Healthy diets as a climate change mitigation strategy

Healthy diets as a climate change mitigation strategy

CHAPTER 13 Healthy diets as a climate change mitigation strategy Michael Clark Introduction GHG emissions resulting from human activities have unden...

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CHAPTER 13

Healthy diets as a climate change mitigation strategy Michael Clark

Introduction GHG emissions resulting from human activities have undeniably altered Earth’s climate systems. Global temperatures have increased by 0.85°C since 1880, precipitation patterns have changed, and extreme weather events have become more common (IPCC, 2014b). These have directly affected Earth’s ecosystems through desertification and flooding, altered ecosystem productivity, and species’ migration (Grimm et al., 2013). Climate change has also directly affected human well-being by changing crop yields (Ray et al., 2015), increasing air pollution ( Jacob and Winner, 2009), and contributing to death via extreme weather events (Coumou and Rahmstorf, 2012). Human activity will continue to change Earth’s climate system unless action is taken. National governments convened in Paris in 2014 to agree to global GHG emissions targets in an effort to limit future climate change to less than 2°C or 1.5°C relative to 1880 (United Nations Treaty Collection, 2015). These targets are aggressive and would require large reductions in GHG emissions. For example, a 50% meeting of the 2°C target would limit global GHG emissions to 25 Gt CO2-e/yr in 2050, whereas a 50% chance of meeting the 1.5°C target would require net zero emissions in 2050 (UNEP, 2015). These are large reductions compared to current global GHG emissions of 52 Gt of CO2-e/yr in 2014 (European Commission, 2016). Meeting these targets, especially for the 1.5°C pathway, would require immediate and aggressive intervention across numerous industries and sectors. Most national and international climate policies omit agricultural activities and dietary choice (Wynes and Nicholas, 2017) even though agriculture is one of the largest global contributors of GHG emissions and emit between 25% and 30% of global GHG emissions (13–15.6 Gt CO2-e/yr; IPCC, 2014b). The three main sources of GHG emissions in agriculture are production and application of nitrogen-based fertilizers; methane emissions Environmental Nutrition https://doi.org/10.1016/B978-0-12-811660-9.00014-X

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from production of ruminant meats; and carbon dioxide emissions from land use and land use change. Integrating agriculture activities and dietary choice into climate policies could result in large reductions in global GHG emissions. Dietary choices are also a leading cause of poor health. Diet-related noncommunicable diseases (NCDs) such as heart disease, diabetes, and overweight and obesity are the leading cause of morbidity and mortality globally (Hay et al., 2017; Naghavi et al., 2017). In addition, global prevalence of NCDs has increased rapidly over the past several decades because of dietary shifts toward diets high in calories, sugars, oils, and animal products (Drewnowski and Popkin, 1997). For example, global prevalence of adult overweight and obesity increased by nearly a third (Ng et al., 2014), while diabetes incidence doubled from 1980 to 2012 (World Health Organization, 2016). The increase of NCD prevalence has been especially rapid in developing nations. For example, diabetes incidence in China increased from <1% to >10% between 1980 and 2008 (Hu, 2011), while prevalence of overweight more than doubled in 30 Sub-Saharan African nations from 1980 to 2014 (World Health Organization, 2017). There is a large gap between current diets and healthy and environmentally sustainable diets. This gap can be leveraged to improve diet-related health and environmental outcomes. This chapter discusses the benefits that including healthy diets as a climate change mitigation policy could have on global GHG emissions. The chapter is divided into five sections. The first section examines the GHG emissions and health impacts of thirteen major food groups. The second section examines forecasts of diet-related environmental and health outcomes to 2050. The third section examines the GHG and health benefits of adopting healthy dietary patterns such as a Mediterranean, Pescatarian, or vegetarian diet. The fourth section discusses the potential for abandoned agricultural land to act as a GHG sink. The fifth and final section briefly examines other changes in the agricultural system that could also be leveraged to reduce agricultural GHG emissions.

GHG emissions and health impacts of different foods This section focuses on the climate change and health impacts of thirteen major food groups. Although the main aim of this chapter is the potential of healthy diets as a climate change mitigation strategy, we assess these food groups because individuals most often purchase individual food items rather than entire diets.

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GHG emissions of different foods

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GHG emissions vary greatly across food groups, sometimes by more than two orders of magnitude (de Vries and de Boer, 2010; Nijdam et al., 2012; Clark and Tilman, 2017; Clune et al., 2017). Whole grains, nuts, legumes, fruits, and vegetables have among the lowest GHG emissions per gram of food produced (Fig. 1A). Rice production emits more GHGs than other plant-based foods because of methane that is released when rice paddies are flooded. Dairy production emits slightly more GHGs than does production of whole grains per gram of food produced, while egg production emits 3–5 times the GHGs of most plant-based foods. Production of poultry emits 3–5 times the GHGs of eggs per gram produced. Average GHG emissions per gram of fish produced are slightly higher than those of poultry. However, GHG emissions in fish production systems vary widely

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Fig. 1 GHG emissions and marginal health impacts for thirteen major food types. (A) GHG emissions per gram of food production plotted on a log10 axis. Bars indicate mean  one standard deviation. (B) Marginal health impact per serving of daily serving of food consumed, reported in relative risk of disease incidence. A relative risk of 1 indicates no change in disease risk; a relative risk greater than 1 indicates an increase in disease risk. Bars are 95% confidence intervals. Food groups are ordered from lowest to highest average relative risk across disease outcomes examined. SSBs, sugar-sweetened beverages; ACM, all-cause mortality; CHD, coronary heart disease. ((A) Data are from Clark and Tilman, 2017. (B) Data are from Larsson and Orsini, 2011; Zheng et al., 2012; Aune et al., 2013a,b, 2016, 2017; Chen et al., 2013; Feskens et al., 2013; Rong et al., 2013; Abete et al., 2014; Afshin et al., 2014; Huang et al., 2014; Tasevska et al., 2014; Yang et al., 2014; Grosso et al., 2015; Imamura et al., 2015; Wang et al., 2015; Wu et al., 2015; Xi et al., 2015; Zhao et al., 2015; Mullie et al., 2016; Wallin et al., 2016.)

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because of the variety of fish production methods (e.g., nontrawling fisheries, recirculating aquaculture, etc.). Producing a gram of red meat (pork, beef, sheep, goat) emits 5 times the GHGs of poultry. In total, producing a gram of beef emits 50 times more GHGs than a gram of wheat and 100 times more GHGs than a gram of maize. This trend is similar when examining GHG emissions per kcal or gram of protein, and is also similar when examining other environmental impacts such as land use and nutrient pollution (Clark and Tilman, 2017). Production of animal-based foods emits more GHGs and in general causes higher environmental impacts than does production of plant-based foods because of the inefficiency at which animals convert feed into human-edible food. Livestock systems that are most efficient at converting feed into human-edible food and therefore require the least amount of feed to produce a unit of food have the lowest GHG emissions per unit of food produced (Clark and Tilman, 2017). Dairy and egg systems, for example, require the lowest feed inputs (Tilman and Clark, 2014) and also have the lowest environmental impacts. In contrast, production of red meat (pork, beef, sheep, and goat) requires the highest feed inputs and also has the highest environmental impacts. Production of ruminant meats (beef, sheep, goat) has especially high GHG emissions per unit of food produced, in part not only because of the high feed inputs required to produce ruminant meats, but also because of methane released during a digestive process called enteric fermentation. This clear trend of GHG emissions across thirteen broad food categories—that plant-based foods having low impacts; dairy, eggs, pork, poultry, and fish having intermediate impacts; and red meats (especially ruminant meat) having high impacts—indicates that diets composed primarily of plant-based foods are likely to have lower GHG emissions than diets that contain larger proportions of animal-based foods, and especially ruminant meats.

Health impacts of different foods Foods also vary in their health impact across several disease outcomes. To show this, data were compiled from a series of prospective cohort studies that examine the dose-response health impact of food consumption (Larsson and Orsini, 2011; Zheng et al., 2012; Aune et al., 2013a,b, 2016, 2017; Chen et al., 2013; Feskens et al., 2013; Rong et al., 2013; Abete et al., 2014; Afshin et al., 2014; Huang et al., 2014; Tasevska et al., 2014;

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Yang et al., 2014; Grosso et al., 2015; Imamura et al., 2015; Wang et al., 2015; Wu et al., 2015; Xi et al., 2015; Zhao et al., 2015; Mullie et al., 2016; Wallin et al., 2016). These analyses examine the marginal health impact of consuming an additional serving of food per day by following populations through time and examining their health outcomes. This collection of dose-response studies across thirteen food groups and four disease outcomes (total mortality, diabetes, coronary heart disease, and stroke) helps to clarify the health impact of thirteen major foods groups. This collection of analyses shows that consuming an additional serving of whole grain cereals (Aune et al., 2013a,b, 2016), fruits (Wu et al., 2015; Aune et al., 2017), vegetables (Wu et al., 2015; Aune et al., 2017), nuts (Afshin et al., 2014; Grosso et al., 2015), legumes (Afshin et al., 2014), and fish (Larsson and Orsini, 2011; Zheng et al., 2012; Zhao et al., 2015) per day would offer large health benefits across multiple disease outcomes (Fig. 1B). Consumption of whole grains is especially beneficial for diabetes and coronary heart disease (Aune et al., 2016), whereas consumption of nuts has the largest positive impact on total mortality (Grosso et al., 2015). Fruits and vegetables have either a small (fruit) or no (vegetable) benefit on diabetes incidence (Wu et al., 2015), an intermediate benefit for CHD incidence and total mortality (Aune et al., 2017), and a large benefit for stroke incidence (Aune et al., 2017). Legumes reduce the risk of coronary heart disease incidence but have no significant effect on diabetes or stroke incidence (Afshin et al., 2014). Fish consumption reduces the risk of stroke (Larsson and Orsini, 2011), coronary heart disease (Zheng et al., 2012), and mortality from all causes (Zhao et al., 2015). Consumption of dairy (Aune et al., 2013a,b; Mullie et al., 2016), eggs (Rong et al., 2013; Wallin et al., 2016), and chicken (Feskens et al., 2013; Abete et al., 2014) often has small health benefits or no significant impact on health (Fig. 1B). Consuming an additional serving of dairy per day has no significant effect on any health outcome examined (Aune et al., 2013a, 2013b; Mullie et al., 2016). Low-fat dairy, however, reduces diabetes risk when consumed in quantities as low as 50 g per day (Aune et al., 2013a, 2013b). Small additional amounts of egg consumption have no significant effect on any health outcome examined in healthy populations (Rong et al., 2013; Wallin et al., 2016), but increase risk of CHD and total mortality among diabetic individuals (Wallin et al., 2016). In addition, larger amounts of egg consumption increase diabetes risk by 42% (Shin et al., 2013) and total mortality by 23% (Djousse and Gaziano, 2008). An additional serving of chicken per day has no significant impact on diabetes incidence

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(Feskens et al., 2013), cardiovascular disease mortality (Abete et al., 2014), or total mortality (Abete et al., 2014). Consumption of red meat (beef, sheep, goat, pork) (Chen et al., 2013; Feskens et al., 2013; Wang et al., 2015), processed red meat (e.g., salami, bologna) (Chen et al., 2013; Feskens et al., 2013; Wang et al., 2015), added sugar (Tasevska et al., 2014; Yang et al., 2014), and sugar-sweetened beverages (SSBs) (Huang et al., 2014; Imamura et al., 2015; Xi et al., 2015) consistently has a negative impact on health. This is especially true for processed red meats. An additional 50g serving of processed red meats per day increases the risk of total mortality by 15% (Wang et al., 2015), diabetes incidence by 32% (Feskens et al., 2013), stroke incidence by 11% (Chen et al., 2013), and CHD incidence by 15% (Wang et al., 2015). Consumption of red meats increases the risk of stroke (Chen et al., 2013) and has a negative but nonsignificant impact on diabetes (Feskens et al., 2013), CHD (Wang et al., 2015), and mortality from all causes (Wang et al., 2015). One analysis showed that reducing consumption of red and processed red meat by substituting 42 g per day with a different protein source such as chicken, low-fat dairy, fish, and legumes would reduce total mortality by 7%–19% (Pan et al., 2012). Consumption of added sugars increases the risk of CHD incidence (Yang et al., 2014), while consumption of SSBs increases the risk of CHD incidence by 16% (Huang et al., 2014) and diabetes incidence by 27% (Imamura et al., 2015) but has no significant impact on stroke incidence (Xi et al., 2015). These data show that additional consumption of plant-based foods often has better health outcomes than does additional consumption of dairy, eggs, and meat. In addition, when combined with the GHG emissions per unit of food produced (Fig. 1), these data further indicate that diets primarily composed of plant-based foods with some fish, dairy, eggs, and chicken would likely be healthier and have lower GHG emissions than would diets that contain more dairy, eggs, and meat. The health outcomes compiled here are the marginal health effect of consuming an additional serving of food per day. It is important to note that increasing consumption of one food type without decreasing consumption of another food type can lead to excess caloric intake and related negative health outcomes. Eating more of a food without reducing consumption of another food may not be beneficial to health. Further, additional consumption of many of these foods when they are already consumed in large quantities may have smaller or no additional health benefits. Additional consumption of fruits and vegetables beyond 300g per day, for example, has

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smaller additional benefits for total mortality and no additional benefit for stroke or CHD incidence (Aune et al., 2017); additional consumption of whole grains beyond 90 g per day has smaller benefits for total mortality and stroke and no additional benefit for diabetes (Aune et al., 2016); while additional consumption of fish above 60 g per day has no additional benefit on total mortality or CHD (Zheng et al., 2012; Zhao et al., 2015).

Forecasts of diet-related environmental and health outcomes The previous section discusses the GHG and health impacts of thirteen major food groups and shows that foods that have low GHG emissions are often healthy while foods that have high GHG emissions are unhealthy. This section examines how current dietary trends toward diets higher in calories, refined sugars, fats, and animal products—foods that are less healthy and often less sustainable—will influence diet-related environmental and health outcomes in 2050 if current dietary trends continue (Fig. 2).

Forecasts of diet-related environmental impacts Dietary trajectories and population growth are forecast to lead to a 70%– 100% increase in global agricultural production between 2005 and 2050, largely because of increased per capita meat demand as populations become more affluent. As a result, diet-related GHG emissions from food production are forecast to increase by 50%–80% by 2050, to a global total of 11.4–15 Gt CO2-e/yr (Tilman et al., 2011; Bajzelj et al., 2014; Tilman and Clark, 2014; Springmann et al., 2016). The increase in diet-related GHG emissions, both per capita and absolute, will be highly spatial. The largest total and per capita increases will occur in developing countries because of their rapid growth in per capita income and population. In contrast, per capita and total dietrelated GHG emissions will remain relatively constant in developed nations because of their comparatively slow dietary shifts and population growth (Springmann et al., 2016). Land use and land use change that results from expansion of diets and populations also contribute to global GHG emissions. Indeed, land use and land use change currently account for 10% of global GHG emissions (IPCC, 2014a), and will continue to emit GHGs as agricultural areas continues to expand to meet growing populations and growing diets. Two independent forecasts estimate that diet-driven land use and land use change will emit 2.2–7 Gt CO2-e/yr between 2009 and 2050 (Tilman and Clark, 2014;

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Bajzelj et al., 2014), bringing total agriculture-related GHG emissions in 2050 to 13.6–22 Gt CO2-e/yr. Diet-related GHG emissions in 2050 will account for 54%–88% of the annual global GHG budget under the 2°C pathway if current diet and population trajectories continue. Continuing to not incorporate diets and agriculture into climate policies would be a significant oversight. Current dietary trends and population growth have environmental impacts beyond GHG emissions. Cropland use, for instance, is projected to increase by 300–700 million hectares by 2050 (Alexandratos and Bruinsma, 2012; Bajzelj et al., 2014; Schmitz et al., 2014; Tilman and Clark, 2014), the majority of which is forecast to occur in tropical developing nations. Expansion of croplands also has several indirect environmental impacts such as biodiversity loss (Tilman et al., 2017) and decreased provision of other ecosystem services (Dobson et al., 2006; Schr€ oter, 2005). Agricultural water, nitrogen, and phosphorus are also forecast to increase several-fold to meet future food demand (Tilman et al., 2001).

Forecasts of diet-related health impacts Current dietary trajectories will also increase the prevalence of diet-related noncommunicable diseases such as diabetes, overweight, obesity, and heart disease. This is especially true in developing nations that are undergoing rapid dietary and lifestyle shifts. Diabetes prevalence is forecast to increase by at least 55% by 2035 in the most conservative estimates, with a near doubling of diabetes prevalence in the Middle East and North Africa and a >70% increase in South and Southeast Asia (Popkin, 2016), while cardiovascular disease prevalence is projected to increase >50% in China (Moran et al., 2010) and the United States (Heidenreich et al., 2011) by 2050. In total, by 2030, diet-related NCDs are projected to become three-quarters of the global burden of disease if current dietary trends continue (Beaglehole and Bonita, 2008).

Healthy diets as climate change mitigation strategies The previous section discussed the expected GHG and health outcomes if current dietary shifts toward less healthy and less sustainable foods continue to 2050. This section highlights the potential health and environmental benefits if healthy diets were instead to be adopted globally.

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Environmental benefits of adopting healthy dietary patterns Several recent analyses have also shown that adopting healthy dietary patterns would have large environmental benefits (Fig. 2). Tilman and Clark (2014), for example, showed that global adoption of a Mediterranean, pescetarian, or vegetarian diet would reduce diet-related GHG emissions from food production by 40% relative to expected dietary patterns in 2050. In addition, adoption of one of these diets would also prevent the release of 2.2 Gt CO2-e/yr of diet-related land use change emissions. Bajzelj et al. (2014) also showed that adoption of a healthy diet would reduce diet-related GHG emissions by 10.9 Gt CO2-e/yr, of which 57% (6.2 Gt CO2-e/yr) would be through reduced land use change emissions and 43% (4.7Gt CO2-e/yr) through reduced food production emissions (Bajzelj et al., 2014).

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Fig. 2 Diet-related GHG emissions from different dietary patterns. Data include emissions from food production and land use and land use change. “Current” indicates GHG emissions from global diets in 2010; “BAU 2050” indicates expected emissions if diets and population continue to shift along current trajectories; “Mediterranean,” “Pescatarian,” “Vegetarian,” and “Vegan” indicate GHG emissions if healthier alternative diets are adopted. The horizontal dashed line at 25Gt CO2-e/yr indicates the global GHG budget under the 2°C pathway. Land use and land use change emission in the “Mediterranean,” “Pescatarian,” “Vegetarian,” and “Vegan” scenarios are assumed to be 2.6 Gt CO2-e per year, but could be smaller if policies designed to minimize expansion of agricultural lands in tropical regions are implemented. (Data are from Bajzelj et al. 2014; Tilman and Clark, 2014; Springmann et al., 2016.)

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Springmann et al. (2016) examined a wider range of healthy dietary patterns and also showed that adoption of healthier diets would reduce diet-related GHG emissions by 29%–70%, with diets that contain the least amount of meat offering the largest reduction in GHG emissions (Springmann et al., 2016). Analyses at smaller spatial scales have reached similar conclusions (Harwatt et al., 2017; Peters et al., 2016; Meier and Christen, 2013). Harwatt et al. (2017), for example, showed that substituting beans for beef in the United States would meet 46%–74% of the reductions required for the United States to meet its 2020 GHG emissions target. In total, global adoption of healthier plant-based diets would reduce dietrelated GHG emissions to 3.4–8.3 Gt CO2-e/yr 2050, or 14%–33% of the global emissions target in 2050 for the 2°C pathway. This is a large reduction relative to expected global diet-related GHG emissions if current diet and population trajectories continue (13.6–22Gt CO2-e/yr). Adoption of healthy dietary patterns would also have other environmental benefits, such as reducing demand for agricultural land. Current dietary trends and population growth are forecast to increase agricultural cropland demand by 300–700 Mha by 2050 (Alexandratos and Bruinsma, 2012; Bajzelj et al., 2014; Schmitz et al., 2014; Tilman and Clark, 2014). Adoption of healthy dietary patterns, in contrast, would greatly reduce future expansion of agricultural land, and in some cases would result in reductions of agricultural cropland extension relative to current levels (Bajzelj et al., 2014; Tilman and Clark, 2014). Adoption of healthier diets that contain less meat and dairy would also reduce demand for pastureland. Rapid adoption of healthy dietary patterns would free substantial amounts of cropland and pastureland for other uses, raising the question of how abandoned agricultural lands could be used.

Health benefits of adopting healthy dietary patterns Global adoption of healthier diets would also have significant health benefits. Tilman and Clark (2014) performed a meta-analysis and showed that adoption of a Mediterranean, pescetarian, or vegetarian diet would decrease diabetes incidence by 15%–40%, cancer incidence by 7%–13%, coronary mortality by 20%–25%, and total mortality by up to 20% relative to westernized dietary patterns that are becoming more common globally as populations become more affluent. Using a different methodology, Springmann et al. (2016) showed that adoption of an omnivorous, vegetarian, or vegan diet that meets international dietary recommendations would avoid 5.1–8.1

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million deaths per year (6%–10% of total deaths) and would save 79–129 million years of life relative to expected dietary trajectories, largely through reduced consumption of red meat and increased consumption of fruits and vegetables.

Abandoned agricultural land as a potential carbon sink Adoption of healthy dietary patterns would greatly reduce global expansion of agricultural lands, and might reduce agricultural land use in some temperate nations (Bajzelj et al., 2014; Tilman and Clark, 2014; Tilman et al., 2017). Management decisions on abandoned agricultural lands—for example leaving it empty, using it to grow biofuels, or restoring it into seminatural ecosystems—will have a large impact on whether global emissions targets are met. This section discusses the potential that restoring abandoned agricultural lands into seminatural ecosystems could have in meeting global climate change targets. Abandoned agricultural lands have the potential to sequester large amounts of carbon. Long-term studies examining carbon sequestration in abandoned croplands have recorded soil carbon sequestration of 0.15–0.5 tonnes of carbon per hectare per year in temperate regions and up to 5 tonnes of carbon per hectare in tropical regions (Post and Kwon, 2000). However, these estimates might be closer to the lower bound rather than average of the potential rates of carbon sequestration in abandoned agricultural lands because carbon sequestration in aboveground biomass, especially in forest ecosystems, is much larger than is carbon sequestration in soils (Poulton et al., 2003). The exact amount of potential carbon sequestration in abandoned agricultural lands is difficult to estimate because it is highly dependent on historical and future patterns of agricultural land demand and abandonment. Current estimates of carbon sequestration in forests on recently abandoned agricultural, however, provide an approximation of potential long-term carbon storage in abandoned agricultural lands. Forests in abandoned agricultural lands currently sequester 8.6 Gt CO2-e/yr, largely because of forest regrowth in tropical regions on abandoned agricultural lands (Pan et al., 2011). Note, however, that this estimate of carbon sequestration on abandoned agricultural lands corresponds with current dietary trends. Adopting healthier and less land-intensive diets would free more land that was previously in agriculture and thereby increase the global potential for forest regrowth on abandoned agricultural lands to sequester carbon. In

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addition, careful management of abandoned agricultural lands would increase their potential to sequester carbon (Wollenberg et al., 2016). Current agricultural lands could also sequester large amounts of carbon if properly managed. One recent study estimated that 1.5–8 Gt CO2-e/yr could be sequestered in current crop and pasturelands through a variety of management techniques and technological improvements (Paustian et al., 2016), although the lower estimate is likely more feasible because the upper estimate requires major technological advancements and no economic constraints. Agroforestry schemes could sequester another 0.39 Gt CO2-e/yr (Wollenberg et al., 2016). Careful planning of policies and management practices is necessary to maximize carbon sequestration in current and abandoned agricultural lands. The type of treeplanted in abandoned agricultural lands influences the maximum amount of carbon that can be sequestered, with broadleaf trees often sequestering more carbon than pines or conifers (Laganie`re et al., 2010). Past land history also influences carbon sequestration in abandoned lands. Laganie`re et al. (2010), for example, found that reforesting abandoned pastures and rangelands does not increase and could decrease rates of carbon sequestration. Long-term planning is also necessary to maximize carbon sequestration in abandoned agricultural lands: shifts in management strategies have been shown to reduce rates of carbon sequestration (Freibauer et al., 2004), and in some cases may result in net releases of greenhouse gases (Saarsalmi et al., 2010).

The need for other improvements in agricultural systems Dietary shifts toward healthy diets combined with management of abandoned agricultural lands for carbon sequestration may not be adequate to meet global GHG emissions targets in 2050 if they are adopted in isolation of other potential improvements to the global agricultural system. This section discusses three examples of how to improve environmental sustainability of the global agricultural system using existing technologies and management techniques.

Sustainable increases in crop yields Crop yields in many nations are much lower than what they could be because of lack of access to fertilizers, pesticides, and improved seeds (Mueller et al., 2012; Global Yield Gap and Water Productivity Atlas, 2016). The difference between current and potential yields is called the yield

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gap. The yield gap is especially large in Sub-Saharan Africa, where six nations (Angola, Eritrea, Mozambique, Namibia, Rwanda, and Zimbabwe) have average yields less than one-quarter of their potential yields. Yield gaps are also large in many South Asian, Southeast Asian, and South American nations. Increasing crop yields by decreasing yield gaps in underyielding nations would have large environmental benefits. Tilman and Clark (2014), for example, showed that every 1% reduction in yield gaps would decrease global land demand by 8.5 million hectares by 2050. Tilman et al. (2017) showed that closing the yield gap by 80% by 2060 would reduce global expansion of cropland by 60% (390 million hectares), with the largest reductions in the biodiverse regions of Sub-Saharan Africa, Southeast Asia, and South America. Sustainable increases in crop yields would also improve food security, especially in regions with limited access to markets (Godfray et al., 2010). Sustainable increases in crop yields are possible with current technologies and management techniques. Planting legumes to increase nitrogen availability and soil fertility (Robertson and Vitousek, 2009), using manure and cover crops to increase soil organic matter (Robertson and Vitousek, 2009), increasing access to fertilizer inputs such as seeds (Sa´nchez, 2010), fertilizers and pesticides (Godfray et al., 2010; Foley et al., 2011; Mueller et al., 2012), and better nutrient management techniques that mesh crop nutrient demand with nutrient application have increased yields across a variety of crops (Robertson and Vitousek, 2009). These policies and management techniques are applicable at the country-level. Malawi (Dorward et al., 2011), Ghana, Mali, Rwanda, Senegal, and Zambia (Druilhe and Barreiro-hurle, 2012), for instance, have increased average national crop yields by 20%–80% by improving access to fertilizer and seeds.

Reducing GHG emissions per unit of food Production and use of agricultural inputs such as fertilizers and feed account for the majority of GHG emissions from food production (Clark and Tilman, 2017). Fertilizer production and application, for example, accounts for 50%–90% of GHG emissions in plant-based systems, while feed production accounts for 45%–80% of GHG emissions in egg, poultry, and pork production systems. Feed production accounts for a lower proportion (25%–35%) of GHG emissions in dairy, beef, and goat production systems because of methane released during the digestion process of ruminants.

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Reducing the amount of fertilizer and feed used per unit of food produced would reduce diet-related environmental impacts (Robertson and Vitousek, 2009; Clark and Tilman, 2017) and is possible with known technologies and management practices. Precision agriculture, or where fertilizer is applied to spatially and temporally match crop nutrient needs, reduces fertilizer inputs across a variety of crops without reducing crop yields (Robertson and Vitousek, 2009). Crop rotation, especially with nitrogenfixing crops, also reduces fertilizer inputs by reducing nutrient leaching and fixing nitrogen into soils (Robertson and Vitousek, 2009). Reducing feed inputs is possible by reformulating diets to better match livestock nutrient needs. Adding amino acids to pig diets, for example, reduced GHG emissions by 10% while having other environmental benefits (Ogino et al., 2013). Adding amino acids to livestock feeds also reduces GHG emissions in other livestock systems (Robertson and Vitousek, 2009).

Reducing food waste Thirty-forty percent of global food production is wasted (Gustavsson et al., 2011), and the environmental impacts embedded in food waste are substantial. For example, the GHG emissions embedded in food waste are greater than the GHG emissions of any nation except China and the United States (CAIT, 2015), while 24% of freshwater, 23% of cropland, and 23% of fertilizer is used to produce food that is ultimately wasted (Kummu et al., 2012). The source of food waste differs between nations; affluent nations primarily waste food at retail stores and households; while developing nations tend to waste food during production, transportation, and processing because of a lack of infrastructure (Gustavsson et al., 2011). Reducing food waste in half would reduce water, land, and fertilizer used to produce food that is ultimately wasted by 44%, 39%, and 42%, respectively (Kummu et al., 2012). Several interventions have successfully reduced food waste in a variety of nations. Intermarche, a French supermarket, ran a campaign named “Inglorious Fruits and Vegetables” to sell fresh produce that would have otherwise been wasted at a 30% discount. In two days, Intermarche sold 1.2 tonnes of produce per store that would have been wasted while increasing foot traffic by 24%. Five of Intermarche’s main competitors ultimately started similar offers as a result of Intermarche’s success (Intermarche, 2017). Several nations, including Italy and France, now have laws that encourage or mandate restaurants and grocery stores to donate food that would be wasted. Reducing food loss and waste is also possible in developing nations. Improving crop harvest and storage techniques would reduce food loss

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during agricultural production, while increasing access to refrigeration would reduce food waste during transportation and at retail stores and households (Gustavsson et al., 2011).

Conclusions There is a large gap between current dietary trajectories and healthy and sustainable diets. Current dietary trajectories are environmentally unsustainable and unhealthy. Diet-related GHG emissions in 2050 are forecast to be 54%–88% of the global GHG budget under the 2°C pathway, and much larger than the global carbon budget for the 1.5°C pathway. In addition, diets are becoming less healthy globally, and by 2030 are forecast to be the cause of three-quarters the global burden of disease (Beaglehole and Bonita, 2008). Adoption of healthier diets would instead emit fewer GHGs (Bajzelj et al., 2014; Tilman and Clark, 2014; Springmann et al., 2016) and simultaneously reduce total global mortality by 6%–10% (Springmann et al., 2016). Adoption of healthier diets would also free large amounts of agricultural land (Bajzelj et al., 2014; Tilman and Clark, 2014) that could be used as a large carbon sink that could offset other sources of GHG emissions. Altered management techniques and adoption of improved technologies could also sequester 1.5–8 Gt CO2-e/yr in current crop and pasturelands (Paustian et al., 2016). Yet, despite the magnitude of current and expected future diet-related GHG emissions, most national and international climate mitigation policies do not include healthy diets as a climate mitigation strategy (Wynes and Nicholas, 2017). Leveraging the gap between current and expected diets and healthy and sustainable diets by incorporating healthy diets into climate change mitigation strategies would result in large reductions in global GHG emissions and free large amounts of agricultural land for carbon sequestration while simultaneously improving health outcomes. Adoption of healthier dietary patterns is likely necessary to meet the global carbon budget for both the 2°C and 1.5°C emissions pathways.

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