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Christian J. Peters, Jamie Picardy, Amelia F. Darrouzet-Nardi, Jennifer L. Wilkins, Timothy S. Griffin, Gary W. Fick; Carrying capacity of U.S. agricultural land: Ten diet scenarios. Elementa: Science of the Anthropocene 1 January 2016; 4 000116. doi:
This line of thinking is not new. The equation I=PAT, conceived in the 1970s, proposes that environmental impact is a function of population, affluence, and technology (Parris and Kates, 2003). Calls for considering the environmental impacts of food consumption through changes in diet were made decades ago both in popular (Lappé, 1971) and academic literature (Gussow and Clancy, 1986). However, for most of the 20th Century the predominant agricultural science paradigm focused on increasing yield and production efficiency, expanding in the 1980s and 1990s to include ecological impacts of farming but not focusing on food systems (Welch and Graham, 1999). Likewise, nutritional sciences and dietary advice over most of the past century have been guided almost exclusively by evidence on the relationships among nutrients, foods, diets and human health (King, 2007). If strategies for sustainability must address both food consumption and production, then analyses that link agriculture and nutrition are needed.
The second lesson is cautionary. While livestock production is the largest land user on Earth, simplistic thinking about dietary change must be avoided (Herrero and Thornton, 2013). Reviews of life cycle assessments of livestock systems and protein products show, definitively, that land use per unit of protein is generally lower with plant than animal sources (de Vries and de Boer, 2010; Nijdam et al., 2012). However, they also demonstrate a wide range among individual livestock products and among different systems producing the same livestock product. In addition to this variability in area of land required, the quality of land required differs as well. Modeling studies suggest that the largest fraction of land needs for ruminant animals are from forages and grazing lands (Wirsenius et al., 2010; Peters et al., 2014), which are often grown on non-arable land. Thus, reducing the most land-intensive products in the diet does not necessarily equate to freeing up land for cultivation. Finally, the land needs for producing animals do not always follow linear patterns, and can change rapidly when supplies of residual forage (Keyzer et al., 2005) or oilseed byproducts (Elferink et al., 2008) have been exhausted. When it comes to interpreting the land impacts of dietary change, caution is warranted.
A biophysical simulation model (the U.S. Foodprint Model based on Peters et al., 2007) that represents the conterminous U.S. as a closed food system was designed to calculate the per capita land requirements of human diets and the potential population fed by the agricultural land base of the continental United States. To do this, three sets of calculations were performed (Fig. 1). The first set of calculations estimated the annual, per capita food needs of the population based on daily food intake, the individual food commodities that comprise each food group, the weight of a serving of food, losses and waste that occur across the food system, and the conversion of raw agricultural commodities into processed food commodities. The second set of calculations estimated the individual land area required for each agricultural commodity in the diet based on yield data for each component crop and the feed requirements of all livestock. The third set of calculations estimated the potential carrying capacity of U.S. agricultural land, accounting for the aggregate land requirements of a complete diet, the area of land available, and the suitability of land for different agricultural uses. At key points in these calculations, marked with an asterisk in the diagram, additional calculations were performed to account for interdependencies in the food system. A description of the primary calculations and data sources is described below, and additional detail is provided in the Supplementary material.
Cropland was further partitioned to limit the percentage of the total area that can be used for cultivated crops in a given year. Scientists have long recognized that soils vary in their inherent suitability for intensive agriculture. The U.S. land capability classification system was first developed in the 1930s and refined over several decades to categorize land into grades based on their suitability for agriculture (Helms, 1997). The system distinguishes between arable land, land suitable only for grazing or forestry, and land entirely unsuited to commercial plant production. It further divides arable land into four grades (Classes I through IV). Sustainable land management on all but Class I soils (the highest grade of arable land) requires attention to crop choice or production practices, and these requirements become increasingly restrictive at each change in capability class (see USDA Natural Resources Conservation Service, 2013).
Diet scenarios were structured based on intake of food groups, as shown in Table 2. The first set of calculations performed in the U.S. Foodprint model translated each of the diet scenarios into estimates of the mass of primary food commodities needed to supply each diet, as well as the equivalent quantities of agricultural commodities from which the foods are derived.
2.4.2.1 Individual foods. Annual per capita land requirements (LR) for individual foods were calculated (in ha yr-1) based on the quantities of agricultural commodities needed to support food intake and estimates of the respective agricultural yields. For plant-based foods (Eq. 3a), the land requirement (LR) for each individual food commodity (j) equaled the quantity (QA) of agricultural commodity (k) required (in kg yr-1) divided by the average U.S. yield (Y) of that commodity (in kg ha-1) over the time period 2000-2010. In this equation, agricultural commodities are synonymous with crops.
Annual per capita land requirements for animal-based foods were calculated for each individual feed ingredient (Eq. 3b). The land area required (LR) for each feed ingredient (l) needed to produce an animal-based food (j) is equal to the quantity of feed crop needed divided by the associated crop yield. The quantity required of each individual feed crop equals the product of three factors: the quantity (QA) of agricultural commodity (k) required supply the food in the diet (in kg yr-1), the amount of feed ingredient (l) in the ration (R) fed to livestock (in kg feed kg livestock product-1) and a conversion factor (P) to account for any processing losses in deriving the feed ingredient from the source crop (e.g. soybean meal from soybean (Glycine max)). The quantity of feed crop required is divided by the yield of the crop (in kg ha-1) to calculate land requirements.
The findings of this study build upon one another sequentially. Estimates of the annual per capita land requirements of complete diets are foundational and are thus presented first (section 3.2). Assumptions regarding the area of agricultural land available in each pool and the utilization of available land are discussed next (section 3.3). Carrying capacity of the U.S. agricultural land base is compared across each diet scenario in the final section (3.4).
Total per capita requirements for agricultural land varied widely across the diet scenarios, with a factor of eight separating the least land intense and most land intense diets (Fig. 2). The baseline scenario had the highest total land use, 1.08 ha person-1 year-1, followed closely by the positive control, 1.03 ha person-1 year-1. Land requirements decreased steadily across the five healthy omnivorous diets, from 0.93 to 0.25 ha person-1 year-1, and the total land requirements for the three vegetarian diets were all similarly low, 0.13 to 0.14 ha person-1 year-1. However, differences in total per capita land requirements are only part of the story.
To calculate potential carrying capacity, all diet scenarios were restricted to the areas available within each pool of productive agricultural land. The aggregate area available for food production was estimated to be 95 million ha cultivated cropland, 134 million ha total cropland, and 299 million ha grazing land (Dataset S1). Aggregate land use in each scenario was estimated as the product of carrying capacity and the annual per capita land requirements.
Not all diets equally exploited each pool of land (Fig. 3). The five diets containing the largest quantities of meat (baseline, positive control, 100% health omnivorous, 80% healthy omnivorous, and 60% healthy omnivorous) used the entire available area, both cropland and grazing land. The five diets containing the least meat (or no meat) used the maximum allowable area of cultivated cropland and varied widely in their use of the remaining agricultural land. The 40% healthy omnivorous diet and the 20% healthy omnivorous diet used some of the available grazing land (214 and 75 million ha, respectively) and most of the cropland restricted to perennial forages (35 and 24 million ha, respectively). The ovolacto- and lacto-vegetarian diets used about half of the cropland restricted to perennial forages, while the vegan diet used none of the restricted cropland. None of the vegetarian diets used any grazing land (dairy rations were modeled with cows fed only harvested feeds and forages, see Peters et al., 2014). 2ff7e9595c
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