|Fig. 1: Glorious food. We are addicted to this stuff. (Source: Wikimedia Commons)|
One of the main problems with people, environmentally speaking, is our insistence on being fed. We are so committed to the idea, in fact, that if we are not fed, we will shrivel up and die. While this result reduces the world food burden, it is considered undesirable for reasons unrelated to the environment. Feeding people, then, is important, and it is one of the largest and most energy-intensive pursuits on the planet. Recently, some focus has been given to the amount of distance food travels between its growth and its consumption, sometimes referred to as "food-miles". 
Over the last fifty years, increasing globalization has enabled produce to shift much more freely around the globe, as witnessed in the US by the shift to year-round availability of formerly seasonal items. This has necessarily also increased the distances traveled by our food - to take one particular instance, grapes coming to Iowa traveled an average of 1,590 miles in the early 70s and an average of 2,848 miles in the late 80s.  This additional travel obviously increases the fuel costs involved in transporting the food, and so impacts the environment through greenhouse gas emissions.
The greenhouse impacts of certain specific products can be dominated by these transportation costs, particularly when that product requires relatively small amounts of fuel and fertilizer to produce and especially if it must then travel by air to reach its destination on time. One UK study determined that runner beans, gala apples, and watercress had environmental impacts which were dominated by transport costs when imported.  This held true even after accounting for freezing costs to maintain a year-round domestic supply.
|Table 1: Greenhouse gas emissions and consumer food loss figures for various categories of food. [1,4]|
It stands to reason, however, that food which is less efficiently produced and more efficiently transported would not have such high relative transportation costs. To evaluate the total impact of food-miles, it is useful to examine the food system more holistically. Weber and Matthews estimate the cost of production relative to the cost of transportation for food production over the entire US economy, and find that 83% of food's greenhouse gas emissions come from its production, while only 4% come from the cost of moving food from the farm to the dinner table (other costs include transportation of goods and personnel within the supply chain of food production facilities).  They suggest that a small shift in diet away from egregiously energy-intensive foods (such as red meat) would equal or surpass the benefits obtainable by buying all food perfectly locally.
When considering choices that might reduce the environmental impact of food, it is also interesting to consider the results of Buzby et al.: they conclude that roughly 19% of food value produced is lost at the consumer level.  Part of this food loss is due to inedible portions of food such as peach pits, but the rest is due to losses in cooking, plate waste, and spoilage which can be reduced through consumer action.
|Fig. 2: A visual display of the relative greenhouse gas impacts of food by category, transportation cost, and waste, using the data in Table. 1. [1,4] Trailer size corresponds to relative emissions from production, wheel size to relative emissions from transportation, and red shading to relative amount of food product wasted at the consumer level. The two studies do not have identical categories for food products, so some judgement was exercised in estimating food waste. The goal is to provide a first-order approximation of relative greenhouse emissions from a consumer perspective.|
In Fig. 2, I have compiled results from Weber and Matthews and from Buzby et al. (summarized in numerical form in Table. 1) in a way that visually demonstrates the environmental impact of consumer choices in diet, local purchasing, and food waste. [1,4] Each of the food categories (with the exception of restaurants and fast food) are shown as a tractor-trailer rig.  The total area covered by the trailers and their wheels is equal to the amount of greenhouse gas emissions caused in the United States by the production and final delivery transportation (food-miles) of all these food categories. The trailers are proportional to the production cost of each category, while the size of the wheels represent the relative energy costs from transport. For all but two of the categories, the back portion of the trailer is shaded to represent an estimate of the amount of energy used in that category to produce food which is eventually wasted.  (I was unable to find categories that were reasonable matches for "Other Miscellaneous" and "Beverages". )
Looking at this figure, we can start to get a rough sense for how our choices might shape the environmental impact of our food. As pointed out by Weber and Matthews,, red meat requires large amounts of energy to produce, and so a reduction in red meat consumption has an outsize impact on our food emissions.  Red meat also accounts for a large share of wasted production, so more careful attention to our scraps of beef and increased vigilance against spoilage could also make a difference. Foods like cereals, fruits, and vegetables have relatively high transport costs relative to their production costs, so buying locally makes a larger difference compared to other categories.
Eating is one of humanity's most environmentally taxing activities, but it is also one which we seem determined to continue doing. One of the side-effects of eating which has changed over the course of the last generation is the longer average distances traveled by many food items. Looking at a larger cross-section of the system of food production and consumption enables us to see the relative importance of this shift in the context of the many choices consumers can make to affect the greenhouse emissions caused by their food.
© Morgan Pope. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 C. L. Weber and H. S. Matthews, "Food-Miles and the Relative Climate Impacts of Food Choices in the United States," Environ. Sci. Technol. 42, 3508 (2008).
 R. Pirog et al., "Food, Fuel, and Freeways," Leopold Center for Sustainable Agriculture, Iowa State University, June 2001.
 S. Sim et al., "The Relative Importance of Transport in Determining an Appropriate Sustainability Strategy for Food Sourcing," Int. J. Life Cycle Assess. 12, 422 (2007).
 J. C. Buzby et al., "The Value of Retail and Consumer Level Fruit and Vegetable Losses in the United States," J. Consum. Aff. 45, 492 (2011).