Energy Efficiency of Foods
We understand that food contains energy as described by the number of Calories.1 It also takes energy to make food and that energy can be estimated. The energy efficiency of a food item can then be defined as the total amount of energy required to produce the food divided by the total amount of energy the food contains.2 Published estimates show that vegetables and grains such as potatoes, rice, and corn have an energy intensity of about 1.2 to 2.5, whereas the energy intensity of meats such as chicken, pork, and beef ranges from 16 to 68.3 So, in general, it takes significantly more energy to make food from animal products than it does to grow vegetables.
In terms of understanding the impact of food on climate, we note that, in addition to energy, the production of animal-based foods generates significant emissions of methane, an important greenhouse gas. So when we compare different food items, the carbon intensity, or the carbon emissions (associated both with the energy to grow the food and any animal-related emissions) per 100 Calories of food item, is a more accurate method for measuring the impact of food on climate.4 Let's compare the carbon intensity of different foods using Figure 1.5 For example, whereas 100 Calories of corn emits 11 grams (0.024 pounds) of carbon dioxide, 100 Calories of pork emits 308 grams (0.7 pounds). Red meats (beef, pork, and lamb) have the largest carbon intensity; chicken and dairy are more efficient.6 The carbon intensity of fish products varies; farmed salmon has about the same as most red meat; farmed shrimp has a higher carbon intensity (more than 400 grams (0.9 pounds) CO2 /100 Calories)7 , and other fish, such as herring, have a lower carbon intensity (about 25 grams (0.025 pounds) CO2 /100 Calories). Vegetables and fruits, in comparison, generally have the lowest carbon intensity.
Figure 1. Estimates of the carbon dioxide intensity of various food products given in grams of CO2e per 100 Calories. Calculation includes emissions of methane and nitrous oxide. For example, 100 Calories of milk produces 247 grams of CO2e, while 100 Calories of potatoes produces 23 grams of CO2e.
Although issues relating to how products are farmed, processed, and packaged affect these estimates, generally speaking, the carbon intensity of animal products is much higher than that of vegetable products. We also note that these estimates come from analyses of commercial farms, and that organic farms may use less energy and thus produce fewer emissions in growing these foods.8
1. North Americans commonly think about food energy in terms of "Calories." By definition, a calorie of energy is actually quite small, so common practice is to refer to Calories in multiples of 1,000 or as kilocalories (kcal). It is also common practice to use the term, Calorie (with an uppercase 'C') to mean kcal, which we follow in this book. We note that most other countries report food energy in kilojoules, where 1 Calorie = 1kcal = 4.18 kilojoules (kJ).
2. The energy required to produce the food incorporates all aspects of growing, including farm machinery, irrigation, production, and application of fertilizers and pesticides.
3. In this case, energy intensity is defined as the ratio of energy required to produce the product divided by the amount of protein energy in the food. See D. Pimentel and M. Pimentel, Food, Energy and Society, 3rd ed. (Boca Raton, FL: CRC Press, 2008).
4. The calculation of carbon intensity includes the emissions associated with energy used to grow the food item, and any methane emissions associated with animal products. Nitrous oxide emissions due to fertilization of cropland have not been accounted for in this analysis and thus these calculations serve as a lower range for these intensities.
5. Figures for each food item come from estimates of Pimentel and Pimentel (2008). The input energy associated with each food item has been converted into CO2 emissions based on U.S. national emissions and energy-use statistics in a manner similar to that described in Eshel and Martin (2006). The agriculture-related methane or nitrous oxide emissions are also included through a conversion into CO2 equivalent using the procedure of G. Eshel and P. A. Martin, "Diet, Energy, and Global Warming,"Earth Interactions 10 (2006), 1-17. See also U.S. Department of Energy, Energy Information Administration, Emissions of Greenhouse Gases in the United States 2003 (Dept. of Energy, 2004), http://tonto.eia.doe.gov/FTPROOT/environment/057303.pdf, accessed Jan. 25, 2008.
6. There are significant uncertainties in these estimates depending on both the method of growing the food and on the methodology of calculating the emissions. For example, Pimentel (2008) finds that energy inputs may be reduced by up to 50 percent or more for free-range beef and sheep. Comparisons with other published estimates of energy intensity can differ by less then 20 percent up to 200 percent or more as described by Carlsson-Kanyama (2003).
7. The relatively large emissions from some fish products reflect the relatively large energy demands of long-distance voyages required for fishing particular species.
8. At present, studies offer differing conclusions regarding the energy and yield differences between conventional and organic agriculture, although consensus is found on the improved soil health and water quality associated with organic agriculture. See P. Maeder, et al., "Soil Fertility and Biodiversity in Organic Farming,"Science, 296 (2002), 1694-1697; C. Forster, et al., "Environmental Impacts of Food Production and Consumption: A Report to the Department for Environment, Food and Rural Affairs (DEFRA)" (Manchester Business School, 2006), http://www.defra.gov.uk/science/project_data/DocumentLibrary/EV02007/EV02007_4601_FRP.pdf, accessed Oct.31, 2007; and J. Ziesemer, "Energy Use in Organic Food Systems" (Food and Agriculture Organization of the United Nations, 2007), http://www.fao.org/docs/eims/upload/233069/energy-use-oa.pdf, accessed Oct. 31, 2007.