Journal of Cleaner Production 17 (2009) 222–230
Contents lists available at ScienceDirect
Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro
Decisions to reduce greenhouse gases from agriculture and product transport:
LCA case study of organic and conventional wheat
Kyle Meisterling a, *, Constantine Samaras a, b, Vanessa Schweizer a
a
b
Department of Engineering and Public Policy, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
Department of Civil and Environmental Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 December 2007
Received in revised form 22 April 2008
Accepted 24 April 2008
Available online 12 June 2008
A streamlined hybrid life cycle assessment is conducted to compare the global warming potential (GWP)
and primary energy use of conventional and organic wheat production and delivery in the US. Impact
differences from agricultural inputs, grain farming, and transport processes are estimated. The GWP of
a 1 kg loaf of organic wheat bread is about 30 g CO2-eq less than the conventional loaf. When organic
wheat is shipped 420 km farther to market, organic and conventional wheat systems have similar impacts.
These results can change dramatically depending on soil carbon accumulation and nitrous oxide emissions
from the two systems. Key parameters and their variability are discussed to provide producers, wholesale
and retail consumers, and policymakers metrics to align their decisions with low-carbon objectives.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Comparative LCA
Agriculture
Food miles
Greenhouse gases
Organic production
1. Introduction
The sale of organic foods in the United States (US) has grown at
an annual rate of 20% since 1990 [1]. In the US, organic products are
produced using permissible substances and processes, as defined
by the US Department of Agriculture [2]. While organic agriculture
improves soil quality and biodiversity [3], responsible farming
practices (whether organically certified or not) can reduce soil
erosion and nutrient runoff and leaching from crop production [4].
Other advantages commonly associated with organic agricultural
methods are the reduction or elimination of synthetic chemical
residue on foods for human consumption and improved nutrition.
Thus, consumers may purchase organic products to realize potential environmental and personal health benefits.
While consumers may purchase organic products because fewer
synthetic chemicals are used, they may not have access to credible
information about environmental impacts associated with production and transport of their food [5]. It is not uncommon for
organic goods in the US to travel farther from farm gate to point-ofsale than conventional goods, with the additional transport
resulting in supplementary greenhouse gas (GHG) emissions.
Despite the growing popularity of organic food products, conventional agriculture is still more prevalent in the US. This is especially
* Corresponding author. Tel.: þ1 412 268 2670; fax: þ1 412 268 3757.
E-mail address: [email protected] (K. Meisterling).
0959-6526/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jclepro.2008.04.009
evident in US wheat production, as less than 1% of planted wheat
acres in the US are organic [6]. Thus, a tradeoff may exist between
GHG emissions from production and emissions from transport.
As indicated by Andersson and Ohlsson [7], Blanke and Burdick
[8], and Sim et al. [9], transportation can be an important contributor to life cycle GHG emissions of fruits and vegetables and
cereal-based food products. Pretty et al.’s [10] study of the environmental costs of agricultural production practices and food miles
in the UK indicates that domestic road transport from farm to
point-of-sale comprises the largest share of externalities (29%)
attributed to the British food basket. It is for these reasons that food
miles, or the distance food must travel from farm to plate, have
captured the attention of decision-makers, particularly in the UK
[10,11].
When calculating environmental impacts of food products,
a holistic supply-chain perspective is desirable [12]. The SETAC
North America Streamlined Life Cycle Assessment (LCA) Workgroup provides guidelines for streamlining what can otherwise be
cumbersome data requirements for complete LCAs [13]. By focusing
on material flows and/or environmental impacts of particular
interest to the audience, meaningful comparisons of food product
life cycles can be conducted with a fraction of the effort of complete
LCAs.
This hybrid LCA [14] combines process-based methods and the
EIO-LCA (economic input–output life cycle assessment) tool [15]. A
pure EIO analysis would be subject to aggregation error, particularly in the grain farming sector (which includes wheat, maize, and
K. Meisterling et al. / Journal of Cleaner Production 17 (2009) 222–230
rice). An all process-based analysis would omit some supply-chain
impacts that are captured with appropriate use of EIO. Thus, a hybrid LCA can benefit from the strengths of both approaches [14].
With this study we perform a comparative LCA of whole-wheat
flour produced with conventional or organic management conditions in the US, and we include product transport to the point-ofsale. Wheat is a staple crop in the American diet and ranks as the
most consumed cereal grain in the US with an annual per capita
consumption of approximately 62 kg [16]. The main goal of this
study is to determine the difference in life cycle global warming
potential (GWP) impacts between conventional and organic goods,
and to compare the magnitude and variability of these differences
through metrics that would be most useful to decision-makers.
Previous work that compares organic and conventional practices typically does not include transport processes [3,17,18], even
though these impacts can be substantial. This study illustrates the
degree to which wheat product transport affects the difference in
GWP impact between organic and conventional wheat production.
Therefore, we determine the additional shipping distance for which
organically produced wheat flour has equivalent environmental
impact to conventionally produced wheat flour. We refer to this
metric as the equivalent impact transport distance.
Another important contribution of our work is the comparison of
key agricultural GWP impacts with transport impacts. This study
details the GWP differences between the production practices, and
the uncertainties that affect comparative impacts in the organic and
conventional systems. Nitrous oxide emissions are investigated, as is
global warming mitigation through soil carbon storage. We demonstrate the magnitude of these variable and uncertain impacts,
compared to transport impacts and the difference between the two
production practices.
2. Goal and scope
2.1. Goal of the study
The aim of this study is to determine the primary energy use and
GWP differences between conventional and organic product life
cycles, and we focus on processes that differ between the two
systems. The reason for this study is to contribute to the larger
policy discussion of GWP footprints related to food. Often this
policy discussion is collapsed to a simple measure of food miles (the
total distance a food item must travel from farm to plate), but we
investigate how the GWP impacts of different production practices
balance against the GWP impact of food transport.
Like the work of Narayanaswamy et al. [19], this study is
intended to provide decision-makers (producers, consumers, and
policymakers) a systems level framework for comparing energy
inputs and GHG emissions and to identify life cycle stages with
substantial energy and GWP impacts. The intended application of
this study is to identify key parameters and their variability to help
producers in the bread supply chain, wholesale and retail consumers, and policymakers align their practice and purchasing
decisions with low-carbon objectives.
223
The functional unit of 0.67 kg of whole-wheat flour is defined as
follows:
(a) Spatial context: organic or conventional whole-wheat flour
produced in the US for sale domestically
(b) Temporal context: It is important to note that farms (both conventional and organic) might not harvest the same crop every
year, or every season. Rotations with other crops can increase
soil quality and reduce nitrogen fertilizer requirements in crop
production. It is acknowledged that organic wheat production
may require different crop rotations than conventional production, and there may be more wheat harvests in conventional
production than in organic. However, in this analysis, it was
assumed that during wheat off-seasons, the alternative crops
grown have equivalent economic value in the two systems.
Therefore, the yields consider only the season during which
wheat is grown.
The system boundary includes production of differentiated
grain farming inputs and their transport, fuel use during grain
farming, machinery production, and transport of milled wheat to
the processing center and point-of-sale (Fig. 1). Impacts from fuel
production, transport vehicle manufacturing and end-of-life, as
well as transport infrastructure life cycle phases are also included.
For wheat inputs that are outputs of other processes (e.g. manure
from animal production), only the processing necessary to utilize
the input are allocated to wheat. Table 1 shows the wheat production inputs considered in the reference case analysis. The per
hectare use of these inputs shown in the table are specific to wheat,
but the life cycle energy and emissions per unit are not. Thus, these
parameters can be used as a framework for considering other types
of crop production. Sensitivity of results to changes in the reference
case parameters are explored in the Interpretation (Section 5).
Since we employ a streamlined method, our cut-off criteria are
processes that are similar for conventional and organic bread production. These processes include wheat milling, bread baking,
packaging, and local distribution and procurement. Andersson and
Ohlsson report that food processing can contribute up to one third
of total impacts from the conventional wheat bread life cycle [7].
However, since the aforementioned processes are the same for
organic and conventional wheat bread, omitting them does not
affect the primary energy and GWP differences between the two
systems.
2.3. Data sources and data quality requirements
We have relied on peer-reviewed sources, publicly available
models, and reports and databases compiled by US Government
surveys of industry. The data used in our analysis are representative
of wheat production and economic activity in the US for the years
between 1997 and 2005. As shown later in the paper, our results for
primary energy use in wheat agriculture are similar to other
studies. Details on methods and data not discussed in the main text
are presented in Appendix A.
2.2. Functional unit and system boundary
2.4. Impact categories
The target product is a 1 kg loaf of whole-wheat bread, and the
functional unit for analysis is 0.67 kg of wheat flour (the approximate amount of flour needed for a 1 kg loaf of bread). Bread is an
end-use product in the wheat life cycle and provides better context
for decision making than wheat alone. Based on the assumption
that whole-wheat flour is used for the bread (and 0.67 kg of
harvested wheat is equal to 0.67 kg of whole-wheat flour), all
inputs and outputs are normalized to the production of 0.67 kg of
harvested wheat.
Global warming potential (GWP) is adopted as the impact category for this study. GWP per functional unit is characterized in g
CO2-equivalents (CO2-eq) on a 100-year time scale, using factors
recommended by the IPCC [20]. We also analyze primary energy
use (J) per functional unit. While other impacts can be important
when comparing agricultural production systems [21], energy and
GHGs are of increasing interest to producers, consumers and policymakers aiming to reduce the energy and carbon intensities of
their products and decisions.
224
K. Meisterling et al. / Journal of Cleaner Production 17 (2009) 222–230
TRANSPORT
VEHICLES AND
INFRASTRUCTURE
FUEL PRODUCTION
BAKING,
PACKAGING,
AND SALES
PESTICIDE
PRODUCTION
TRANSPORT
OF WHEAT
PRODUCTION
INPUTS
FERTILIZER
PRODUCTION
WHEAT
FARMING AND
HARVESTING
FLOUR
TRANSPORT
MANURE STORAGE
AND HANDLING
0.67 KG
WHEAT
FLOUR
Wheat
flour
System Boundary
Wheat grain
SEED
PRODUCTION
FARM
MACHINERY
AND IRRIGATION
GRAIN
PROCESSING
AND TRANSPORT
Fig. 1. Reference case product system boundary for the streamlined comparative assessment.
This streamlined analysis highlights solely the energy and
GHG tradeoffs associated with conventional and organic wheat
production and flour delivery. A thorough LCA comparing the
impacts of conventional and organic agricultural production
would include acidification, eutrophication, photochemical smog,
terrestrial and aquatic toxicity, human health, resource depletion, land use, and perhaps biodiversity, soil quality (e.g. soil
organic matter), soil erosion, and community/rural development
[22].
3. Life cycle inventory
3.1. Grain farming inputs
Although nutrient sources are different for conventional and
organic grain farming, it was assumed that nutrient requirements
per kg of harvested wheat are the same for both systems. Nitrogen
(N) and phosphorus (P) fertilizers were considered in this analysis.
For the organic wheat system, N is supplied via leguminous cover
Table 1
Parameters used in the reference case comparison (org ¼ organic; conv ¼ conventional; kg CO2-eq ¼ kg CO2-eq, 100 year time horizon; a.i. ¼ active ingredient)
Unit
Use (per ha)
Primary energy
coefficient (MJ per unit)
GHG emissions
(kg CO2-eq per unit)
Transport
distance (km)
References
and notes
Wheat life
cycle inputs
N fertilizer: conv
P fertilizer: conv
kg N
kg P
66
8.7
45 direct; 5 fuel cycle
23 direct; 2 fuel cycle
2.3 direct; 0.5 fuel cycle
1.6 direct; 0.2 fuel cycle
1500
2200
Pesticide: conv
kg a.i.
0.4
240
18
1500
Phosphate rock:
org
Manure: org
kg P
3.3
19 direct; 1.5 fuel cycle
1.4 direct; 0.2 fuel cycle
2200
[33]; direct
energy from [60,61]; fuel
cycle and GHG from [30,56,62];
energy for P fertilizer
production assumed to be diesel
[33]; production
impacts from [30]; 50% a.i.
weight basis [63]
Adapted from [60,61] and [25]
kg of
manure P
3.3
0
5
30
Diesel (org & conv)
Gasoline (org & conv)
l
l
41
9.3
39 direct; 3.1 fuel cycle
35 direct; 3.2 fuel cycle
2.7 direct; 0.4 fuel cycle
2.4 direct; 0.4 fuel cycle
Not included
Not included
Nutrient content
adapted from [29];
GHG from [26,27]
[30,40,56]
[30,40,56]
Transport
Truck transport
t km
n.a.
[30,40,55–57]
t km
n.a.
0.15 direct; 0.02 fuel
cycle; 5.9E03 vehicle
prod; 5.4E03 infrastructure
0.02 direct; 2.6E03 fuel
cycle; 2.5E03 train
prod; 2.1E03 infrastructure
n.a.
Rail transport
2.1 direct; 0.17 fuel
cycle; 0.10 vehicle
prod; 0.11 infrastructure
0.28 direct; 0.02 fuel
cycle; 0.03 train
prod; 0.04 infrastructure
n.a.
[30,40,55–57]
Since some of these parameters are quite variable, we investigate the impact of changes in the Interpretation section. Impacts of wheat production inputs at production gate.
Impacts of input transport can be added using transport factors. Reference case transport distances shown.
K. Meisterling et al. / Journal of Cleaner Production 17 (2009) 222–230
crops, and may be augmented with manure. The analysis does not
include use of potassium fertilizer, which comprises less than 2% of
the total life cycle energy used in conventional wheat production
[23], or lime, which may be used in either system, depending on
residue management practices (see Appendix A).
Half of P requirements for organic wheat are satisfied by raw
rock phosphate and half by manure. It was assumed that the
application of organic fertilizer ensures an adequate pool of P in the
soil available to plants. Energy used to mine and crush raw rock
phosphate accounts for 84% of the net energy required to produce P
fertilizer (adapted from [24]).
GWP impacts from manure can vary widely depending on
manure management and storage practices [25,26], and on allocation procedure [27]. In our analysis, GHG emissions from manure
storage and composting are assigned to the organic wheat production system, and are adapted from Hansen et al. [25] and Sommer
and Moller [26]. Manure nutrient content was from ASABE [28]. Our
reference case emission factor is 5 kg CO2-eq emitted during manure
handling per kg of P in the manure. This is a moderate value, as
emissions from poorly managed manure can be much higher.
In the reference case analysis, machinery manufacturing
impacts are assigned to be the same for the two systems. However,
since organic wheat cultivation may require increased tillage, the
effect of different machinery use rates is investigated with a sensitivity analysis.
For conventional agriculture, the production and transport of
conventional pesticides are considered. Pesticide production
impacts are from EIO-LCA [29], following the procedure outlined in
Appendix A. Of the four organic pesticides that are monitored by
the US government (copper, oil, sulfur and Bt, a toxin from the soil
bacteria Bacillus thuringiensis), none is used in wheat farming [30].
Organic farms primarily plow or till as a mechanical method of
controlling weeds, instead of using herbicides.
3.2. Grain farming
Wheat yields for organic agriculture are typically lower than
conventional wheat yields (per harvested ha). Mader et al. [3]
report organic wheat yields that are 80% of conventional yields, but
note that the average in Europe is 67%. For this study, we assume
that organic yields are 75% of conventional yields. Since studies of
organic wheat yield in the US are limited, organic yield is a key
parameter in this assessment. In some cases, organic yields can be
similar to or larger than conventional yields, especially in drought
conditions [31]. Thus, the impact of wheat yield variations on
equivalent impact transport distances is explored in the sensitivity
analysis.
Conventional wheat yield used in this study is 2.8 t grain/ha (42
bushel/acre). This is the US average from nine years (1997–2005), as
reported by USDA databases [32]. This study considered generic
wheat and made no distinction between winter wheat (planted in
autumn, harvested the following summer) and spring wheat
(planted in the spring). Since only 5% of all wheat acres are irrigated
[33], this study does not include energy required to supply irrigation water.
Fuel is used in farm equipment to prepare fields, plant seeds,
manage pests and weeds, and harvest the crop. The amount of fuel,
as well as the types of farm equipment used during these processes
varies from farm to farm. Previous work has argued both that organic farming is less fuel-intensive [3,34] and more fuel-intensive
[35] per hectare. In the reference case, it was assumed that organic
and conventional wheat production consume the same amount of
liquid fuel [18] and use farm equipment at the same rate per
hectare. The sensitivity analysis investigates the impacts of differences in fuel and farm equipment use between conventional and
organic wheat farming.
225
Nitrous oxide (N2O) emissions make up a considerable portion
of the GWP from farm systems [36–38]. Agroecosystems and nonmanaged ecosystems both emit N2O. However, most agricultural
systems enhance inputs of reactive N, compared to unmanaged
landscapes, increasing N2O emissions. The reactive N inputs that
enhance N2O emissions can come from synthetic N fertilizer,
through biological nitrogen fixation (e.g. leguminous crops in
rotation or cover crops), or through organic amendments (e.g.
manure). N2O is emitted directly from fields, as well as from N that
leaves the field and enters other ecosystems via volatilization and
leaching.
For this analysis, organic and conventional wheat are supplied
the same amount of N per kg of wheat produced. For the reference
case, we assume that N2O emissions amount to 1.3% of total N supply
for both systems (this includes emissions directly from the field, and
indirect emissions from N leached or volatilized from the fields [39]).
The source and fate of nitrogen in various farming systems, and
subsequent N2O emissions, are uncertain [40]. Furthermore, IPCC
N2O emission factors are being debated [41]. For these reasons, the
impacts of higher N2O emission variations are shown explicitly in
the Interpretation section.
By transferring carbon (C) from the atmosphere to soil matter,
agricultural soils can be a CO2 sink [42]. Organic practices
[38,43,44] and reduced-tillage in conventional agriculture [40] can
both increase the carbon content of soils, compared to conventional
tillage. Location-specific management practices will affect soil C in
wheat production systems [45,46]. Uncertainties exist about the
efficacy of no till wheat practices [47], the longevity and stability of
the C sink [48,49], and the impact of sampling depth on measured
and actual C enrichment [50,51]. Finally, C and N cycling in the
environment are coupled, and enriching soil C may increase N2O
emissions from soils [52,53]. Due to these numerous uncertainties,
we assume in the reference case that soil C storage potential is the
same for both systems. The impact of a range of potential soil C
enrichment is, however, explored as a sensitivity analysis, since this
impact is potentially quite large.
3.3. Transport processes
This assessment considers the long-haul transport of milled
wheat flour and grain farming inputs. Energy and GWP impacts are
expressed as a function of each tonne-kilometer (t km) traveled.
Shipping impacts were analyzed with both truck and rail modes.
Conventional and organic wheat farms are assumed to be equidistant from the grain-processing center (shipping origin), and
both truck and rail modes depart from a multimodal terminal at the
shipping origin. It is assumed that the wheat arrives by truck or rail
at a multimodal terminal at the shipping destination. In the reference case, milled wheat is transported 2000 km to the shipping
destination, where it is baked, packaged, and distributed. This
shipping distance allows the wheat to reach either New York City or
Los Angeles from Wichita, Kansas, representing the two most
populous US metropolitan statistical areas [54]. Transport of the
wheat from the shipping destination to the final baking center/
point-of-sale was assumed to be similar between the two systems,
and was omitted.
Energy intensity of freight modes was adapted from Vanek and
Morlock [55], who use both an aggregated and disaggregated
analysis. GHG emissions of fuel combustion are obtained from the
2006 IPCC guidelines for GHG inventories [39] and the US Energy
Information Administration (EIA) [56]. Fuel use (direct energy
intensities) for Class I rail and intercity truck freight were 0.28 and
2.09 MJ/t km, respectively. Direct energy intensities in freight
transport will vary based on the utilization rate of vehicles, terrain,
vehicle and payload characteristics, and other factors. Several
studies report lower freight energy intensities for trucking [57,58],
226
K. Meisterling et al. / Journal of Cleaner Production 17 (2009) 222–230
while the US Department of Energy reports a higher value for
trucking [59]. Freight energy intensity is explored in the Interpretation section. GHGs and energy use associated with fuel
production are collected from the petroleum refineries sector in
EIO-LCA [29]. Upstream impacts associated with truck and rail
vehicle and infrastructure manufacturing, maintenance, and roadway end-of-life can represent up to 20% of transport impacts [57]
and are included in calculating an equivalent impact transport
distance.
3.4. Omitted processes
Impacts from seed production, electricity use on farm, wheat
processing, bread baking, and bread storage are assumed to be
similar for the two systems and are omitted from our analysis, even
though they can account for sizable fractions of total bread GWP
impacts [7,23]. Thus, we do not account for differences in milling
techniques, or procurement and consumption patterns between
the two systems. The extent and impact of these differences could
be the subject of further research. Impacts from wheat transportation could also be affected by differing distances from farm to
mill. Our results can be used to estimate the impacts from differences in wheat transportation distance between conventional and
organic systems, whether differences occur from farm to mill or
from mill to baking centers.
4. Life cycle impact assessment
In our streamlined LCA, the GWP impact of producing 0.67 kg of
conventional wheat flour (for a 1 kg bread loaf), not including
product transport, is 190 g CO2-eq, while the GWP of producing the
wheat organically is 160 g CO2-eq (reporting two significant figures). These are streamlined impacts, and so the difference of about
30 g CO2-eq per bread loaf is of interest. For a 50/50 modal share
between truck and rail, 0.67 kg of wheat flour transported 420 km
is associated with 30 g CO2-eq. Thus, our analysis indicates that
GWP impacts would be equivalent between organic and conventional systems if the organic wheat were freighted about 420 km
farther than the conventional wheat.
The table in Appendix A shows the energy and GWP impacts of
the system processes, assuming that wheat in both systems travels
2000 km. The GWP impact of transporting the 0.67 kg of wheat
2000 km is 140 g CO2-eq, more than four times the difference
between the two systems. Thus, transport distance from farm to
point-of-sale could affect the relative impact of organic and conventional bread. Fig. 2 shows the streamlined GWP impacts of
wheat production, along with the impacts of transport.
Our streamlined results for energy used in wheat production at
the farm gate amount to 2.2 MJ/kg of wheat, and are similar to
previous studies. Piringer and Steinberg estimate energy used in
conventional wheat production in the US to be 3.9 MJ/kg wheat
[23]. Other researchers generally report that between 1.8 and
3.5 MJ of primary energy are consumed to produce 1 kg of wheat
[3,17,18,64–66].
5. Interpretation
Since there is considerable variability in the impacts of processes considered in our analysis, it is important to investigate the
degree to which changes can affect equivalent impact transport
distances. In Table 2, we show ranges for various input parameters,
and the effect these ranges have on the equivalent impact transport
distances. It is clear that N2O emissions and wheat transport mode
can materially affect comparative impacts.
The GWP associated with N2O emissions from N management in
agriculture is substantial but uncertain. The IPCC default
Fertilizer & pesticide
Fuel use on farm
Wheat transport
Machinery
N2O from fields
Conventional
(reference case)
Organic (reference
case)
Wheat transport:
420 km
Wheat transport:
2,000 km
0
50
100
150
200
GWP (g CO2-eq / kg bread)
Fig. 2. Results. Top two bars show streamlined global warming potential (GWP) of
wheat production for the reference cases, without wheat product shipping. Bottom
two bars show GWP associated with two transport distances. Wheat transported
420 km corresponds to the equivalent impact transport distance; 2000 km corresponds to the distance necessary to transport wheat from the farm in Kansas to
population centers in New York or Los Angeles. Transport considered is a 50/50 truck/
train modal share.
volatilization and leaching factors indicate that 10% and 30% of applied mineral N is lost by volatilization and leaching, respectively,
while 20% and 30% of organic N is lost [39]. These factors indicate
that organic wheat farming would emit slightly more N2O per kg of
wheat: 1.3% of anthropogenically added N is converted to N2O in
conventional, compared to 1.4% in organic. Using these factors, the
equivalent impact transport distance would be reduced to 320 km
(from 420 km in the reference case). If, however, the percentage of
supplied N that is emitted as N2O reaches 2% for the organic case, but
is maintained at 1.3% for the conventional system (as in the high
organic impact case shown in Table 2) the conventional wheat system would have a lower GWP impact than organic wheat.
While most LCAs of agricultural systems assume N2O emission
factors between 1% and 2%, N2O emissions from agricultural system
could be as high as 5% of reactive N added [41]. Fig. 3 shows a range
of GWP impacts, reflecting emissions factors between 1% and 5% of
reactive N emitted as N2O. Tailoring organic N inputs to improve
crop uptake is a challenge for organic agriculture [67], but could
reduce N losses and subsequent N2O emissions. At the same time,
improved fertilizer use efficiency could reduce N2O emissions from
conventional agriculture [41].
For the reference case, soil carbon enrichment was assumed to
be equal between the two systems and omitted (see Section 3.2).
The magnitude of the potential carbon sink in agricultural soils is
large, however, and could offer an important CO2 sink in the near
term [42]. A range of GWP impacts from the soil C sink is shown in
Fig. 3. The ranges reflect the following carbon sequestration rates:
0 to 1700 kg CO2-eq/ha/year for organic (adapted from [38,43,44]),
and 250 to 1100 kg CO2-eq/ha/year for conventional (adapted
from [38,40,45,68]). The negative sequestration value indicates net
emissions of soil carbon, allowing the possibility that soil C is lost
due to tillage in conventional wheat production.
The figure shows clearly that soil C and N2O emissions are
important parameters in agricultural LCAs, and that the comparative GWP impact of conventional and organic production may be
heavily influenced by these two processes. It is important to
remember that carbon and nitrogen cycling in soils are coupled.
Thus, the ranges of N2O and soil C impacts shown in Fig. 3 indicate
the magnitude of the uncertainty these processes could introduce
to an LCA of agricultural products.
K. Meisterling et al. / Journal of Cleaner Production 17 (2009) 222–230
227
Table 2
Sensitivity of the GWP equivalent impact transport distance to variations of uncertain parameters
Process
Parameter
description (units)
Parameter values
Reference
case
Organic wheat yield
Nitrogen fertilizer
manufacturing impacts
GHG impact from manure storage
Manure use in conventional
Manure transport distance
Fuel use on farm
Rate of farm machinery use
N2O emissions from organic N supply
Organic wheat transport mode
% of conventional yield
MJ fuel/kg N
kg CO2-eq per kg P in manure
% of P from manure
km
% of baseline (liters diesel
and gasoline per ha)
% of baseline
% of organic N supplied emitted as N2O
% by rail (remainder by truck)
Equivalent impact wheat transport
distance for GWP (km)
Low organic
impact
High organic
impact
Reference
case
75%
45
120%
60
60%
30
420
420
680
600
240
210
5
0%
30
100%
0
100%
5
80%
20
na
100
150%
420
420
420
420
490
550
440
550
190
na
360
70
150%
2.0%
30%
420
420
420
440
760
1300
100%
1.3%
50%
80%
1.0%
80%
Low organic
impact
High organic
impact
360
290
170
The ‘‘Nitrogen fertilizer manufacturing impacts’’ and ‘‘Manure use in conventional’’ processes apply to conventional wheat production, all other processes apply to organic. The
Low and High organic impact columns indicate values that increase and decrease the difference between organic and conventional, relative to the reference case. Negative
values in the rightmost column indicate situations where the ‘‘high organic impact’’ is larger than the conventional reference impact, so conventional wheat must travel farther
than organic to equalize GWP impact.
6. Conclusions
Organic and conventional wheat grown in the US are compared
in this analysis using a streamlined hybrid life cycle assessment.
The study estimates differences in energy use and GWP impacts
from agricultural inputs, grain farming, and transport processes.
Key parameters and their variability are discussed to provide
metrics to producers, wholesale and retail consumers, and policymakers to evaluate and align their agricultural processes, purchasing decisions, and policies with low-GWP objectives.
When conventional and organic wheat are transported the same
distance to market, the organic wheat system produces about 30 g
less CO2-eq per 0.67 kg wheat flour (1 kg loaf of bread) than the
conventional wheat system. The impact from synthetic nitrogen
used for conventional wheat is primarily responsible for this outcome. However, nitrous oxide emissions and soil C storage have the
potential to considerably alter this comparative result. Uncertainty
and variability related to these processes may make it difficult for
producers and consumers to definitively determine comparative
GHG emissions between organic and conventional production. In
addition, altering the transport mode of finished products and
500
GWP [g CO2-eq / kg bread]
400
300
200
soil carbon
100
0
N2O
emissions
-100
-200
wheat
transport
Reference
case GWP
difference
(conv-org)
conventional
-300
-400
-500
-600
organic
Fig. 3. Reference case GWP difference (conventional impacts minus organic impacts;
far right bar) compared with impacts from soil carbon (C), N2O, and wheat transport.
Grey bars show GWP allocated to parameters in the reference case for both the organic
and conventional systems (wheat transport assumed to be 2000 km in the reference
case; N2O emission are 1.3% of applied N). The uncertainty bars represent the range of
plausible impacts from N2O (1–5% of applied N lost as N2O) and soil C (as in text). Note
that the uncertainty ranges for GWP from soil carbon and N2O emissions are considerably larger than the GWP difference between the two systems calculated in the
reference case.
shipping distance of the wheat could cancel or enhance the organic
GWP advantage.
Parameters that have little bearing on reference case results are the
increased use of machinery by organic farmers and greater manure
transport distance. Organic wheat yield, the greenhouse gas intensity
of fertilizer manufacturing, and manure handling vary such that the
GWP advantage of organic wheat could be decreased but not eliminated. Fuel combustion in farming equipment was also identified as
an important contributor to GWP for both organic and conventional
wheat. Organic farming practices sometimes rely on mechanical weed
control rather than chemical control commonly practiced in conventional agriculture. Efforts to optimize and reduce fuel use during
farming activities would reduce the GWP of wheat at the farm gate.
We estimate that when organic wheat is shipped 420 km farther
to the point-of-sale, organic and conventional wheat systems have
similar GWP impacts. When the modal share of wheat transport shifts
to primarily by truck or rail, life cycle GWP impacts are considerably
increased or decreased, respectively. Hence, efforts to increase the
utilization of rail freight for US grain products would measurably reduce the life cycle GWP impacts of both conventional and organic
wheat products. The energy intensity of grain product shipment has
been steadily increasing due to a modal shift between rail and trucks.
The share of wheat tonnage shipped domestically by rail in the US has
declined from 70.4% in 1987 to 45.8% in 2000. Wheat rail freight has
shifted to truck transport, which increased its share from 27.1% to
52.7% over the same period (with the remaining small fraction shipped by barge) [69]. Decision-makers interested in reducing GHG
emissions should identify methods to maximize utilization of rail
freight transport for wheat and other appropriate commodities. A
challenge to increasing the share of wheat products shipped by rail is
that farmers have categorized rail freight as often unreliable and
uneconomical during harvest peaks [70]. More localized production
(where applicable) could ameliorate some of these logistical issues.
N2O emissions and the potential for soil C sequestration could
considerably increase or decrease the GWP impact of agricultural
products. The fate of nitrogen in agricultural ecosystems is subject to
uncertainty [36,39,41], and best management practices should be
developed to minimize N2O emissions on both organic and conventional farms. Likewise, soil C levels can be affected by land management changes. The relative GWP impacts of N2O emissions and the
efficacy of the soil C sink in organic and conventional agricultural
systems will require revisiting as new research in this area develops.
These findings suggest that though food miles are an important
parameter for reducing the GWP impacts of agriculture, the geographic
origin of one’s food alone is not necessarily most important. Farming
practices such as fuel use, fertilizer management, and tillage matter
228
K. Meisterling et al. / Journal of Cleaner Production 17 (2009) 222–230
greatly when discussing the difference between organic and conventional products. Similarly, choices among transport modes bear important consequences for GWP. This underscores the important role of
best farming and transport practices in a carbon-constrained world.
Acknowledgements
We gratefully acknowledge the work of Catherine Sheane in the
early stages of this paper. This work was supported by the Climate
Decision Making Center, which was created through a cooperative
agreement between the National Science Foundation (SES0345798) and Carnegie Mellon University. K.M. acknowledges
support from the Cooperative Research Network of the National
Rural Electric Association (CRN R0312); C.S. thanks the Teresa Heinz
Scholars for Environmental Research Program; V.S. was also supported by the Clare Booth Luce Graduate Fellowship Program. We
also thank three anonymous reviewers and H. Scott Matthews and
Christopher Weber for helpful comments and discussions.
Appendix. A
A.1. Procedure for EIO-LCA
EIO-LCA is a linear economic input–output model based on the 491
industry sectors of the US economy. An input–output analysis identifies the purchases from each economic sector required to produce
output from any other given sector. Sector activities are linked to primary energy use, GWP, and other impacts through EIO-LCA [29]. EIOLCA is based on sector purchases within the economy as reported to the
US Department of Commerce. The latest data available for the US
input–output model is from 1997. Thus to utilize the EIO-LCA model,
current retail prices had to be converted to 1997 producer prices using
1997 benchmark input–output accounts [71]. Producer prices were
converted to 1997 dollars using US producer price indexes [72].
A.2. Detail for grain farming inputs
Fertilizer, pesticide, and fuel use in conventional wheat production were compiled from US Department of Agriculture (USDA)
databases [32,33]. Herbicides (i.e. weed control) comprise the
majority of pesticide used in wheat farming (approximately 90% or
greater by mass) [33]. The majority of conventional herbicides used
in US wheat farming contain either Glyphosate or 2,4-D and contain 50% active ingredient (a.i.; weight basis) [32,63].
For conventional wheat, nitrogen (45% N by mass) and phosphorus (20% P by mass) fertilizers were considered. Fertilizer and
pesticide requirements that were used are averages for wheat production in the US. P sources for organic wheat are rock phosphate
and manure. Natural gas was assumed to be the fuel used to produce
N fertilizer; diesel was assumed to be the fuel used to produce P
fertilizer. Upstream impacts of natural gas production and supply
are from Jaramillo et al. [62]; upstream impacts of diesel production
and pesticide production are from EIO-LCA.
Limestone is added to agricultural soils to maintain healthy soil
pH. When rain or irrigation leaches cations out of the soil, it
becomes acidic. In addition, removal of harvested material tends to
lower pH by removing the alkaline material contained in the plant
matter. Since much of the alkalinity in wheat is present in the
wheat straw (as opposed to the grain), straw removal will tend to
acidify soil, and necessitate lime additions [73]. The USDA organic
standards require that organic producers manage soil fertility by
recycling crop residues. However, lime is not a prohibited substance
[2]. Conventional wheat farmers may recycle wheat straw, and
organic farmers may harvest some straw. Thus, unlike Robertson
[38], we assume that conventional and organic farms use the same
amount of lime, and omit lime use impacts from our analysis.
Impacts from farm equipment manufacture are from Piringer
and Steinberg [23].
Once farming inputs are manufactured, they must be distributed
to farms. Kansas is the largest producer of wheat in the US [32], thus
the conventional wheat for the base case was assumed to be produced in Kansas. LCI data for transportation of grain farming inputs
to farm were calculated based on the distances to the farm from the
primary production centers of the various inputs. About half of the N
fertilizer consumed in the US is imported. Trinidad and Tobago (54%
of imports) and Canada (18% of imports) are the largest suppliers.
Over half of domestic ammonia production occurs in Louisiana,
Texas or Oklahoma because of the availability of natural gas [74]. To
simplify the analysis, it is assumed that N fertilizer was produced in
Louisiana and travels 1500 km to the farm. If the N fertilizer were
imported, there would be additional fertilizer transport impacts. The
majority of phosphate rock produced in the US comes from Florida,
which was estimated to be 2200 km from the farm. Manure is
obtained within the general vicinity of the farm (30 km for the base
case), due to the density and associated shipping costs of manure.
Manure transport does not exceed 100 km, since transport beyond
this distance is not typically economically feasible [75]. Pesticides
are assumed to travel 1000 km from production to the farm (adapted
from [76]). The primary transportation modes used are Class I rail for
fertilizers and truck transport for pesticides and manure. Modal
shares for fertilizers and pesticides are adapted from [77].
A.3. Transport detail
Long-haul distances traveled were obtained by an Internet
highway directions interface [78]. While highway kilometers may
not yield an ideal representation of railroad route kilometers, they
provide a useful comparison between the energy and emissions
impacts between the two modes. Modal fractions for wheat
transport are adapted from the USDA [69], and are assumed to be
50% by truck and 50% by rail in the reference case analysis.
Primary energy use (J) and global warming potential (GWP, measured in g CO2-eq, 100 year time frame) of the streamlined wheat
production and delivery system of the 670 g of wheat required for
a 1 kg loaf are as follows (reporting two significant figures):
Process
Fertilizer production
Nitrogenous
Phosphatic
Pesticide production
Fertilizer & pesticide
transport
Fuel use
Fuel production
Wheat farming
Tillage
Fuel production
N2O emission from soil
GHG from manure
storage
Farm machinery
production
Conventional wheat
(reference case)
Organic wheat
(reference case)
Energy
use (J)
Energy
use (J)
Global warming
potential
(g CO2-eq)
Global warming
potential
(g CO2-eq)
820
770
50
22
29
46
42
3.8
1.6
2.1
21
0.0
21
0.0
31
1.7
0.0
1.7
0
2.2
22
7.0
490
450
37
n.a.
n.a.
1.5
0.5
36
32
4.4
96
n.a.
25
5.4
650
600
49
n.a.
n.a.
1.8
0.4
48
42
5.8
96
5.1
85
7.3
85
7.3
Subtotal
Flour transport (2000 km)
Fuel use
Fuel production
1400
1900
1600
310
190
140
110
25
790
1900
1600
310
160
140
110
25
Total
3300
330
2700
300
K. Meisterling et al. / Journal of Cleaner Production 17 (2009) 222–230
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