Volume 10 (2006)
Paper No. 9
Copyright © 2006, Paper 10-009; 6,923 words, 3 Figures, 0 Animations, 6 Tables.
Diet, Energy, and Global Warming
Gidon Eshel* and Pamela A. Martin
Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois
Received 16 May 2005; Final form 12 December 2005
The energy consumption of animal- and plant-based diets and,
more broadly, the range of energetic planetary footprints spanned by reason-
able dietary choices are compared. It is demonstrated that the greenhouse gas
emissions of various diets vary by as much as the difference between owning
an average sedan versus a sport-utility vehicle under typical driving conditions.
The authors conclude with a brief review of the safety of plant-based diets, and
find no reasons for concern.
Diet; Energy consumption; Public health
As world population rises (2.5, 4.1, and 6.5 billion individuals in 1950, 1975,
and 2005, respectively; United Nations 2005), human-induced environmental pres-
sures mount. By some measures, one of the most pressing environmental issues is
global climate change related to rising atmospheric concentrations of greenhouse
gases (GHGs). The link between observed rising atmospheric concentrations of
CO and other GHGs, and observed rising global mean temperature and other
climatic changes, is not unequivocally established. Nevertheless, the accumulating
evidence makes the putative link harder to dismiss. As early as 2000, the United
Nations–sponsored Intergovernmental Panel on Climate Change (Houghton et al.
2001) found the evidence sufficiently strong to state that “there is new and stronger
* Corresponding author address: Gidon Eshel, Dept. of the Geophysical Sciences, University
of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637.
E-mail address: firstname.lastname@example.org
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evidence that most of the warming observed over the last 50 years is attributable
to human activities” and that “[t]he balance of evidence suggests a discernible
human influence on global climate.”
If one views anthropogenic climate change as an undesirable eventuality, it
follows that modifying the ways we conduct various aspects of our lives is re-
quired in order to reduce GHG emissions. Many changes can realistically only
occur following policy changes (e.g., switching some transportation volume to less
CO -intensive modes). However, in addition to policy-level issues, energy con-
sumption is strongly affected by individual personal, daily-life choices. Perhaps
the most frequently discussed such choice is the vehicle one drives, indeed a very
important element of one’s planetary footprint. As we show below, an important
albeit often overlooked personal choice of substantial GHG emission conse-
quences is one’s diet. Evaluating the implications of dietary choices to one’s
planetary footprint (narrowly defined here as total personal GHG emissions) and
comparing those implications to the ones associated with personal transportation
choices are the purposes of the current paper.
2. Comparative energy consumption by food production
In 1999, Heller and Keoleian (2000) estimated the total energy used in food
production (defined here as agricultural production combined with processing and
distribution) to be 10.2 × 1015 BTU yr?1. Given a total 1999 U.S. energy con-
sumption of 96.8 × 1015 BTU yr?1 (Table 1.1 in U.S. Department of Energy
2004a), energy used for food production accounted for 10.5% of the total energy
used. In 2002, the food production system accounted for 17% of all fossil fuel use
in the United States (Horrigan et al. 2002). For example, Unruh (2002) states that
delivered energy consumption by the food industry, 1.09 × 1018 J in 1998, rose to
1.16 × 1018 J in 2000 and is projected to rise by 0.9% yr?1, reaching 1.39 × 1018
J in 2020. Unruh (2002) also reports that delivered energy consumption in the
crops and other agricultural industries (the latter consisting of, e.g., animal and
fishing) increases, on average, by 1% and 0.9% yr?1, respectively. Thus, food
production, a function of our dietary choices, represents a significant and growing
To place energy consumption for food production in a broader context, we
compare it to the more often cited energy sink, personal transportation. The annual
U.S. per capita vehicle miles of travel was 9848 in 2003 (Table PS-1 in U.S.
Department of Transportation 2004). Using the same source, and focusing on cars
(i.e., excluding buses and heavy commercial trucks), per capita vehicle miles
traveled becomes 8332, of which an estimated 63% are traveled on highways
(Table VM-1 U.S. Department of Transportation 2004). According to the U.S.
Department of Energy’s (2005) table of most and least efficient vehicles (http://
www.fueleconomy.gov/feg/best/bestworstNF.shtml) and considering only highly
popular models, the 2005 vehicle miles per gallon (mpg) range is bracketed by the
Toyota Prius’ 60:51 (highway:city) on the low end and by Chevrolet Suburban’s
11:15. At near average is the Toyota Camry Solara’s 24:33 mpg. The salient
transportation calculation (Table 1) demonstrates that, depending on the vehicle
model, an American is likely to consume between 1.7 × 107 and 6.8 × 107 BTU
yr?1 for personal transportation. This amounts to emissions of 1.19–4.76 ton CO2
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Table 1. Energy consumption for personal travel.
Miles per gallon
Ton CO c
a Based on 63% highway driving.
b The conversion of gallons consumed to BTU consumed is based on an average of 1 U.S. gallon of fuel ?
115 000 BTU. Many sources report a conversion factor of 1 U.S. gallon of fuel ? 125 000 BTU, but this
assumes a so-called high heating value, which is not appropriate for motor vehicles’ internal combustion
engines [Oak Ridge National Laboratory bioenergy conversion factors; http://bioenergy.ornl.gov/papers/
c The conversion of BTU consumed to CO emissions is based on the total of the U.S. emissions, as described
in the text.
based on the estimated conversion factor of 7 × 10?8 ton CO BTU?1 derived from
the 2003 U.S. total energy consumption, 98.6 × 1015 BTU (U.S. Department of
Energy 2004a), and total CO emissions of 6935.9 × 106 ton (U.S. Department of
Next, we perform a similar energetic calculation for food choices. Accounting
for food exports, in 2002 the U.S. food production system produced 3774 kcal per
person per day or 1.4 × 1015 BTU yr?1 nationwide (FAOSTAT 2005). (The
difference between 3774 kcal per person per day and the needed average ?2100
kcal per person per day is due to overeating and food discarded after being fully
processed and distributed.) In producing those 1.4 × 1015 BTU yr?1, the system
used 10.2 × 1015 BTU yr?1. That is, given both types of inefficiency, food pro-
duction energy efficiency is 100(1.4/10.2) (2100/3774) ? 7.6%. Therefore, in order
to ingest 2100 kcal day?1, the average American uses 2100/0.076 ? 72.6 × 104 kcal
? 4 × 107
In summary, while for personal transportation the average American uses 1.7 × 107
to 6.8 × 107 BTU yr?1, for food the average American uses roughly 4 × 107 BTU
yr?1. Thus, there exists an order of magnitude parity in fossil energy consumption
between dietary and personal transportation choices. This is relevant to climate
because fossil fuel–based energy consumption is associated with CO emissions.
Note that both food production and transportation also release non-CO GHGs
produced during fossil fuel combustion (principally NO conversion to N O), but
these are ignored below. This omission is irrelevant to the comparison between
transportation and food production because these contributions are proportional to
the mass of fossil fuel burned and thus scale with CO emissions. They are
noteworthy, however, as they render our bottom-line conclusion an underestimate
of the range of GHG burden resulting from dietary choices.
The next logical step is quantifying the range of GHG emissions associated with
various reasonable dietary choices. In exploring this question, we note that food
production also releases non-CO GHGs unrelated to fossil fuel combustion (e.g.,
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methane emissions due to animal manure treatment). In comparing below the GHG
burden exerted by various reasonable dietary choices we take note of both con-
3. Plant-based versus animal-based diets
To address the variability in energy consumption and GHG emissions for food,
we focus on a principal source of such variability, plant- versus animal-based diets.
To facilitate a quantitative analysis, we define and consider several semirealistic
mixed diets: mean American, red meat, fish, poultry, and lacto-ovo vegetarian.
These diets are shown schematically in Figure 1. To obtain the mean American
Figure 1. The composition of the diets discussed in this paper. (left) The actual
observed mean U.S. diet based on per capita food disappearance data
(FAOSTAT 2005). The legend at the bottom shows the various components.
The mean diet comprises 3774 gross kcal, of which 1047 kcal are from
animal products. The breakdown of the animal-based portion, shown on
the right side of the panel, is 54% meats, 41% dairy, and 5% eggs. (right)
Schematic depiction of the five semirealistic, hypothetical diets consid-
ered in this paper, all comprising 3774 kcal. The (variable) fraction of the
total from animal products, ? (shown on the right end of the plot), com-
prises the various animal-based food items shown and totals 3774a kcal.
The remaining plant-based portion totals 3774(1 ? a) kcal. Of the animal-
based part of the lacto-ovo diet, 85% of the calories are from dairy, and
15% from eggs. In the remaining four diets (mean American, fish, red
meat, and poultry), 46% of the animal-based calories are from dairy and
eggs, similar to the observed mean American diet shown in the left panel,
with the remaining 54% from either the single sources shown, or the blend
of sources characterizing the mean American diet. Red meat consists of
35.61% beef, 62.61% pork, and 1.78% lamb.
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diet, we use actual per capita food supply data summarized in the Food Balance
Sheets for 2002 (FAOSTAT 2005). Those balance sheets report a total gross
caloric consumption of 3774 kcal per person per day, of which 1047 kcal, or
27.7%, is animal based. Of those 1047 kcal day?1, 41% are derived from dairy
products, 5% from eggs, and the remaining 54% from various meats. For com-
parison, we let all diets, including the exclusively plant-based one (“vegan”),
comprise the same total number of gross calories, 3774 kcal day?1.
The red meat, fish, and poultry diets we consider share similar dairy and egg
portions, 41% and 5% of the animal-based caloric fraction of the diet (Figure 1).
The remaining 54% of the animal-based portion of the diet is attributed to the sole
source given by the diet name. For example, the animal-based part of the red meat
diet comprises 41%, 5%, and 54% of the animal-based calories from dairy, eggs,
and red meat, respectively. For the purposes of this paper, we define red meat as
comprising 35.6% beef, 62.6% pork, and 1.8% lamb, reflecting the proportions of
these meats in the FAOSTAT data. In the lacto-ovo diet, we set the total animal-
based energy derived from eggs and dairy to 15% and 85% based on values from
Table 1 of Pimentel and Pimentel (2003).
Specific diets vary widely in the fraction of caloric input from animal sources
(hereafter ?). For example, Haddad and Tanzman (2003) suggest that lacto-ovo
vegetarian diets in the United States contain less than 15% of their calories from
animal sources, well below the 27.7% derived from animal sources in the mean
American diet. We therefore calculate the energy and GHG impact of each diet
over a range of this fraction, 0% ? a ? 50%, where ? ? 0 corresponds to a vegan
3.1. Greenhouse effects of direct energy consumption
This section addresses the greenhouse burden by agriculture that is directly
exerted through (mostly fossil fuel) energy consumption and the subsequent CO2
release. The fossil fuel inputs treated here are related to direct energy needs such
as irrigation energy costs, fuel requirements of farm machinery, and labor. We are
interested in the range of this burden affected by dietary choices, especially plant-
versus animal-based diets.
We define energy efficiency as the percentage of fossil fuel input energy that is
retrieved as edible energy [e ? 100 × (output edible energy)/(fossil energy input);
see Table 2]. We derive energy efficiency e of various animal-based food items by
combining available estimates of (edible energy in protein output)/(fossil energy
input) (Pimentel and Pimentel 1996a) and the total energy content relative to the
energy from protein. The estimated energy efficiency of protein in animal products
(Pimentel and Pimentel 1996a) varies from 0.5% for lamb to ?5% for chicken and
milk to 3% for beef (second column of Table 2). This wide range reflects the
different reproductive life histories of various animals, their feed, their genetic
ability to convert nutrients and feed energy into body protein, fat, and offspring,
the intensity of their rearing, and environmental factors (heat, humidity, severe
cold) to which they are subjected, among other factors. Accounting for the total
energy content relative to the energy from protein (Table 2; U.S. Department of
Agriculture 2005), these numbers translate to roughly 1%, 20%, and 6% (e ? 0.1,
0.2, and 0.06). The weighted mean efficiency of meat [red meat (consisting of
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Table 2. Energetic efficiencies for a few representative food items derived from
land animals, aquatic animals, and plants.
Beef (grain fed)
a Pimentel and Pimentel (1996a,b); energy input refers to fossil fuels.
b Assuming 1 gram protein ? 4 kcal and using U.S. Department of Agriculture (2005) values.
c For animal products, the product of the previous two columns.
beef, pork, and lamb, as previously defined), fish, and poultry] in the American
diet is 9.32% (U.S. Department of Agriculture 2002; see Table 3). These efficien-
cies are readily comparable with the energy efficiency f of plant-based foods
estimated by Pimentel and Pimentel (1996b,c): 60% for tomatoes, ?170% for
oranges and potatoes, and 500% for oats. The wide range of f reflects differences
in farming intensity, including labor, machinery operation, and synthetic chemical
Table 3. Weighted-mean energetic efficiency of the animal-based portion of the
hypothetical mixed diets considered in this paper.
Weighted mean (%)
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Because of the wide range of efficiencies in both plant- and animal-based foods,
we quantitatively compare plant-based diets with animal-based ones by consider-
E = cd
for the various hypothetical diets shown in Figure 1. In (2),
c = 3774
? 1 377 510
is the U.S. per capita annual gross caloric consumption, and
× ?7 × 10?8?
? 2.778 × 10?7
so that cd ? 0.383 ton CO yr?1 is the annual CO emissions of a person consuming
3774 kcal day?1 using the BTU–CO conversion factor introduced earlier and
assuming perfect efficiency (the deviation from ideal efficiency is accounted for
by the bracketed term). The parameter ? is the fraction of the dietary caloric intake
derived from animal sources. As defined above, e and f are the weighted mean
caloric efficiencies of animal- and plant-based portions of a given diet. Those
efficiencies for the five hypothetical diets considered here, shown in Table 3, are
simply the weighted mean efficiencies derived from the characteristic caloric
efficiency of each component of the diet and the caloric prevalence of those
The efficiencies are e ? 0.1152 (fish), 0.1152 (red meat), 0.1405 (average
American diet), 0.1876 (poultry), and 0.1919 (lacto-ovo). Recall that the red meat,
mean American, fish, and poultry diets derive 41% and 5% of their animal-based
calories from dairy and eggs; thus, the weighted-mean efficiency e of the diets
reflects the higher efficiency of dairy and egg relative to fish or red meats. The
specific (not weighted mean) efficiency of poultry production is between those of
dairy and eggs (Table 2). The notable equality of fish and red meat efficiencies
reflects 1) the large energy demands of the long-distance voyages required for
fishing large predatory fishes such as swordfish and tuna toward which western
diets are skewed, and 2) the relatively low energetic efficiency of salmon farming.
Note that similar e values for two or more diets (such as the poultry and lacto-ovo
above) reflect similar overall energetic efficiency of the total diets only if those
diets also share ?, the animal-based caloric fraction of the diet. However, recall the
aforementioned Haddad and Tanzman (2003) suggestion that American lacto-ovo
vegetarians eat less than 15% of their calories from animal sources, indicating that
the overall energetic efficiency of lacto-ovo diets is higher than that of the average
poultry diet assumed here, with ? ? 0.277, the same fraction as that of the mean
Equation (2) allows us to calculate the total CO burden related to fossil fuel
combustion for various diets characterized by specific ?, e, and f values. However,
our objective is to compute the difference between various mixed diets and an
exclusively plant-based, vegan, diet. To facilitate such comparison, we get an
expression for the difference in CO -based footprint between mixed diets and an
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exclusively plant-based one by subtracting from Equation (2) the expression for a
purely plant-based diet. We get the latter by setting ? ? 0 in Equation (2), yielding
? cd/f, with which
E ? E ? E
= c d ??1 ? .
Figure 2 shows the results of solving Equation (3) with 0 ? a ? 0.5 for three
values of e corresponding to fish and red meat (red), poultry/lacto-ovo (magenta),
and the blend of animal sources characteristic of the average American diet (blue).
For each value of e (each color), we solve for ?E with the three shown values of
f, bracketing the actual efficiency of nearly all plant-derived foods.
Note that the difference among the three diet groups is larger than the range in
efficiencies arising from different values of f for a given mean e. Figure 2 shows
that a person consuming the average American diet, with average caloric efficien-
cies of the animal- and plant-based portions of the diet, releases 701 kg of CO yr?1
beyond the emissions of a person consuming only plants. Compared with driving
Figure 2. Comparison of four mixed diets to an exclusively plant-based one in terms
of additional energy use beyond that of the plant-based diet. The addi-
tional energy use per person per year is reported in two interchangeable
units, tons of CO emissions on the left, and million BTUs on the right, using
the conversion factor introduced in the text. The four animal-based diets
considered are shown in the upper left. The blue curves show the average
animal-based diet composition, with caloric efficiency of e = 13.7%. For
each diet (a given color), three curves are shown, differing from each
other in the caloric efficiency of the plant-based fraction of the diet, f,
where the values considered are 1.2, 2, and 4. The average American
diet, with ? = 0.277 (with 27.7% of calories from animal sources) is shown,
along with the added CO it corresponds to (assuming average efficiency
of 13.7%), 0.726 ton.
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a Toyota Camry under the conditions of Table 1, this amounts to 100 × 0.701/2.24
? 31.3%, that is, roughly a third of the greenhouse costs of personal transportation.
3.2. Greenhouse effects in addition to energy inputs
Of agriculture’s various non-energy-related GHG emissions, we focus below on
the two main non-CO GHGs emitted by agriculture, methane, CH , and nitrous
oxide, N O. In 2003, U.S. methane emissions from agriculture totaled 182.8 × 106
ton CO -eq, of which 172.2 × 106 ton CO -eq are directly due to livestock (U.S.
Department of Energy 2004b). The same report also estimates the 2003 agricul-
ture-related nitrous oxide emissions, 233.3 × 106 ton CO -eq, of which 60.7 × 106
ton CO -eq are due to animal waste. Thus, the production of livestock in the U.S.
emitted methane and nitrous oxide is equivalent to at least 172.2 × 106 + 60.7 ×
106 ? 232.9 × 106 ton CO in 2003. With 291 million Americans in 2003, this
amounts to 800 kg CO -eq per capita annually in excess of the emissions associ-
ated with a vegan diet.
One may reasonably argue that the ?0.8 ton CO -eq per person per year due to
non-CO GHGs does not accurately represent the difference between animal- and
plant-based diets, which is our object of inquiry; if there were no animal-based
food production at all, plant-based food production would have to increase. How-
ever, such a hypothetical transition will produce zero methane and nitrous oxide
emissions in the categories considered above, animal waste management, and
enteric fermentation by ruminants. Ignored categories, principally soil manage-
ment, will indeed have to increase, but over an area far smaller than that vacated
by eliminating feed production for animals. For example, Reijnders and Soret
(2003) report that, per unit protein produced, meat production requires 6 to 17
times as much land as soy. Therefore, the net reduction in methane and nitrous
oxide emissions will have to be larger than our estimate presented here.
Approximately 74% of the total nitrous oxide emissions from agriculture, ?173
× 106 ton CO -eq, are due to nitrogen fertilization of cropland, which supports
production of both animal- and plant-based foods. The exact partitioning of ni-
trogen fertilization into animal feed and human food is a complex bookkeeping
exercise beyond the scope of this paper. Consequently, we ignore this large con-
tribution below. Nevertheless, simple analysis of the Food Balance Sheets
(FAOSTAT 2005) and Agriculture Production Database (FAOSTAT 2005) data
shows that the portion of those 173 × 106 ton CO -eq attributable to animal
production is at least equal to, and probably larger than that attributable to plants,
thereby rendering our estimate of the GHG burden exerted by animal-based food
production a lower bound.
The value of 800 kg CO -eq yr?1 due to non-CO emissions computed above
represents the composition of the actual mean American diet. To calculate the
added non-CO burden of specific diets, we must first compute, from the mean
American diet, the burden for individual food items.
This calculation requires intermediate steps, as available data are for specific
farm animals, not individual food items. Using annual emissions reported by the
U.S. Department of Energy (2004b), in Table 4 we sum the contributions of
methane from enteric fermentation and manure management and the nitrous oxide
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Table 4. Non-CO GHG emissions associated with the production of various food
items. Units are 106 CO -eq yr?1, except column 6.
N O* manure
percentage of total
* Sources: U.S. Department of Energy (2004b, Tables 21, 22, and 28).
from manure management for cattle, pigs, poultry, sheep, and goats. To partition
cattle methane emissions from enteric fermentation [108.72 million ton CO -eq;
Table 21 in U.S. Department of Energy (2004b)] among beef (75.46%) and dairy
(24.54%) cattle, we use emission ratios derived from Table 5-3 of U.S. Environ-
mental Protection Agency (2005) (we apply these ratios to the 2003 data, but we
do not use the absolute values, because the table’s latest entry is 2001). We
similarly use Table 5-5 of U.S. Environmental Protection Agency (2005) to par-
tition nitrous oxide emissions from cattle manure management, 56.3 million ton
CO -eq [Table 28 in U.S. Department of Energy (2004b)], among dairy (39%) and
beef (61%) cattle.
Table 28 in U.S. Department of Energy (2004b) reports emissions of 1.3 million
ton CO -eq from N O due to poultry manure management. Because we do not
have direct information on the partitioning of these emissions among eggs and
poultry meat, we assume this partition in N O is proportional to total manure mass
and thus is roughly similar to the partitioning of methane from poultry manure
management, 47.38% and 52.62% for eggs and meat, respectively (Table 22 in
U.S. Department of Energy 2004b). We thus partition the 1.3 million ton CO -eq
from N O due to poultry manure management as 0.62 and 0.68 million ton CO -eq
due to eggs and poultry meat, respectively.
To obtain the per capita daily emissions associated with food items, we divide
the individual non-CO GHG annual sums (Table 4, fourth numeric column) by
the U.S. 2003 population, 291 million, and 365 days. The results, in grams of
CO -eq per day, are shown in the first numeric column in Table 5. To calculate
emissions per kcal associated with the consumption of individual food items, we
divide the per capita daily emissions (Table 5, first numeric column) by the
respective per capita consumptions (FAOSTAT 2005; Table 5, second numeric
column). These divisions yield the non-CO GHG emissions per kcal reported in
the rightmost column in Table 5. Importantly, the non-CO GHG emissions per
kcal vary by as much as a factor of 70 for the animal-based food items considered,
rendering some animal-based options (e.g., poultry meat) far more benign than
other ones (most notably beef).
Using the emission associated with individual food items (Table 5, rightmost