Clark M, Tilman D. Comparative analysis of environmental impacts of agricultural production systems, agricultural input efficiency, and food choice. Environmental Research Letters. 2017 2017/06/01;12(6):064016.
The authors performed a meta-analysis of life cycle assessments across 164 publications (pre July 2015) to compare the environmental impacts* of different: agricultural production systems, agricultural input efficiencies, and foods. This includes (but is not limited to), an analysis of grass-fed and grain-fed beef; trawling and non-trawling fisheries; and greenhouse grown and field produce. Eighty-six % of the publications were from highly industrialized systems in Europe, North America, and Australia and New Zealand.
*greenhouse gas emissions, land use, fossil fuel energy use, eutrophication and acidification potential
Bottom line for nutrition practice:
The results illustrate that environmental impacts of agricultural production systems are different depending on which systems, food, and environmental indicators are examined. The difference in environmental impacts between foods of different types is large compared to the difference between the same foods produced using different systems. For all environmental indicators and nutritional units assessed, plant-based foods have the lowest environmental impacts – even when analyzed per kilocalorie of food produced.
Organic systems use more land, result in more eutrophication and require less energy per unit of food as compared to conventional systems. Grain fed beef uses less land than grass fed beef, and trolling fisheries have much higher GHG emissions than low-input aquaculture and non-trawling fisheries. Additionally, increasing agricultural input efficiency (the amount of food produced per input of fertilizer or feed) is associated with lower environmental impacts for both crop and livestock systems.
The authors suggest, however, that these results should not be understood to mean that conventional systems are more sustainable than organic systems, as conventional systems require more energy and rely on high nutrient, herbicide, and pesticide inputs which can negatively impact human and environmental health. Rather, they suggest that systems should integrate the benefits of both systems to develop more sustainable agriculture (e.g., organic’s lower use of chemical inputs, and higher yields in conventional systems). Finally, results are relevant only to highly industrialized systems.
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.
Details of results:
Five environmental indicators were examined, including greenhouse gas emissions, land use, energy use, eutrophication potential (a measure of nutrient runoff), and acidification potential (a measure of nutrient loading – for further explanation see “of additional interest”). Other indicators that were not included in the data sets, such as impacts on biodiversity and pesticide use, were not measured. The life cycle assessments used to assess the food’s environmental impact were calculated by weight of food, and also by kilocalorie, gram protein, and USDA serving (serving size recommended by the US Department of Agriculture). Results are outlined above under “bottom line” and within the abstract. Instead, this section will focus on the author’s explanations of the findings.
The authors suggest that the higher land use eutrophication potential in organic systems could occur due to nutrient mismatch. This is explained in detail within the article (p.4), but an example they illustrate is manure application, which releases nutrients that are not matched with crop nutrient demand and thus increases the amount of nutrients not absorbed by plants. The authors also indicate, however, that comparisons between organic and conventional were limited to within the same publication, so results are representative at the local scale and not necessarily at the regional, national, or global scale. Regarding land use, they note that other researchers have found that 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” (p.4). The authors also state that organic production systems may be beneficial to human and environmental health in other ways not examined in this study such as: higher micronutrient concentrations; lower pesticide residues; farm biodiversity; and soil organic carbon. However, if organic agriculture requires clearing of land, then the impact on biodiversity and soil organic carbon would still be greater than for conventional systems.
The authors suggest that one of the reasons that grass fed beef has higher land use and tendency toward higher greenhouse gas emissions occurs due to lower macronutrient densities and digestibility of feeds than grain-fed systems. This in turn means that grass fed beef requires greater feed inputs. Further explanation on the environmental impacts of grass-fed beef is detailed within the article (p.5). For example, a longer life for grass fed cattle results in greater greenhouse gas emissions per unit of food. Conversely, they also suggest ways that grass fed beef might have human and environmental health benefits which they were unable to examine (i.e., soil carbon sequestration, promotion of food security on land not suitable for crop production, within-pasture nutrient cycling which could decrease eutrophication, and increased micronutrient concentration and improved fatty acid profile for human health). And while they suggest that aquaculture-raised fish from non-recirculating systems (e.g., aquaculture in ponds, fjords, rivers, etc.) could decrease pressure on over harvested fisheries and lower greenhouse gas emissions – particularly in relation to trawling fisheries – there are large discrepancies between the impacts of different aquaculture systems (for more details, see p. 6).
To assess agricultural input efficiency, the authors examined the amount of food produced per unit of fertilizer or feed input in non-rice cereal and non-ruminant livestock systems. As noted above, they found that systems with higher agricultural input efficiency are associated with lower environmental impacts. The authors outline technologies and management techniques (p.8) which could increase agricultural input efficiency.
In investigating the environmental impact of various foods, the authors illustrate that “foods with low impact for one environmental indicator tend to have low impacts for all environmental indicators examined”(p.8). Across all indicators, ruminant meat had 20-100 times greater impact than plant-based foods per kilocalorie of food produced, while milk, eggs, pork, poultry, and seafood had 2–25 times greater impact. This trend also held true when foods were assessed per gram protein, USDA serving, or by weight.
Of additional interest:
“Acidification potential… includes acidification potential from sulfur dioxide, nitrogen oxides, nitrous oxide, and ammonia, among others. Acidification potential is a measurement of the potential increase in acidity of an ecosystem, [and occurs from activities such as]… fertilizer application, fuel combustion, and manure management… Excess acidification makes it more difficult for plants to assimilate nutrients, and thus results in decreased plant growth. In addition, nutrient applications not incorporated into plant growth cause eutrophication and acidification, thereby driving the higher eutrophication potential and tendency for higher acidification potential in organic systems” (p.3), (although the latter was not a significant difference).
It is useful to see an article which outlines the nuances of different food production systems, rather than taking a black-and-white look at organic vs. Conventional agriculture This is important as dietitian-nutritionists are often asked about whether organically produced foods are more sustainable. Further, it is helpful to see findings showing that animal foods have a higher environmental impact when measured by kilocalories or grams of protein, as animal producers have used the argument that measuring by weight was not an accurate portrayal of the nutritional importance of differing foods.
Open access link to article:
Conflict of interest/ Funding:
Support for this research came from the Balzan Foundation, the McKnight Presidential Chair, and the University of California, Santa Barbara. The authors notes that some publications conducted by for-profit companies were excluded due to potential biases.
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