Comparison of Land, Water, and Energy Requirements of Lettuce Grown Using Hydroponic vs. Conventional Agricultural Methods — TEI-tagged view

Guilherme Oliveira BarbosapersName, Francisca GadelhapersName, Natalya KublikpersName, Alan ProctorpersName, Lucas ReichelmpersName, Emily A. WeissingerpersName, Gregory Michael WohllebpersName, Rolf U. HaldenpersName (2015date). “Comparison of Land, Water, and Energy Requirements of Lettuce Grown Using Hydroponic vs. Conventional Agricultural Methods.” International Journal of Environmental Research and Public Healthtitle·venue.

Object of study — the land, water and energy required to grow lettuce hydroponically against conventionally, in Yuma, Arizona

Research question — whether hydroponicterm·dom lettuce production is a more sustainable alternative to conventional production on land, water and energy together

Contribution — the first quantitative land-water-energy comparison of conventional and hydroponicterm·dom lettuce in the southwestern United States, identifying energy availability as the deciding factor

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# Abstract The land, water, and energy requirements of hydroponicsterm·dom were compared to those of conventional agricultureterm·dom by example of lettuce production in Yuma, Arizona, USA. Data were obtained from crop budgetsterm·mth and governmental agricultural statistics, and contrasted with theoretical data for hydroponicterm·dom lettuce production derived by using engineering equations populated with literature values. Yields of lettuce per greenhouseterm·dom unit (815 m2) of 41 ± 6.1 kg/m2/y had water and energy demands of 20 ± 3.8 L/kg/y and 90,000 ± 11,000 kJ/kg/y (±standard deviation), respectively. In comparison, conventional production yielded 3.9 ± 0.21 kg/m2/y of produce, with water and energy demands of 250 ± 25 L/kg/y and 1100 ± 75 kJ/kg/y, respectively. Hydroponicsterm·dom offered 11 ± 1.7 times higher yields but required 82 ± 11 times more energy compared to conventionally produced lettuce. To the authors’ knowledge, this is the first quantitative comparison of conventional and hydroponicterm·dom produce production by example of lettuce grown in the southwestern United States. It identified energy availability as a major factor in assessing the sustainabilityterm·dom of hydroponicsterm·dom, and it points to water-scarce settings offering an abundance of renewable energy (e.g., from solar, geothermal, or wind power) as particularly attractive regions for hydroponicterm·dom agriculture.

Keywords: agriculture; hydroponicsterm·dom; sustainabilityterm·dom; water; energy; land use; lettuce; Arizona  <strong>Graphical Abstract</strong> Graphical Abstract 1. Introduction Conventional agricultural practices can cause a wide range of negative impacts on the environment. “Conventional” or “modern industrial agriculture” has been historically defined as the practice of growing crops in soil, in the open air, with irrigation, and the active application of nutrients, pesticides, and herbicides. Some of the negative impacts of conventional agricultureterm·dom include the high and inefficient use of water, large land requirements, high concentrations of nutrients and pesticides in runoff, and soil degradation accompanied by erosion [1, 2]. As the world population continues to grow at a rapid rate, so too must the food production. However, approximately 38.6% of the ice-free land and 70% of withdrawn freshwater is already devoted to agriculture [3, 4]. To sustainably feed the world’s growing population, methods for growing food have to evolve.

The benefits of hydroponicterm·dom agriculture are numerous. In addition to higher yields and water efficiency, when practiced in a controlled environment, hydroponicterm·dom systems can be designed to support continuous production throughout the year [5]. Hydroponicterm·dom systems are very versatile and can range from rudimentary backyard setups to highly sophisticated commercial enterprises. Various commercial and specialty crops can be grown using hydroponicsterm·dom including tomatoes, cucumbers, peppers, eggplants, strawberries, and many more. Leafy vegetables, such as lettuce can also be grown hydroponically and perform best using the nutrient film technique (NFT) [6]. Hydroponicterm·dom NFT production involves the circulation of a nutrient solution through shallow channels in a closed-loop system [7].

In 2012, in terms of production by weight, head lettuce was the second largest vegetable crop in the Unites States, second only to onions [8]. A substantial portion of that production (approximately 29% in 2012) occurs in Arizona, primarily in Yuma [8, 9]. Since Arizona devotes approximately 69% of its current freshwater withdrawals to agriculture [10], investigations into hydroponicterm·dom alternatives could be beneficial in reducing the strain on water resources in such regions. There is considerable research available regarding conventional lettuce production and hydroponicterm·dom lettuce production separately, but few studies that have compared the resource inputs of the two at a commercial level.

Regarding conventional lettuce production, in 2001, the University of Arizona Cooperative Extension developed county-specific crop budgetsterm·mth estimating the operating and ownership costs of producing vegetables in Arizona using representative cropping operations and such resource inputs as water, fuel, and fertilizer [11]. Realizing that the water and energy use for agriculture is substantial, Ackers et al. (2008) [12] performed an “order of magnitude” study and determined a reasonable range of estimates for resources used in the production of Arizona agriculture.

Regarding hydroponicsterm·dom, the Ohio State University developed an enterprise model designed to estimate the revenue, expenses, and profitability associated with a typical hydroponicterm·dom greenhouseterm·dom lettuce production system in Ohio [13]. Various other authors have investigated components of hydroponicterm·dom lettuce production as it relates to water and energy inputs [14, 15].

The objective of this study is to determine whether hydroponicterm·dom lettuce production is a suitable and more sustainable alternative to conventional lettuce production in Arizona. For this study, “a suitable and more sustainable alternative” is one that outperforms (i.e., is more efficient than) conventional agricultureterm·dom in the metrics of land use, water use, and energy use, normalized by yield.

2. Experimental Section Metrics for land, water, and energy use were developed for conventional lettuce production and hydroponicterm·dom lettuce production using a southwestern Arizona location as a case study scenario. Data related to conventional lettuce production was obtained from crop budgetsterm·mth and governmental agricultural statistics, while data regarding the hydroponicterm·dom production of lettuce was developed using engineering equations and values reported in literature which were then applied to a hypothetical 815 square meter enclosed hydroponicterm·dom greenhouseterm·dom located in Yuma, Arizona. This greenhouseterm·dom was assumed to be using the nutrient film technique of growing hydroponicterm·dom lettuce, have temperature controls, supplemental artificial lighting, and water circulation pumps.

2.1. Lettuce Grown Using Conventional Agricultural Methods 2.1.1. Conventional Yield The average yield of head lettuce per acre in Arizona was quantified by taking the average of the past ten years of agricultural census data (2003–2012) that was available from the National Agricultural Statistics Service (NASS) for the United States Department of Agriculture (USDA) [8, 16, 17, 18, 19]. The resulting value was then converted into units of kilograms per square meter per year (kg/m2/y).

2.1.2. Conventional Water Usage The water usage related to the conventional production of head lettuce in Arizona was estimated using county-specific crop budgetsterm·mth developed by the University of Arizona Cooperative Extension. Specifically, the two Yuma County crop budgetsterm·mth for head lettuce were used to produce an average estimate of irrigation use of 965 L per square meter per crop [11]. The average value was then converted to liters per kilogram per year (L/kg/y) using the estimated average yield value for conventional lettuce and assuming one crop of lettuce per year.

2.1.3. Conventional Energy Usage Energy use related to the conventional production of head lettuce in Arizona was limited to the direct use of fossils fuels by farms during operations and the use of electricity in the pumping of irrigation water. An average estimate of fuel use was derived from the Yuma County crop budgetsterm·mth for head lettuce production developed by the University of Arizona Cooperative Extension [11]. The average value was then converted to kilojoules per kilogram per year (kJ/kg/y) using the estimated average yield value for conventional lettuce and assuming one crop of lettuce per year. It was conservatively assumed that all farms require energy to pump irrigation water and that all of the pumps use electricity.

Using an estimate of 1037 kilowatt-hours (kWh) per acre foot pumped [12], a conversion factor of 3600 kilojoules per kWh, the average estimate of water use calculated previously, and the estimated average yield value for conventional lettuce, an average estimate of energy use for the pumping of irrigation water was computed in units of kJ/kg/y.

Together, the average estimates of energy use related to fossil fuels and the pumping of irrigation water were combined to produce an overall average estimate of energy use for the conventional production of head lettuce in Arizona in units of kJ/kg/y.

2.2. Lettuce Grown Using Hydroponicterm·dom Methods 2.2.1. Hydroponicterm·dom Yield The average yield of head lettuce grown using hydroponicterm·dom methods was estimated by evaluating values reported in available literature. Databases searched included; AGRICOLA, Web of Science, Google Scholar, and Science Direct, using keywords “hydroponicsterm·dom” and “head lettuce.” Only publications that contained the necessary information regarding yield, type of hydroponicterm·dom system (e.g., NFT), configuration, production area, and greenhouseterm·dom area were used. Once the information was gathered, the data was standardized.

Configurations from three studies were averaged to get an approximate plant density of 24 plants per square meter of greenhouseterm·dom [7, 13, 20]. With supplemental artificial lighting and temperature controls lettuce production can be continuous year round, with a full harvest cycle completed every 30 days (or approximately 12 harvests per year) [13]. The mass of edible lettuce per plant was averaged from two sources to get an average mass of 144.6 grams per plant [13, 21]. Using the estimates for plant density, number of harvests, and plant mass, the average yield of hydroponicterm·dom lettuce per year was calculated in units of kg/m2/y.

2.2.2. Hydroponicterm·dom Water Usage The water usage related to the hydroponicterm·dom production of head lettuce in Arizona was estimated by considering the average evapotranspiration of each individual plant, which takes into account the water loss due to evaporation and transpiration. Both (2014) [22] reported that lettuce plants grown hydroponically must transpire at least 400 mL per gram of dry mass accumulated to avoid tipburn damage. Using a ratio of dry mass to fresh mass of 0.045 [22] and the average mass per head of lettuce calculated previously, the average evapotranspiration per plant was calculated. This value was then conservatively increased by an additional 10% to account for periodic flushing of the nutrient solution. This estimate was used in combination with the estimated yield values for hydroponicterm·dom lettuce to create an average estimate of water usage in units of L/kg/y.

2.2.3. Hydroponicterm·dom Energy Usage Energy use related to the hydroponicterm·dom production of head lettuce in Arizona focused on the energy demands associated with supplemental artificial lighting, water pumps, and heating and cooling loads.

The energy use related to supplemental artificial lighting was determined by assuming a 24-h photoperiod and a recommended daily Photosynthetically Active Radiation (PAR) of 17 mol/m2/day of both natural and supplemental light [5], for optimal lettuce production. It was also assumed that half of the required radiation could be obtained from natural lighting. Assuming a wavelength of 600 nm (the approximate average wavelength of the high pressure sodium bulbs used in supplemental light), the following equation was used to calculate the energy of a mole of photons: E=h×cλ×6.022×1023mol (1) where - E: Energy per mole of photons (J/mol) - h: Planck’s constant (6.626 × 10−34 J·s) - c: Speed of light (2.998 × 108 m/s) - λ: Wavelength of light (m) The resulting value was then used in conjunction with the estimated yield values for hydroponicterm·dom lettuce to calculate the energy demand from supplemental lighting in units of kJ/kg/y.

The energy use related to the pumping of water associated with the hydroponicterm·dom production of lettuce was estimated using an average pumping time and a representative pump power rating. Lopes da Luz et al. (2008) [15] performed a study indicating that the optimal irrigation regimen for hydroponicterm·dom lettuce resulted in four hours thirty minutes of pumping per day. Regarding the pump power rating, a study [23] suggested that a 0.5 horsepower (HP) pump is sufficient to deliver water to a hydroponicterm·dom system with an average of 2040 heads of lettuce. Together, the average horsepower per plant along with the average pumping time per day was used in conjunction with the estimated yield values for hydroponicterm·dom lettuce to estimate the energy use related to pumping in units of kJ/kg/y.

The energy use related to heating and cooling loads associated with the hydroponicterm·dom production of lettuce was estimated by calculating the design heat load of a hypothetical greenhouseterm·dom in Arizona. The following equation, based on standard heat transfer equations, was used to determine these values [24].

Q=U×SA×(Tin−Tout) (2) where, - Q: Heat lost or gained due to outside temperature (kJ·h−1) - U: Overall heat transfer coefficient (kJ·h−1·m−2·°C−1) - SA: Surface area of greenhouseterm·dom (m2) - Tin: Inside air set point temperature (°C) - Tout: Outside air temperature (°C) For the purpose of these estimations, 23.9 °C was used as the set point temperature of the greenhouseterm·dom, as this is the optimal temperature for lettuce cultivation [5]. Polyethylene is one of the most common materials used in greenhouseterm·dom construction, and the overall heat transfer coefficient for this material (24.5 kJ/h/m2/°C) was used in calculations [24]. The heating and air conditioning efficiency was conservatively estimated to be 75% and the ceilings were estimated to be 3.0 m high. The dimensions of the hypothetical greenhouseterm·dom were estimated to be 63.7 m by 12.8 m. For comparison purposes, it was assumed that the hydroponicterm·dom greenhouseterm·dom would be located in Yuma, Arizona, the same location where the majority of non-hydroponic head lettuce in Arizona is produced [9]; therefore, the average monthly temperatures for this region were used to estimate the temperatures external to the greenhouseterm·dom [25]. The final energy estimates were used in conjunction with the estimated yield values for hydroponicterm·dom lettuce to create a metric in units of kJ/kg/y.

Together, the estimates of energy use related to supplemental artificial lighting, water pumps, and heating and cooling loads were combined to produce an overall estimate of energy use for the hydroponicterm·dom production of head lettuce in Arizona in units of kJ/kg/y.

3. Results and Discussion In terms of yield per area, the hydroponicterm·dom production of lettuce in Arizona was found to be 11 ± 1.7 times greater than that of its conventional equivalent. Specifically, hydroponicterm·dom lettuce production was calculated to result in a yield of 41 ± 6.1 kg/m2/y (±standard deviation, SD, here and in the following), while conventional lettuce production was projected to yield 3.9 ± 0.21 kg/m2/y ( Figure 1).

Figure 1. Modeled annual yield in kilograms per square meter of lettuce grown in southwestern Arizona using hydroponicterm·dom vs. conventional methods (Error bars indicate one standard deviation).

Figure 1. Modeled annual yield in kilograms per square meter of lettuce grown in southwestern Arizona using hydroponicterm·dom vs. conventional methods (Error bars indicate one standard deviation).

Water consumption between the hydroponicterm·dom and conventional production of lettuce in Arizona was comparable on an area basis, but when normalized by yield the average was 13 ± 2.7 times less water demand in hydroponicterm·dom production compared to conventional production. Specifically, hydroponicterm·dom lettuce production had an estimated water demand of 20 ± 3.8 L/kg/y, while conventional lettuce production had an estimated water demand of 250 ± 25 L/kg/y ( Figure 2).

Results for energy consumption found that the hydroponicterm·dom production of lettuce in Arizona requires 82 ± 11 more energy per kilogram produced than the conventional production of lettuce in Arizona. Dominating the hydroponicterm·dom energy use are the heating and cooling loads at 74,000 ± 10,000 kJ/kg/y, followed by the energy used for the supplemental artificial lighting at 15,000 ± 2100 kJ/kg/y. The circulating pumps contributed the least to the total energy use at 640 ± 120 kJ/kg/y. In total, the hydroponicterm·dom energy use was calculated to equal 90,000 ± 11,000 kJ/kg/y ( Figure 3).

The total energy use for the conventional production of lettuce in Arizona was calculated to be 1100 ± 75 kJ/kg/y ( Figure 3). This total was split between the energy use related to fuel usage at 330 ± 20 kJ/kg/y and groundwater pumping at 760 ± 74 kJ/kg/y.

Figure 2. Modeled annual water use in liters per kilogram of lettuce grown in southwestern Arizona using hydroponicterm·dom vs. conventional methods (Error bars indicate one standard deviation).

Figure 2. Modeled annual water use in liters per kilogram of lettuce grown in southwestern Arizona using hydroponicterm·dom vs. conventional methods (Error bars indicate one standard deviation).

Figure 3. ( a) Modeled annual energy use in kilojoules per kilogram of lettuce grown in southwestern Arizona using hydroponicterm·dom vs. conventional methods; ( b) The energy use breakdown related to the hydroponicterm·dom production of lettuce; ( c) The energy use breakdown related to the conventional production of lettuce (Error bars indicate one standard deviation).

Figure 3. ( a) Modeled annual energy use in kilojoules per kilogram of lettuce grown in southwestern Arizona using hydroponicterm·dom vs. conventional methods; ( b) The energy use breakdown related to the hydroponicterm·dom production of lettuce; ( c) The energy use breakdown related to the conventional production of lettuce (Error bars indicate one standard deviation).

The objective of this study was to determine whether hydroponicterm·dom lettuce production is a suitable and more sustainable alternative to conventional lettuce production in Arizona. According to the results presented, summarized in Table 1, while the hydroponicterm·dom production of lettuce results in higher yields and more efficient water use, the controlled environment from which the hydroponicterm·dom system produces its higher yields exhibits a higher energy demand.

Table 1. Summary of modeled annual data with standard deviations (S.D.).

| Production Method | Yield (kg/m2/y) | Water Use (L/kg/y) | Energy Use (kJ/kg/y) | | :-: | :-: | :-: | :-: | | Value | S.D. | Value | S.D. | Value | S.D. | | :-: | :-: | :-: | :-: | :-: | :-: | | Conventional | 3.9 | 0.21 | 250 | 25 | 1100 | 75 | | Hydroponicsterm·dom | 41 | 6.1 | 20 | 3.8 | 90,000 | 11,000 | Table 1. Summary of modeled annual data with standard deviations (S.D.).

Higher yields of hydroponicsterm·dom result from the controlled environmental conditions maintained within the hydroponicterm·dom greenhouseterm·dom, which allow for continuous production year round. These conditions also promote a reduction in the number of days required for each harvest cycle, allowing for multiple crops per year. This benefit of hydroponicterm·dom production is not unique to lettuce alone, but will vary depending on the operational parameters under which the crop is grown.

Similarly, most hydroponicterm·dom systems will utilize water more efficiently than conventional farming. The volume of water consumed per plant in a hydroponicterm·dom system is not different from that grown using conventional methods; however, the hydroponicterm·dom system delivers the water more efficiently, with a larger percentage of the water going to plant evapotranspiration [26]. For example, lettuce has shallow roots, but is primarily irrigated through flood furrow irrigation in southwestern Arizona. Water not quickly absorbed by the roots is lost to percolation. Increases in the use of low-flow and more-targeted irrigation techniques could lower the overall water use of conventional farming.

As mentioned previously, most of the energy use for the hypothetical hydroponicterm·dom greenhouseterm·dom can be attributed to the heating and cooling loads. This is primarily due to the fact that the greenhouseterm·dom was sited in Yuma, Arizona, an area which can have average temperatures of 34.7 °C in the summer and 14.1 °C in the winter [25]. For illustrative purposes, the percent heating and cooling energy demand by month is presented in Table 2. Greenhousesterm·dom located in more moderate climates (i.e., climates closer to the greenhouseterm·dom set point temperature) will experience a lower energy demand. In fact, in certain climates heating and cooling systems may not be required, but instead replaced by a passive ventilation system, thus reducing the overall energy demand considerably. The feasibility of hydroponicterm·dom systems is heavily reliant on the climate of farming locations.

Table 2. Percent heating and cooling energy demand by month.

| Month | January | February | March | April | May | June | July | August | September | October | November | December | | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | :-: | | Average Temperature (°C) | 14.8 | 16.6 | 19.6 | 22.8 | 27.4 | 31.7 | 34.7 | 34.6 | 31.7 | 25.3 | 18.7 | 14.1 | | Percent Energy Demand | 12% | 8% | 6% | 1% | 5% | 10% | 14% | 14% | 10% | 2% | 6% | 13% | Table 2. Percent heating and cooling energy demand by month.

The next highest use of energy for the hypothetical hydroponicterm·dom greenhouseterm·dom is for the supplemental artificial lighting, which is used to maximize crop yield and maintain consistent production year round [7]. Some systems use supplemental lighting to create a 24-h photoperiod, especially during the first few days of plant growth, whereas others may use supplemental lighting for only a few hours a day [5, 13]. In addition, small and low-output systems may not use artificial lighting at all. This study assumed that maximum yield was desired and did not perform a cost-benefit analysis of reducing or eliminating supplemental lighting. There are various studies that have tried to optimize supplemental lighting systems [14, 27, 28]. Progress in this area of research could lead to improved energy efficiency for hydroponicterm·dom lettuce.

Due to the high energy demands, at this time, commercial hydroponicsterm·dom is not a suitable alternative to conventional lettuce production in Yuma, Arizona. One possible way to make commercial hydroponicsterm·dom a more sustainable and suitable alternative would be to relocate the greenhouseterm·dom to an area where there are cheap and renewable sources of energy, such as solar, geothermal or wind power; though keeping in mind that the initial investment required for such technologies may be cost-prohibitive. For instance, a hydroponicterm·dom greenhouseterm·dom could be sited in an area without arable soil, or on previously developed areas with impervious pavement, where land is cheap and could be used for photovoltaic solar panels. Assuming a conversion efficiency of 14% and an average daily solar radiation value of 6.5 kWh per square meter per year [29] the ratio of greenhouseterm·dom to solar panel area that would be required to offset the full energy demand of the hydroponicterm·dom system is approximately 1:3.0, with an understanding that the 24-h production cycle along with seasonal and day-to-day changes in solar radiation would require the system to be connected to the electrical grid. This is still an improvement over conventional production, which uses ten times more land and water on a yield basis and requires arable landterm·dom. Eliminating the need for arable landterm·dom has other benefits including versatility in system siting and a potential reduction in the distance in which food must travel. Performing a life cycle assessmentterm·mth that considers the environmental impact of food transportation could show whether this benefit of hydroponicsterm·dom is significant.

An alternative scenario to consider is one in which much of the energy-intensive characteristics of advanced and commercial hydroponicterm·dom operations are abandoned in favor of simpler systems, a practice known as “simplified hydroponicsterm·dom” or “popular hydroponicsterm·dom.” These operational changes come at the cost of total yield; however, such systems can still out-perform conventional systems by a factor of three to four on an area basis [30]. The Food and Agriculture Organization of the United Nations maintains that such systems can be as small as one square meter in size, but that most household simplified hydroponicterm·dom gardens range between 10 to 20 square meters in size. Communities have also adopted simplified hydroponicterm·dom gardens at sizes greater than 200 square meters—a scale in which it becomes viable to sell excess produce for income [30].

There are several limitations to this study that need to be considered. To begin, the calculations for the conventional production of lettuce assumed that only one crop of lettuce can be grown in a given year. While this might be true, much of the land used to produce lettuce is dedicated to warm season crops when lettuce is not in production. It’s possible that these crops have water and energy requirements that differ from lettuce. Additionally, the hydroponicterm·dom greenhouseterm·dom was assumed to be of commercial scale and was optimized for maximum production. Furthermore, as seen in Equation (2), the greenhouseterm·dom was assumed to be of a specific size and located in Yuma, Arizona (one of the most productive areas for lettuce grown conventionally in the United States). In reality, hydroponicterm·dom greenhousesterm·dom come in a variety of configurations and can be located almost anywhere. As such, it would be reasonable to model multiple hypothetical greenhousesterm·dom. Changing the assumptions surrounding the hypothetical greenhousesterm·dom could produce alternative results. In addition, this study solely focused on direct energy inputs to hydroponicterm·dom and conventional lettuce production and did not consider energy embodied in chemical or material inputs. Performing a full life cycle assessmentterm·mth of hydroponicsterm·dom vs. conventional lettuce production could also produce alternative results, with labor hours potentially figuring prominently into the economic equation. Lastly, this study did not discuss additional factors that might inhibit the successful implementation of hydroponicsterm·dom, such as energy scarcity and higher upfront capital costs.

4. Conclusions Despite its high demand for energy, hydroponicsterm·dom remains a promising technology. Several factors could influence the feasibility of hydroponicterm·dom production of crops, specifically lettuce, in the future. As more sophisticated control devices become available, the cost of maintaining the controlled environment of hydroponicterm·dom greenhousesterm·dom could decrease. The future availability of water, land, and food will also influence feasibility through increased demand. Increasing land and water scarcity will make the more land- and water-efficient hydroponicterm·dom systems more appealing to city planners. Government and local grass-roots support could also influence the future of hydroponicterm·dom farming, as subsidies could be used to offset the high initial cost of hydroponicterm·dom infrastructure or more simplified hydroponicterm·dom systems take hold.

At this point in time, hydroponicterm·dom farming of lettuce cannot be deemed a more sustainable alternative to conventional lettuce farming techniques, but it provides promising concepts that could lead to more sustainable food production. In summary, hydroponicterm·dom gardening of lettuce uses land and water more efficiently than conventional farming and could become a strategy for sustainably feeding the world’s growing population, if the high energy consumption can be overcome through improved efficiency and/or cost-effective renewables.

Acknowledgments The authors of this study would like to gratefully acknowledge Alizée Jenck for her time, guidance, and feedback.

Author Contributions Guilherme Lages BarbosapersName, Francisca Daiane Almeida GadelhapersName, Natalya KublikpersName, Alan ProctorpersName, Lucas ReichelmpersName, Emily WeissingerpersName, and Gregory M. WohllebpersName contributed equally by performing the literature review, conducting the modeling analysis, and co-writing the manuscript. Rolf U. HaldenpersName directed the research and supervised the co-authors who completed this work in partial fulfillment of the requirements of the Fall 2014 Arizona State University course HON490-CEE470/570. All authors have given approval to the final version of the manuscript.

Conflicts of Interest The authors declare no conflict of interest.