Food Miles: Comparative Energy/Emissions Performance of New Zealand's Agriculture Industry," Agribusiness and Economics Research Unit (AERU), Lincoln University research report number 285 (2006), http://www.lincoln.ac.nz/story_images/2328_RR285_s9760.pdf, accessed Jan. 25, 2008.
5. We used Carlsson-Kanyama's (2003) "Food and Life Cycle Energy Inputs" to estimate the energy required to grow cherries.