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Memo on CMU study: “Energy use, blue water footprint, and greenhouse gas emissions for current food consumption patterns and dietary recommendations in the US” (Tom et al., 2015) Diets low in animal‐based foods, including vegetarian diets, are indeed better for the environment By Richard Waite and Tim Searchinger, World Resources Institute May 17, 2016 Summary: A December 2015 study by Carnegie Mellon University (Tom et al., 2015) and associated press release appeared to question other analyses that diets high in animal‐based foods have higher environmental impacts than diets with less meat and dairy. However, four important factors led to this miscommunication: 1) the study modeled alternative diet scenarios that were not vegetarian and actually increase overall animal‐based food consumption relative to the current US diet; 2) the study did not count either the land use implications of diets or the particular inefficiencies of beef; 3) the study relied on data from multiple studies using diverse methods and assumptions, leading to possibly inconsistent results; and 4) the press release did not accurately represent the study’s findings. The World Resources Institute (WRI)’s newly‐released paper, Shifting Diets for a Sustainable Food Future (Ranganathan et al., 2016), uses the GlobAgri‐WRR (“GlobAgri”) model and finds that diets high in meat and dairy do lead to far greater greenhouse gas emissions and land use demands. WRI’s paper finds that in countries like the United States where meat and dairy consumption is high, shifting to diets with less beef and other animal products would cause fewer greenhouse gas emissions and free up resources to sustainably feed a growing world population. 1. The “healthy diet” scenarios the CMU study modeled actually increase overall consumption of animal‐based foods relative to the current US diet. The three scenarios modeled in Tom et al. (2015) are shown in Figure 1 as daily per capita shifts in calories relative to the current US diet. Not surprisingly, Scenario 1, which reduced all food consumption by 9 percent relative to the current diet, resulted in a 9 percent decrease in energy use, water use, and GHG emissions. However, much press attention focused on Scenario 3. It modeled a shift to USDA dietary recommendations (and an overall reduction in calories) that actually led to an increase in resource use and environmental impacts relative to the current US diet. Under Scenario 3, the CMU study found that energy use increased by 38 percent, water use by 10 percent, and GHG emissions by 6 percent. 1 Figure 1. Shifts in average daily caloric consumption from the current US diet to three dietary scenarios (Tom et al., 2015)
Scenario 3 was not actually a low livestock product scenario. Although this scenario modeled a 33 percent decrease in meat consumption (beef, pork, and poultry), it also modeled a 78 percent increase in dairy consumption and a tripling in seafood consumption—leading to an overall 13 percent increase in animal‐based food consumption (as measured by calories). Although many low‐impact foods were greatly reduced (sugars, oils), and some moderate‐to‐high‐impact foods were reduced (beef, pork, poultry), other moderate‐impact foods (dairy and seafood) were increased. Fruits and vegetables also increased by 300 calories and the CMU study indicated they had high energy and water impacts. One implication of the paper, which is correct, is that just shifting from some meats to dairy will not reduce resource use. But the basic finding of Scenario 3—that increasing overall animal‐based food consumption leads to increased resource use—was not surprising. 2. The CMU study did not count the land use implications of diets or the particular inefficiencies of beef. Two particular characteristics of the CMU study also led to an underestimate of the environmental significance of dietary choices, particularly when analyzed in a scenario that increases dairy and fish consumption. First, the CMU study lumps together beef, lamb and pork in one “meat” category (Figure 2). Beef and lamb produce far more greenhouse gas emissions than pork because they are inefficient converters of feed to edible food, and because cows produce methane. By contrast, pork is one of the most efficient meats—similar to poultry, eggs, and farmed fish. (Figure 3, showing results from the GlobAgri model, illustrates the extreme inefficiency of beef.) Switching from beef to dairy does reduce emissions, but switching from pork to dairy does not. 2 Figure 2. Indices of average energy use, blue water footprint, and GHG emissions per calorie of food for each food group (Tom et al., 2015) 3 Figure 3. Production of animal‐based foods is generally more impactful on the planet than plant‐based foods (Ranganathan et al., 2016) Source: GlobAgri model (land use and greenhouse gas emissions), authors’ calculations from Mekonnen and Hoekstra (2011, 2012) (freshwater consumption) and Waite et al. (2014) (farmed fish freshwater consumption). Notes: Data presented are global means. Entries are ordered left to right by amount of total land use. Indicators for animal‐
based foods include resource use to produce feed, including pasture. Tons of harvested products were converted to quantities of calories and protein using the global average edible calorie and protein contents of food types as reported in FAO (2015). “Fish” includes all aquatic animal products. Freshwater use for farmed fish products is shown as rainwater and irrigation combined. Land use and greenhouse gas emissions estimates are based on a marginal analysis (i.e., additional agricultural land use and emissions per additional million calories or ton of protein consumed). Based on the approach taken by the European Union for estimating emissions from land‐use change for biofuels, land‐use change impacts are amortized over a period of 20 years and then shown as annual impacts. Land use and greenhouse gas emissions estimates for beef production are based on dedicated beef production, not beef that is a coproduct of dairy. Dairy figures are lower in GlobAgri than some other models because GlobAgri assumes that beef produced by dairy systems displaces beef produced by dedicated beef‐production systems. 4 Second, the CMU study does not examine the land use consequences of diets or the GHGs arising from land‐use change, which are captured by GlobAgri and shown in Figure 3. Livestock products in general, and beef in particular, require far more land per kilocalorie or gram of protein than plant‐based foods. Reducing land‐use demands, which also reduces emissions from deforestation and other land use change, is the single most important environmental benefit of a low‐meat diet. Although the CMU result for Scenario 3 was not necessarily surprising in light of the large increases in dairy and fish, we were curious if the result would be the same using the GlobAgri model, which does count land use and separates out the effects of beef from pork. By our analysis, Scenario 3 led to slightly reduced resource use and environmental impacts compared to the current US diet described in the CMU study despite its overall increase in animal‐based protein—because of a 36 percent reduction in beef consumption. Scenario 3 reduced overall land use by 7.4 percent (cropland by 5.5 percent, pasture by 8.1 percent). Total GHG emissions fell by 7.5 percent compared to the current US diet described in the CMU study, including direct emissions from agricultural production by 10.9 percent, and emissions from land use change by 7.2 percent. Why does our GlobAgri model estimate a 7‐11 percent reduction in GHG emissions under Scenario 3, while CMU estimated a 6 percent increase? The answer is probably based heavily on the benefits of reducing beef consumption, which the CMU study does not capture because it combines beef and pork in one category. 3. CMU’s meta‐analysis of life‐cycle assessment (LCA) data, assessing environmental effects by food type, has a high risk of inconsistent results. Studies that use one consistent model to estimate environmental effects show that animal‐based foods are generally more impactful on the planet than plant‐based foods. The CMU study relies on LCAs by different authors of different foods and averages them together. For example, the study used one set of LCAs for one food group, but different studies for other food groups. Different studies generally are based on different methods, assumptions, and boundaries (Table 1), as evidenced by the large error bars in Figure 2 (showing different values given by different studies) even for studies of the same food types. Averaging many different LCAs in this way has a high chance of producing misleading results because the differences in emissions or impacts attributed to different food types will almost certainly also reflect differences in method or boundaries among the different LCAs. Although other studies using the LCA approach have still found higher emissions from diets high in animal‐based foods (Springmann et al. 2016), there are advantages in consistently using one model to estimate the effects of different diets. Stehfest et al. (2009) finds a similar result to GlobAgri using a consistent but simpler model, and Eshel et al. (2014) finds such results using a detailed model, but only focused on the United States. 5 Table 1. Meta‐Analysis of Cumulative Energy Intensity Values (example for “meat” category) (Tom et al., 2015, supplementary material) Food Product
Meat
Beef
Beef
Beef
Beef
Beef
Beef, fresh, cooked
Beef, frozen, cooked
Cow, fresh, cooked
Beef stew, cooked
Veal
Pork
Pork
Pork
Pork, fresh, cooked
Pork, frozen, cooked
Pork, frozen, cooked
Pork sausage, fresh, cooked
Pork stew, cooked
MJ/kg
Location
LCA Boundary
Author
64
48
43
44
28
70
75
26
24
-32
39
17
40
43
44
34
17
United States
United States
United States
Multiple locations
UK
Sweden
Central Europe
Sweden
Sweden
-Sweden
Sweden
UK
Sweden
Sweden
Central Europe
Sweden
Sweden
Farm to farm gate
Farm to farm gate
Farm to farm gate
Farm to fork
Farm to farm gate
Farm to fork
Farm to fork
Farm to fork
Farm to fork
-Farm to retailer gate
Farm to fork
Farm to farm gate
Farm to fork
Farm to fork
Farm to fork
Farm to fork
Farm to fork
Pelletier et al, 2010
Pelletier et al, 2010
Pelletier et al, 2010
Foster et al, 2006
Williams et al, 2006
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
-Carlsson-Kanyama, 1998
Cederberg, 2003
Williams et al, 2006
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Lamb
32
Europe
Farm to fork
Williams et al, 2006,
Carlsson-Kanyama and Faist, 2000
Lamb
39
Europe
Farm to fork
Williams et al, 2006,
Carlsson-Kanyama and Faist, 2000
Lamb, fresh, cooked
Lamb, frozen, cooked
Lamb, frozen, cooked
Lamb sausage, fresh, cooked
Lamb stew, cooked
Lamb
Lamb
43
46
52
30
18
11
46
Sweden
Sweden
Overseas
Sweden
Sweden
New Zealand to UK
UK
Farm to fork
Farm to fork
Farm to fork
Farm to fork
Farm to fork
Farm to processing
Farm to farm gate
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Carlsson-Kanyama et al, 2003
Saunders and Barber, 2008
Saunders and Barber, 2008
4. The press release of the CMU study’s findings did not accurately reflect the study. The press release associated with the CMU study did not properly reflect the study’s findings. The CMU press release’s headline stated: “Vegetarian and ‘Healthy’ Diets Could Be More Harmful to the Environment: Carnegie Mellon Study Finds Eating Lettuce Is More Than Three Times Worse in Greenhouse Gas Emissions than Eating Bacon.” In addition to the three points above, there are two additional reasons this headline is inaccurate: a. “Vegetarian…Diets Could Be More Harmful to the Environment.” This statement was inaccurate because the CMU study did not model any vegetarian diets. Scenario 3, the closest example, evaluated a diet with substantial meat consumption as well as large increases in both fish and dairy relative to the average US diet. A comparison of vegetarian diets and animal‐based diets should be based on realistic diets, and cannot be extrapolated from individual foods, such as lettuce alone (see below). In addition to our analysis (shown in Figure 4), other studies such as 6 Scarborough et al. (2014) and Springmann et al. (2016) find significant reductions in emissions and resource use from realistic vegetarian diets. b. “Lettuce Is More Than Three Times Worse in Greenhouse Gas Emissions Than Eating Bacon.” This comparison is misleading because meat and lettuce serve different purposes in people’s overall diets. Even more than other vegetables, people do not consume lettuce for calories, but rather for its micronutrient content and taste (lettuce is high in vitamins A and K, among others). By contrast, people do consume bacon and other animal products for their calories and protein. CMU’s data taken from the NHANES survey of American consumers show that all vegetables provided only 5 percent of calories consumed by Americans in 2007‐10, while 30 percent of calories were from animal‐based products. Although a diet consisting entirely of lettuce would be resource‐intensive, no one would eat such a diet. We calculate that animal‐based products were responsible for roughly 85‐90 percent of the land use and GHG emissions associated with the average American diet in 2009, while all plant‐based products were associated with the remaining 10‐15 percent (Figure 4).i The implication from Figure 4 is that the most effective way to reduce resource use and environmental impacts of the average US diet would be to reduce consumption of animal‐based foods—especially beef, which on its own accounts for nearly half of the land use and GHGs associated with the average American diet. Although the press release was inaccurate, the CMU study itself shows some awareness of these issues. Tom et al. (2015) wrote: “In light of the growing evidence that meat production has negative environmental implications, a number of studies…examine the impacts of reducing meat consumption on resource use and emissions...The results of these studies…demonstrate that adopting a vegetarian diet or even reducing meat consumption by 50% is more effective in reducing energy use, the blue water footprint, and GHG emissions through the food supply system than adopting a healthier diet based on regional dietary guidelines.” Conclusion: Although study results do have some differences, and we are partial to our own analysis because of its methodological details, the evidence is strong that—among the world’s wealthier populations—holding down meat and dairy consumption in general, and beef in particular, is an important element of achieving a sustainable food future. 7 Figure 4. Shifting the average American to a realistic vegetarian diet would reduce per capita land use and GHG emissions by about half (Ranganathan et al., 2016) (per capita values, 2009) Source: GlobAgri model. Note: All “US” data are for United States and Canada. Land‐use change emissions are amortized over a period of 20 years and then shown as annual impacts. Calculations assume global average efficiencies (calories produced per hectare or per ton of CO2e emitted) for all food types. “Other animal‐based foods” includes pork, poultry, eggs, fish (aquatic animals), sheep, and goat. The vegetarian diet scenario, which uses data from Scarborough et al. (2014), includes small amounts of meat, as “vegetarians” were self‐reported. 8 i.
We are not clear where the headline points come from. Nowhere in the CMU study or supplementary material is the word “bacon” mentioned, although pork products are discussed. “Lettuce” is only discussed in the supplementary material in terms of energy and water impacts per kilogram (not calorie). This limited data makes the headline hard to verify. References: Eshel, G., A. Shepon, T. Makov, and R. Milo. 2014. “Land, irrigation water, greenhouse gas, and reactive nitrogen burdens of meat, eggs, and dairy production in the United States.” Proceedings of the National Academy of Sciences of the United States of America 111 (33): 11996–12001. FAO (Food and Agriculture Organization of the United Nations). 2015. “FAOSTAT.” Rome: FAO. Mekonnen, M. M., and A. Y. Hoekstra. 2011. “The green, blue and grey water footprint of crops and derived crop products.” Hydrology and Earth System Sciences 15: 1577–1600. Mekonnen, M. M., and A. Y. Hoekstra. 2012. “A Global Assessment of the Water Footprint of Farm Animal Products.” Ecosystems 15: 401–415. Ranganathan, J., D. Vennard, R. Waite, T. Searchinger, P. Dumas, and B. Lipinski. 2016. “Shifting Diets for a Sustainable Food Future.” Working Paper, Installment 11 of Creating a Sustainable Food Future. Washington, DC: World Resources Institute. Scarborough, P., P. N. Appleby, A. Mizdrak, A. D. M. Briggs, R. C. Travis, K. E. Bradbury, and T. J. Key. 2014. “Dietary greenhouse gas emissions of meat‐eaters, fish‐eaters, vegetarians and vegans in the UK.” Climatic Change 125: 179–192. Springmann, M., H. C. J. Godfray, M. Raynera, and P. Scarborough. 2016. “Analysis and valuation of the health and climate change cobenefits of dietary change.” Proceedings of the National Academy of Sciences of the United States of America (Early Edition): www.pnas.org/cgi/doi/10.1073/pnas.1523119113. Stehfest, E., L. Bouwman, D. P. van Vuuren, M. G. J. den Elzen, B. Eickhout, and P. Kabat. 2009. “Climate benefits of changing diet.” Climatic Change 95: 83–102. Tom, M. S., P. S. Fischbeck, and C. T. Hendrickson. 2015. “Energy use, blue water footprint, and greenhouse gas emissions for current food consumption patterns and dietary recommendations in the US.” Environment Systems and Decisions DOI 10.1007/s10669‐015‐9577‐y. Waite, R., M. Beveridge, R. Brummett, S. Castine, N. Chaiyawannakarn, S. Kaushik, R. Mungkung, S. Nawapakpilai, and M. Phillips. 2014. “Improving Productivity and Environmental Performance of Aquaculture.” Working Paper, Installment 5 of Creating a Sustainable Food Future. Washington, DC: World Resources Institute. 9