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Comparative analysis of environmental impacts of agricultural production systems, agricultural input efficiency, and food choice

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Global agricultural feeds over 7 billion people, but is also a leading cause of environmental degradation. Understanding how alternative agricultural production systems, agricultural input efficiency, and food choice drive environmental degradation is necessary for reducing agriculture's environmental impacts. A meta-analysis of life cycle assessments that includes 742 agricultural systems and over 90 unique foods produced primarily in high-input systems shows that, per unit of food, organic systems require more land, cause more eutrophication, use less energy, but emit similar greenhouse gas emissions (GHGs) as conventional systems; that grass-fed beef requires more land and emits similar GHG emissions as grain-feed beef; and that low-input aquaculture and non-trawling fisheries have much lower GHG emissions than trawling fisheries. In addition, our analyses show that increasing agricultural input efficiency (the amount of food produced per input of fertilizer or feed) would have environmental benefits for both crop and livestock systems. Further, for all environmental indicators and nutritional units examined, plant-based foods have the lowest environmental impacts; eggs, dairy, pork, poultry, non-trawling fisheries, and non-recirculating aquaculture have intermediate impacts; and ruminant meat has impacts ~100 times those of plant-based foods. Our analyses show that dietary shifts towards low-impact foods and increases in agricultural input use efficiency would offer larger environmental benefits than would switches from conventional agricultural systems to alternatives such as organic agriculture or grass-fed beef.
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LETTER
Comparative analysis of environmental impacts of agricultural
production systems, agricultural input efficiency, and food
choice
Michael Clark
1,4
and David Tilman
2,3
1
Natural Resources Science and Management, University of Minnesota, St Paul, Minnesota 55108, United States of America
2
Department of Ecology, Evolution and Behavior, University of Minnesota, St Paul, Minnesota 55108, United States of America
3
Bren School of Environmental Science and Management, University of California Santa Barbara, California 93106, United States of
America
4
Author to whom any correspondence should be addressed.
E-mail: maclark@umn.edu
Keywords: life cycle assessment, environmental sustainability, food, organic, agricultural efciency
Supplementary material for this article is available online
Abstract
Global agricultural feeds over 7 billion people, but is also a leading cause of environmental
degradation. Understanding how alternative agricultural production systems, agricultural input
efciency, and food choice drive environmental degradation is necessary for reducing agricultures
environmental impacts. A meta-analysis of life cycle assessments that includes 742 agricultural
systems and over 90 unique foods produced primarily in high-input systems shows that, per unit
of food, organic systems require more land, cause more eutrophication, use less energy, but emit
similar greenhouse gas emissions (GHGs) as conventional systems; that grass-fed beef requires
more land and emits similar GHG emissions as grain-feed beef; and that low-input aquaculture
and non-trawling sheries have much lower GHG emissions than trawling sheries. In addition,
our analyses show that increasing agricultural input efciency (the amount of food produced per
input of fertilizer or feed) would have environmental benets for both crop and livestock
systems. Further, for all environmental indicators and nutritional units examined, plant-based
foods have the lowest environmental impacts; eggs, dairy, pork, poultry, non-trawling sheries,
and non-recirculating aquaculture have intermediate impacts; and ruminant meat has impacts
100 times those of plant-based foods. Our analyses show that dietary shifts towards low-impact
foods and increases in agricultural input use efciency would offer larger environmental benets
than would switches from conventional agricultural systems to alternatives such as organic
agriculture or grass-fed beef.
Introduction
Global agriculture feeds over 7 billion people, but is
also a major cause of multiple types of environmental
degradation. Agricultural activities emit 25%33% of
greenhouse gases (Steinfeld et al 2006, Edenhofer et al
2014, Tubiello et al 2014); occupy 40% of Earths land
surface (FAO 2016a); account for >70% of freshwater
withdrawals (Molden 2007), drive deforestation and
habitat fragmentation (Ramankutty and Foley 1999)
and resultant biodiversity loss (IUCN 2016); and
eutrophy and acidify natural aquatic and terrestrial
ecosystems with agrochemicals (Vitousek et al 1997).
These impacts are likely to increase globally over the
next several decades because of increases in population
growth and income-dependent dietary shifts towards
more meat-based diets (Tilman et al 2011, Bajzelj et al
2014, Tilman and Clark 2014, Springmann et al 2016).
We need to understand the linkages between diets,
agricultural production practices, and environmental
degradation if we are to reduce agricultures environ-
mental impacts while providing a secure food supply
for a growing global population. To quantify these
processes and linkages, we review and synthesize
published information from 742 food production
systems of over 90 foods from 164 published life cycle
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REVISED
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ACCEPTED FOR PUBLICATION
12 April 2017
PUBLISHED
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Environ. Res. Lett. 12 (2017) 064016 https://doi.org/10.1088/1748-9326/aa6cd5
©2017 IOP Publishing Ltd
assessments (LCAs). LCAs are an internationally
recognized way to account the inputs, outputs, and
environmental impacts of a food production system.
Using our meta-analysis of LCAs, we examine the
comparative environmental impacts of different food
production systems, different agricultural input
efciencies, and different foods.
Food production systems such as organic agricul-
ture and grass-fed beef have been proposed as
potential ways to reduce agricultures environmental
impacts (e.g. Ponisio et al 2014). Organic agriculture,
for example, is often promoted as having lower
environmental impacts relative to high-input conven-
tional systems because it replaces agrochemical inputs
with natural inputs such as manure or with ecosystem
services such as pest control (Azadi et al 2011). Recent
analyses examining the comparative impacts of
organic and conventional systems have, of necessity,
been limited to a few environmental indicators or in
statistical strength of their inferences because of small
sample size (Mondelaers et al 2009, Seufert et al 2012,
Tuomisto et al 2012, Ponisio et al 2014). Recent
increases in the number of published LCAs enables
more complete analysis of the comparative impacts of
organic and conventional systems across a range of
environmental indicators and foods. In addition, we
combine de novo analyses to determine the compara-
tive environmental impacts of three other sets of
production systems: grass-fed and grain-fed beef;
trawling and non-trawling sheries; and greenhouse
grown and open-eld produce.
Increases in agricultural input efciency, or the
amount of food produced per unit of fertilizer or feed
input, may also reduce agricultures environmental
impact (e.g. Robertson and Swinton 2005). Agricul-
tural systems depend on fertilizer and feed inputs to
obtain and/or maintain high productivity. However,
excessive application of these inputs increases
agricultures environmental impact without increas-
ing yields or farmer prots (Vitousek et al 2009). Our
analyses examine the extent to which increases in
agricultural input efciency could reduce the
environmental impact of producing a given type of
food.
Previous analyses have shown that foods can differ
greatly in their environmental impact (e.g. Clune et al
2017). However, these have been limited to animal-
based foods (de Vries and de Boer 2010, Nijdam et al
2012) or to a single environmental indicator (e.g.
Mekonnen and Hoekstra 2010, Clune et al 2017). It is
thus currently unclear how foods differ in their
impacts across a range of environmental indicators,
and whether foods with low impacts for one
environmental indicator have similarly low impacts
for other environmental indicators. Our meta-analysis
enables us to make these comparisons for ve
environmental indicators: greenhouse gas emissions
(GHGs), land use, fossil fuel energy use, eutrophica-
tion potential, and acidication potential.
The analyses and results presented here expand on
current knowledge of how food production system,
agricultural input efciency, and food choice affect
agricultures environmental impacts. The results can be
used to create a more sustainable agricultural future.
Methods
Publication selection and issues covered
We searched Web of Knowledge, PubMed, AGRICOLA,
and Google Scholar for food LCAs published before July
2015. We excluded several publications because a lack of
dened system boundaries made direct comparisons
with other LCAs impossible. In addition, some LCAs
conducted by for-prot companies were excluded
because of potential biases. In total, we used 164
publications that analyzed 742 unique food production
systems a (supplementary table 1 available at stacks.iop.
org/ERL/12/064016/mmedia). We used ve different
environmental indicators in our analyses. These
indicators are greenhouse gas emissions, land use,
energy use, acidication potential (a measure of nutrient
loading), and eutrophication potential (a measure of
nutrient runoff) to give a broad overview of the
environmentalimpacts of food production. The data for
other environmental indicators, such as biodiversity
impacts, were not present in adequate amounts to
include in our analyses.
Our analyses include all relevant pre-farm and on-
farm activities (fertilizer production and application,
seed production, farm energy use, feed and fodder
production, manure production (when used for
fertilizer), manure management, infrastructure con-
struction, etc) and their associated environmental
impacts up until a food leaves the farm. Our analyses
are thus of cradle-to-farm gateactivities; a paucity of
data on post-farm gate impacts limited our ability to
analyze them in a balanced manner, although a
previous analysis showed that the vast majority of a
foods greenhouse gas emissions stem from cradle-to-
farm gateactivities (Weber and Matthews 2008).
In-depth examples of the activities included in cradle-
to-farm gatesystem boundary can be found in
Pelletier (2008), Hokazono and Hayashi (2012), and
Torrellas et al (2012).
The majority of LCA publications included in
these analyses are from agricultural systems in Europe,
North America, and Australia and New Zealand (86%
of systems are from these regions). Systems from
China (2%), Japan (2%), the rest of Asia (5%), South
America (4%), and Africa (.4%) are much less
common. The results presented here are therefore
indicative of highly industrialized systems and should
be interpreted with this in mind. However, because the
majority of systems analyzed here are highly industri-
alized systems, comparisons across publications will be
more indicative of environmental differences between
foods than if production systems were highly variable.
Environ. Res. Lett. 12 (2017) 064016
2
We found sufcient data to compare the environ-
mental impacts of four sets of alternative production
systems: organic versus conventional systems; grass-
fed versus grain-fed beef; trawling versus non-trawling
sheries; and greenhouse-grown versus open-eld
produce. We were also able to examine how
agricultural input efciency, or the amount of food
produced per unit of agricultural input, affects a foods
environmental impact, as well as how foods differ in
their environmental impacts across the ve environ-
mental indicators examined.
Description of environmental indicators
Five environmental indicators were used in this
analysis: greenhouse gas emissions, land use, energy
use, acidication potential, and eutrophication po-
tential. The analyses were limited to these indicators
because a very limited number of publications
reported data for other indicators such as human
health, ecotoxicity, or biodiversity. An explanation of
the indicators included in the analyses is below.
Greenhouse gas emissions (GHGs) are reported in
carbon dioxide equivalents, and include the green-
house gas emissions from carbon dioxide, methane,
and nitrous oxide. GHGs from activities in the results
presented include, but are not limited to, fertilizer
production and application, manure management,
enteric fermentation.
Energy use is reported in kilojoules and includes the
energy used during pre-farm and on-farm activities
including, but not limited to, fertilizer production,
infrastructure construction and machinery use.
Land use is a measurement of how much land is
occupied during food production. It accounts for land
used to grow crops and/or livestock feed, to house
animals, and to pasture ruminants.
Acidication potential is reported in SO
2
equiv-
alents and includes acidication potential from sulfur
dioxide, nitrogen oxides, nitrous oxide, and ammonia,
among others. Acidication potential is a measure-
ment of the potential increase in acidity of an
ecosystem. Excess acidication makes it more difcult
for plants to assimilate nutrients, and thus results in
decreased plant growth. Activities such as fertilizer
application, fuel combustion, and manure manage-
ment are included in the results presented here.
Eutrophication potential (a measure of nutrica-
tion) is reported in PO
4
equivalents and includes
eutrophication potential from phosphate, nitrogen
oxides, ammonia, and ammonium, among others.
Eutrophication is a measurement of the increase in
nutrients entering an ecosystem. Eutrophication has
substantial environmental impacts including, but not
limited to, algal blooms and aquatic dead zones.
Alternative production systems
To control for environmental and agronomic differ-
ences between publications, as well as differences in
nutrient contents between foods, we compared
alternative production systems by food item within
publication. We rst calculated the ratio of impacts of
different production systems by food item within each
publication, and then calculated the response ratio by
taking the natural log of the ratio of impacts (Hedges
et al 1999). We then aggregated foods into groups of
similar food types (cereals; fruits; vegetables; pulses,
nuts and oil crops; dairy and eggs; and meats) to
improve the power of statistical tests. We tested for
signicant differences between alternative production
systems using t-tests on the response ratio.
Agricultural input efciency
In determining how agricultural input efciency, or
the amount of food produced per unit of agricultural
input, affects a foods environmental impact, we
performed regressions between a foods environmen-
tal impact and its nutrient use efciency in crop
systems or its feed use efciency in livestock systems.
We limited analyses to non-rice cereal crops and non-
ruminant livestock because ooding in rice paddies
and digestive processes in ruminants do not make
them directly comparable with other crop and
livestock systems. There is not adequate data to
perform similar analyses limited to ruminant systems:
comparisons would be severely limited for beef (n¼7
for GHGs and n<5 for all other indicators), and only
three studies provide feed use efciency in dairy
systems. For the analysis on nutrient use efciency, we
excluded crop systems that applied manure because
the variable nitrogen content of manure made it
impossible to calculate nitrogen inputs in these
systems. In total, we examined the agricultural input
efciency of 49 non-rice cereal production systems
and 53 non-ruminant livestock production systems.
Different foods
LCAs commonly report a foods environmental impact
on a per mass basis (e.g. impacts per kg of food).
However, because the nutritional values of foods come
from their calories, protein, and/or micronutrients, and
not from mass per se, we also calculated a foods
environmental impacts per kilocalorie, gram protein,
and USDA serving (2016). To compare differences
between broad types of foods, we aggregated foods into
13 food groups composed of similar foods (supplemen-
tary table 2).
Results and discussions
Environmental impacts of alternative food
production systems
Organic versus conventional agriculture
Organic agriculture is a fast-growing sector in many
western nations, perhaps because it is perceived as
being more sustainable or healthier than conventional
agricultural systems (Rigby and Cáceres 2001). Our
analyses based on 46-paired organicconventional
Environ. Res. Lett. 12 (2017) 064016
3
systems examine the comparative environmental
impacts of these agricultural systems across ve
environmental indicators and a broad range of foods.
We found that organic systems require 25%110%
more land use (p<0.001; n¼37), use 15% less
energy (p¼.0452; n¼33), and have 37% higher
eutrophication potential (p¼.0383; n¼20) than
conventional systems per unit of food. In addition,
organic and conventional systems did not signicantly
differ in their greenhouse gas emissions (p¼.5923;
n¼44) or acidication potential (p¼.299; n¼26),
although these were 4% lower and 13% higher in
organic systems, respectively (gure 1).
The differences in environmental impacts between
organic and conventional systems are primarily driven
by differences in nutrient management techniques.
Organic agriculture is largely dependent on manure as
a nitrogen input in contrast to conventional agricul-
tures use of synthetic fertilizers. Application of
manure, which releases nutrients in response to
environmental conditions and not crop nutrient
demand (Seufert et al 2012), often results in temporal
mismatches between nutrient availability and nutrient
demand and thereby increases the proportion of
nutrients that are not assimilated by plants (Cassman
et al 2002). These temporal mismatches in organic
systems result in reduced crop growth and yields and
thus in increased land use. In addition, nutrient
applications not incorporated into plant growth cause
eutrophication and acidication, thereby driving the
higher eutrophication potential and tendency for
higher acidication potential in organic systems. In
contrast, energy use is lower in organic systems
because of organics reduced reliance on energy-
intensive synthetic fertilizer and pesticide inputs.
GHG emissions are similar in organic and conven-
tional systems because of the trade-off between
application of synthetic fertilizer in conventional
systems and use of manure in organic systems. Indeed,
while production of conventional fertilizer is energy-
and GHG-intensive, mismatches between nutrient
availability and demand in organic systems dependent
on manure increase the portion of reactive nitrogen in
organic systems that turns into nitrous oxide, a potent
greenhouse gas (Myhre et al 2013), causing organic
and conventional systems to have similar GHG
emissions. Because we limited comparisons to within
publication, the results presented here are therefore
indicative of comparative environmental differences of
organic and conventional systems at a local scale. It is,
however, possible that the comparative environmental
impacts of organic and conventional systems might
differ at a regional, national, or global scale (e.g.
Bengtsson et al 2005 and Phalan et al 2011).
Previous analyses have shown that increasing
nutrient application and adopting techniques such as
rotational farming, cover cropping, multi-cropping,
and polyculture in organic systems can halve the land
use difference between organic and conventional
systems (Seufert et al 2012, Ponisio et al 2014).
Additionally, while the overall pattern is for higher
land use in organic systems, organic systems have
similar land use for legumes and perennial crops while
the land use difference between organic and conven-
tional systems is smaller in rain-fed systems and in
systems with weakly-acidic to weakly-alkaline soils
(Pimentel et al 2005, Seufert et al 2012).
Organic systems might offer health and environ-
mental benets we could not investigate with our data
set. Organic foods have higher micronutrient con-
centrations (Hunter et al 2011, Palupi et al 2012) and
lower pesticide residues (Baker et al 2002) than
conventional foods, although these differences may
not translate into improved human health outcomes
(Dangour and Lock 2010, Hunter et al 2011). On-farm
and near-farm biodiversity (Mäder et al 2002,
Bengtsson et al 2005, Hole et al 2005) tends to be
higher in organic agricultural systems, probably
because of its lower fertilizer, herbicide and pesticide
inputs. In addition, soil organic carbon is higher in
organic systems (Gattinger et al 2012) because manure
application promotes carbon storage in agricultural
soils. However, organic agriculture would likely have a
net negative impact on biodiversity and soil organic
Figure 1. Response ratio of the environmental impacts of organic and conventional food production systems. Comparisons were
made within publication to control for agronomic and environmental differences between publications. Plotted on a log base 2 scale,
where a ratio greater than one indicates organic systems have higher impacts; a ratio less than one indicates organic systems have lower
impacts. Bars are means and standard errors.
Environ. Res. Lett. 12 (2017) 064016
4
carbon at larger spatial scales because of the greater
land clearing required under organic agriculture and
because biodiversity (Balmford et al 2005, Phalan et al
2011) and carbon stocks (Gilroy et al 2014) decrease
dramatically with conversion from natural habitats.
Although organic systems have higher land use
and eutrophication potential and tend to have higher
acidication potential, this should not be taken as an
indication that conventional systems are more
sustainable than organic systems. Conventional
practices require more energy use and are reliant on
high nutrient, herbicide, and pesticide inputs that can
have negative impacts on human health (Townsend
2003, Schwarzenbach et al 2010, Mostafalou and
Abdollahi 2013) and the environment (Vitousek et al
2009, Foley et al 2011). Developing production
systems that integrate the benets of conventional,
organic, and other agricultural systems is necessary for
creating a more sustainable agricultural future.
Grass-fed versus grain-fed beef
We quantitatively analyzed the environmental differ-
ences between grass-fed and grain-fed beef using 7
paired grass- and grain-fed systems. We dene grass-fed
systems as those where beef is raised solely on pasture or
seasonally on pasture and supplemented diets of grass,
silage, and fodder while overwintering. We found that
grass-fed beef had higher land use requirements than
grain-fed beef (p¼.0381, n¼4). Grass-fed and grain-
fed beef had similar impacts per unit food for the other
environmental impacts examined (p>.05 for all other
indicators), although grass-fed beef had, on average,
19% higher GHGs (p¼.2218; n¼7) per unit food than
grain-fed beef (gure 2).
The higher land use and tendency for higher GHG
emissions in grass-fed beef stem from the lower
macronutrient densities and digestibility of feeds used
in grass-fed systems (Feedipedia 2016) because they
cause grass-fed beef to require higher feed inputs per
unit of beef produced than grain-fed systems.
Furthermore, the nutritional yields (e.g. kcal ha
1
)
of grass, silage, and fodder are often lower, possibly
because the land on which they are grown is often less
fertile than that used to produce feed (e.g. maize, soy,
etc) used in grain-fed systems. The combination of
higher feed inputs and lower nutritional crop yields for
feeds drive the higher land use observed in grass-fed
systems. Additionally, because grass-fed cattle grow
slower and are slaughtered 612 months older than
grain-fed cattle, lifetime methane emissions, and thus
GHGs per unit of food, tend to be higher for grass-fed
beef. The source of GHGs in grass-fed and grain-fed
systems further supports this explanation. Indeed,
30% and 52% of GHGs in grain-fed systems result
from feed production and enteric fermentation,
respectively. In contrast, feed production and enteric
are responsible for 20% and 61% of GHGs,
respectively, in grass-fed systems.
Grass-fed beef may have environmental and
human health benets we could not analyze with
our data. For example, grass-fed systems promote soil
carbon sequestration (Derner and Schuman 2007) and
within-pasture nutrient cycling while simultaneously
decreasing eutrophication (Smith et al 2013). Addi-
tionally, grass-fed beef has higher micronutrient
concentrations and a fatty acid prole that might
lead to improved human health outcomes relative to
consumption of grain-fed beef (Daley et al 2010).
Furthermore, grass-fed beef may promote food
security in cropland-scarce regions because it can be
grown on land not suitable for crop production (Smith
et al 2013).
Trawling versus non-trawling sheries versus
aquaculture
We classied commercial sheries into trawling
sherieswhere nets are physically dragged across
a seabedand non-trawling sheries (midwater
trawl, short and long-line shing, and seine nets).
Our analyses of 10 paired systems of trawling and
non-trawling sheries show that trawling sheries
emit 2.8 times more GHGs than non-trawling
sheries (p¼.004; n¼10) (gure 3)becauseof
the high fuel requirements of dragging a net across a
seabed. Response ratios differ greatly between sh,
with non-schooling sh (at sh) having compara-
tively higher impacts under trawl sheries than do
sh that form schools (mackerel, cod). Previous
analyses have also shown that trawl sheries
negatively impact non-targeted species through high
bycatch rates relative to other sh capture methods
and through ecosystem degradation from dragging a
net across a seabed (Dayton et al 1995). Shifting from
trawling to non-trawling sheries would thus
simultaneously decrease GHGs, bycatch rates, and
ecosystem degradation.
Aquaculture, which accounts for 45% of global
sh production, could be a sustainable alternative to
wild-caught sheries (FAO 2016b). Our examination
of 142 shery and aquaculture systems indicates that,
Figure 2. Response ratio of the environmental impact of
grass-fed and grain-fed beef. Comparisons were made within
publication to control for agronomic and environmental
differences between study locations. A ratio greater than one
indicates grass-fed beef has higher impacts; a ratio less than
one indicates grass-fed beef has lower impacts. Bars are means
and standard errors.
Environ. Res. Lett. 12 (2017) 064016
5
on average, non-recirculating aquaculture (e.g. aqua-
culture in ponds, fjords, rivers, etc) and non-trawling
sheries emitted similar GHGs per unit of food and
had emissions similar to pork, poultry, and dairy
(gures 4and S1). In contrast, trawling sheries and
recirculating aquaculture (in tanks and other systems
in which pumps and lters are used) emitted several
times more GHGs than non-trawling sheries and
non-recirculating aquaculture because of their high
energy requirements (gure 4). Aquaculture-raised
sh from non-recirculating systems could thus be a
lower-emission alternative to trawling sheries, an
equal-emission alternative to non-trawling sheries,
and could alleviate pressure on over-harvested
sheries (Costello et al 2012).
There can be marked differences in environmental
impacts even among the lower-impact non-recirculat-
ing aquaculture systems. For instance, aquaculture at
high sh densities can eutrophy closed bodies of water
and cause gene exchange between farmed and wild sh
varieties (FAO 2016b). In addition, shrimp aquacul-
ture systems that require deforestation of mangroves
have high environmental impacts while integrated
rice-catsh agriculture-aquaculture systems have
comparatively low impacts (Folke and Kautsky
1992, Páez-Osuna 2001).
Greenhouse grown versus open-eld produce
Greenhouse systems allow crops to be grown in
climates and regions not suitable for crop production.
Our analysis of ve paired greenhouseopen-eld
systems shows that greenhouse production systems
tend to emit almost three times more GHGs (gure 5;
p¼.089) because of the energy required to maintain
greenhouses at ideal growing conditions. While our
analyses show that, on average, greenhouse produc-
tion systems tend to have higher energy use than open-
eld systems, it is important to note that energy
requirements and thus greenhouse gas emissions can
differ greatly between greenhouses. For example,
greenhouses that are both heated and lighted will
require substantially more energy to maintain than
will greenhouses that are neither heated nor lighted.
Land use in greenhouse systems was, on average, one
quarter of that in open elds, but this difference was
not signicant (p¼.166; n¼3). Crop yields are
higher, and thus land use lower, in greenhouse systems
because they are maintained at ideal conditions for
plant growth. The limited sample size of these analyses
prevents concrete conclusions from being drawn.
Future analyses examining the environmental differ-
ences between greenhouse and open-eld production
systems are needed to fully elucidate their comparative
environmental impacts.
Environmental impacts of agricultural input
efciency
We found large differences among studies in the
environmental impacts of producing the same food
(supplemental gure 1). To examine why foods may
vary in their environmental impacts, we analyzed
agricultural input efciency, or the amount of food
Figure 3. Response ratio of the greenhouse gas emissions of
trawling and non-trawling sheries (e.g. line, purse and seine
net). A ratio greater than one indicates trawling sheries have
higher greenhouse gas emissions than non-trawling sheries.
Bars are means and standard errors.
Figure 4. Greenhouse gas emissions from non-trawling (e.g.
line, purse and seine net) and trawling sheries, and from
non-recirculating (e.g. pond, bag, ow-through) and
recirculating aquaculture systems per gram protein.
Signicant differences are denoted by letters and were
calculated using a Tukey post-hoc test.
Figure 5. Response ratio of environmental impacts of
greenhouse grown and open eld produce. Comparisons
were made within publication to control for agronomic and
environmental differences between study locations. Bars are
means and standard errors. A ratio greater than one indicates
greenhouse systems have higher impacts; a ratio less than one
indicates greenhouse systems have lower impacts.
Environ. Res. Lett. 12 (2017) 064016
6
produced per unit of fertilizer or feed input, in 49 non-
rice cereal production systems and 53 non-ruminant
livestock systems. We found that higher agricultural
input efciency is consistently associated with lower
environmental impacts for both non-rice cereal
systems (gure 6) and non-ruminant livestock systems
(gure 7). While the ts shown in gures 6and 7are
across all food items, ts for individual food by
environmental indicator are almost always downward
sloping and signicant. Increasing agricultural input
efciency reduces a foods environmental impact
because of the environmental impacts of producing
agricultural inputs such as fertilizer, pesticides, and
livestock feeds. However, the environmental benets
of increasing agricultural input efciency would not be
equal across all systems, with improvements in
Figure 6. Correlations between nitrogen use efciency, or calories produced per g of nitrogen input, and the environmental impacts of
non-rice cereal crops. Regression lines are reciprocal ts between nitrogen use efciency and a foods environmental impact. All
relationships are signicant at p<.05 except for acidication potential.
Figure 7. Correlations between feed use efciency, or kcal of food produced per kcal of feed input, and environmental impacts in non-
ruminant livestock systems. Regression lines are reciprocal ts between feed use efciency and a foods environmental impacts. All
relationships are signicant at p<.05.
Environ. Res. Lett. 12 (2017) 064016
7
agricultural input efciency having the largest
environmental benet in the least efcient systems.
Further, improving efciency in more efcient systems
may only be possible at an economic cost. Emphasis
should therefore be placed on improving efciency in
less efcient systems, although efciency improve-
ments in more efcient systems would still have
environmental benets.
Several technologies and management techniques
can increase agricultural input efciency. Precision
farming, where nutrient and pesticide inputs are
temporally and spatially applied to match crop
requirements, has increased fertilizer input efciency
and farmer prots without decreasing crop yields for a
variety of crops in geographically diverse areas
(Robertson and Vitousek 2009). Conservation tillage
and cover cropping, particularly with nitrogen xing
crops because they simultaneously reduce required
nitrogen inputs, also increase fertilizer input efciency
by reducing nutrient loss from agricultural systems
(Robertson and Vitousek 2009, Ponisio et al 2014).
Feed input efciency in livestock systems can also be
increased. For example, pork from pigs fed diets
supplemented with amino acids required less feed and
emitted 5% fewer GHGs and had 28% lower
eutrophication potential than pork from pigs fed
unsupplemented diets (Ogino et al 2013). Similar
benets have also been found in poultry, beef, and
dairy systems (Robertson and Vitousek 2009). In
addition, using agricultural wastes and byproducts as
animal feeds could reduce the environmental impacts
of livestock production by 20% without reducing food
quality or farmer prots (zu Ermgassen et al 2016).
The location of food production can also inuence
its environmental impact because differences in
climatic and soil conditions often affect agricultural
input efciency. Indeed, spatially locating food
production in areas with the most suitable climatic
and soil conditions for a crop can increase agricultural
input efciency and decrease environmental impacts
(Polasky et al 2008, Johnson et al 2014, Chaplin-
Kramer et al 2015). For example, preferentially
locating agricultural land to maximize single ecosys-
tem services would increase carbon stores by
6 billion metric tonnes (worth $1 trillion 2012
USD) (Johnson et al 2014) and substantially decrease
projected rates of agriculturally-driven biodiversity
loss (Chaplin-Kramer et al 2015). Globally leveraging
environmental and soil conditions to increase
agricultural input efciency could thus provide
substantial environmental benets.
Environmental Impacts of different foods
Many analyses have shown that dietary choice can
greatly inuence the environmental impacts of the
agricultural food system (de Vries and de Boer 2010,
Nijdam et al 2012, Tilman and Clark 2014, Clune et al
2017), although these analyses were limited to animal-
based foods or a single environmental indicator. Our
analyses expand on these earlier studies and show that
foods with low impact for one environmental
indicator tend to have low impacts for all environ-
mental indicators examined (gure 8). Indeed, for all
indicators examined, ruminant meat (beef, goat and
lamb/mutton) had impacts 20100 times those of
plants while milk, eggs, pork, poultry, and seafood had
impacts 225 times higher than plants per kilocalorie
of food produced. This clear trend of ruminant meat
Figure 8. Environmental impacts of broad groups of foods per kilocalorie. The environmental indicators examined are greenhouse
gas emissions, land use, energy use, acidication potential (Acid. Pot.) and eutrophication potential (Eut. Pot.). Bars show means and
standard errors. Plant-based foods are in green; dairy and eggs are in grey; meats are in red; and seafood is in blue. Data from foods
grown in greenhouses are not included when plotting this gure. Trawl Fishery ¼bottom-trawling sheries; NT Fishery ¼all other
sheries (e.g. line, purse net, seine net, etc); Recirc Aqua ¼recirculating aquaculture; NR Aqua ¼non-recirculating aquaculture (e.g.
pond, net pen, ow-through, etc).
Environ. Res. Lett. 12 (2017) 064016
8
having high impacts and other animal-based foods
having intermediate impacts also holds when foods are
examined per gram protein, USDA serving, or unit
mass (supplemental gure 1). Isocaloric shifts from
high-impact to lower-impact but nutritionally similar
foods, such as shifts from ruminant meats to sh,
pork, poultry, or legumes, would have large diet-
related environmental benets while also improving
human health outcomes (e.g. Tilman and Clark 2014).
These dietary shifts, however, would likely decrease the
total cost of the diet; it is possible that increased
consumption of other material goods could offset the
environmental benets of consuming lower-impact
foods.
Most of the 742 LCA food analyses used were
based on high-input systems in Europe and North
America; the results presented here are thus indicative
of the impacts of high-input systems in developed
nations. In contrast, the impacts of low-input systems
common in developing nations are not yet well
studied, although a recent analysis indicates that
GHGs may be higher in these systems because of lower
agricultural input efciency (Herrero et al 2013). LCA
analyses on less-studied but nutritionally and cultur-
ally important foods such as quinoa, cassava, and
millet, as well as analyses from additional regions and
management regimes would provide further insight
and a clearer understanding of the environmental
impacts of different foods and food systems globally.
Conclusions
Our analyses show that the comparative environmen-
tal impacts of agricultural production systems differ
depending on the systems, food, and environmental
indicator examined. Per unit of food produced,
organic systems had higher land use and eutrophica-
tion potential, tended to have higher acidication
potential, did not offer benets in GHGs, but had
lower energy use; trawling sheries emitted almost
3 times more GHGs than non-trawling sheries; grass-
fed beef required more land and tended to emit more
GHGs than grain-fed beef; and high agricultural
efciency was consistently correlated with lower
environmental impacts. Combining the benets of
different production systems, for example organics
reduced reliance on chemical inputs with the high
yields of conventional systems would result in a more
sustainable agricultural system.
Agricultural input efciency, or the amount of
food produced per unit of input, is inversely correlated
with a foods environmental impact in non-rice cereal
systems and non-ruminant livestock systems. Increas-
ing agricultural input use efciency would have
environmental benets without necessitating dietary
change. However, because the marginal environmental
benets of increasing agricultural input efciency is
larger in less efcient systems, special emphasis should
be placed on improving efciency in the least efcient
agricultural systems.
The difference in environmental impacts between
foods is large compared to the difference between
production systems and systems with different
agricultural input efciencies producing the same
food. Ruminant meats, for example, have impacts that
are 310 times those of other animal-based foods and
20100 times those of plant-based foods for all
indicators examined. Because the majority of produc-
tion systems included in these analyses are from
Europe and North America, the results presented here
are indicative of trends in highly industrialized and
high-input agricultural systems. Analyses of the
environmental impacts of low-input agricultural
systems are necessary to elucidate the extent to which
the trends observed here also apply to lower-input
agricultural systems.
The analyses presented here greatly expand current
knowledge of the environmental impacts of food
production. However, there are still large knowledge
gaps which, if addressed, would further our under-
standing of agricultures environmental impacts. For
example, analyses on the environmental impacts of
agricultural systems in low-income countries, on
staple crops not common in Westernized diets
(quinoa, yams, sorghum, millet, etc), on sh produced
via aquaculture, and on agricultural input efciency in
non-cereal crops and in ruminant systems are limited.
In addition, agricultural production has a multitude of
environmental impacts beyond the ve environmental
indicators analyzed here; few LCAs analyses have
examined agricultures other environmental impacts
such as water use, pesticide use, or impact on
biodiversity. Analyses into these, and other, under-
studied aspects of agricultures environmental impacts
are needed to more fully elucidate agricultures entire
environmental impact.
Despite current knowledge gaps, it is clear that
current agricultural trajectories would substantially
increase agricultures environmental impacts by
midcentury (Tilman et al 2001,Tilman et al 2011,
Bajzelj et al 2014, Tilman and Clark 2014). Many
interventions would, however, greatly reduce agricul-
tures future environmental impacts. Adoption of low-
meat and no-meat diets in nations with excess meat
consumption (Springmann et al 2016), sustainable
increases in crop yields (Foley et al 2011, Mueller et al
2012), and adoption of low-impact and otherwise
more efcient agricultural systems (Robertson and
Vitousek 2009), would offer large environmental
benets. In addition, over 30% of food production is
wasted; reducing food waste would offer environmen-
tal benets without requiring shifts in production
practices or diets (Foley et al 2011). Implementing
policy and education initiatives designed to increase
adoption of lower-impact foods, of lower-impact
production systems, and of systems with high
agricultural input efciency is necessary before
Environ. Res. Lett. 12 (2017) 064016
9
agriculture causes substantial, and potentially irre-
versible, environmental damage.
Acknowledgments
We would like to thank Kaitlin Kimmel, George Furey,
Adam Clark, David Williams, and James Gerber for
comments on the manuscript. We would also like to
thank the Balzan Foundation, the McKnight Presi-
dential Chair (DT), and the University of California,
Santa Barbara for support.
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