Frequently Asked Questions about climate change and agriculture: Part 2

Last week I introduced a series of frequently asked questions and began by addressing the first:

The EPA says agriculture only accounts for 6% of US greenhouse gas emissions. Shouldn’t we focus our efforts on bigger problems such as coal fired power plants and automobile emissions instead?

Today I’ll address the following:

Do “food miles” – the distance that food travels from producer to consumer – really matter to the climate?

Not only is this a “frequently asked question”, but it has become a fairly contentious issue in both the commercial retail industry as well as in local / community food systems discussions. As with many catchy slogans, the phrase “food miles” hints at a larger truth but is actually too simple to adequately address issues of transport of food.

Most early studies on the topic of transportation of food and energy use / greenhouse gas emissions focused on simple, “single product” comparisons. These simple studies led to more sophisticated assessments that looked at multiple products (e.g. a bag of groceries) or a multi-ingredient product (e.g. yogurt). These types of studies are valuable for teasing out the principles of energy use or climate impact of the transportation decisions behind any given product (e.g. distance vs. mode of transportation). Rich Pirog and colleagues at the Leopold Center for Sustainable Agriculture at Iowa State University have completed a number of these kinds of studies as they related to products that are or could be produced and sourced locally from Iowa farms rather than imported. While these single product methodologies are useful for telling the relative difference of transportation (distance and/or mode) for two otherwise equal products, they don’t provide enough information to understand the relative importance of transportation (in all of its dimensions) in the carbon footprint of the product or the difference between the same product produced under different circumstance in different locations.

Heller and Keoleian (2003) indicate that ALL transportation related to food only consumes 16% of the total energy used in the food system in the US. Weber and Matthews (2008) found that ALL transportation accounts for only 11% of climate related emissions in the US food system, and “final delivery” (from producer to retail) is only 4% of emissions. We also must realize that it is not only total miles we must consider, but also the “mode of transportation” in considering the energy use and emissions associated with transportation (Pirog, 2001, Whitleg 1993). For instance, a semi-truck full of tomatoes may travel a substantial distance to wholesale and retail markets, but the “last five miles” that hundreds of consumers travel from the retail outlet to home with a few pounds of those tomatoes each may represent a substantial fraction of the total transportation energy and carbon footprint.

While farm production is not a “transportation” factor, it does represent a significant portion of the energy and carbon footprint of our food system, accounting for 21% of energy use (Heller and Keoleian 2003) and a whopping 83% of climate emissions (Weber and Matthews 2008). So, the relative potential for a difference in emissions between production systems for similar products is likely to be greater for many food products than any difference caused by a change in transportation. While a substantial body of research exists (and is growing) that evaluates the relative energy and carbon footprints of various agricultural production systems, very little of this research to date has been assessed in the context of the energy or carbon footprints of the “food products” that are outputs of these systems. The CSANR Climate Friendly Farming Team conducted an initial study for wheat produced in different production systems in Washington State and plans to continue working on this kind of assessment. Finally, Weber and Matthews (2008) also show that the relative difference between “food groups” is significant and is likely a more important determinant than transportation of the carbon footprint of any given diet.

So while decisions about purchasing locally may reduce the total miles a given food product travels, it may or may not reduce the total energy or carbon footprint of the product. In fact, the decisions a consumer makes about the “mode of transportation for final delivery” (e.g. car, bicycle, walking), the production system the food comes from, and our dietary choices are likely to have a greater influence than “food miles” on reducing the energy and carbon footprint of our food.


Heller & Keoleian. 2003. Assessing the Sustainability of the US Food System: a life cycle perspective, Agricultural Systems, 76: 1007-1041

Pirog, R., 2001. Food, Fuel, and Freeways: An Iowa perspective on how far food travels, fuel usage, and greenhouse gas emissions.

Weber, Christopher L., and H. Scott Matthews. 2008. Food-Miles and the Relative Climate Impacts of Food Choices in the United States. Environ. Sci. Technol., 2008, 42 (10), pp 3508–3513.

Whitlegg, J. 1993. Transport for a Sustainable Future: The Case for Europe. Bellhaven Press, London.


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