Decisions to reduce greenhouse gases from agriculture and product transport: LCA case study of organic and conventional wheat

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Abstract

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.

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-of-sale 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 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 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-of-sale. 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.

Section snippets

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

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 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

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

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

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.

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 (SES-0345798) 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

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