|Fig. 1: Lettuce at an indoor farm. (Source: Wikimedia Commons)|
Since the advent of agriculture some ten millennia ago, humans have been attempting to improve and expand upon the basic principle of a predictable and controllable food source. Many thinkers and leaders, beginning most notably with Thomas Malthus, have worried that the exponential growth of humanity would at some point outstrip the natural productive capacity of the Earth and human population growth would stagnate.  So far, the ability of humanity to innovate new agricultural solutions has kept pace with the population, so the agricultural potential of the world has grown and Malthusian mass starvations have been avoided. Technologies like crop rotation, fertilizers, pesticides, GMOs, and industrial equipment have kept production growing in line with demand.  However, unused arable land in the world is at a record low and the distribution of land does not align with the distribution of human population. 
In considering what humanity's next successful agricultural innovation will be (if there is one) much has been made of indoor farming. Indoor farming in its entirety encompasses many principles, but in general it can be divided into partially indoor farming and fully indoor farming. Partially indoor farming is not a particularly new idea; greenhouses have been employed since Roman times to keep plants warm while still allowing them full access to sunlight.  Fully indoor farms are more novel, and encompass such modern concepts as urban farming, vertical farming, and hydroponics. Here we will critically examine the current and potential future energy use of many of these systems to discern whether any can be economically or environmentally feasible.
Greenhouses in their simplest form (i.e. without additional lighting, climate control, etc.) do not add to the energy costs associated with growing produce in a traditional outdoor farm, except as far as harvesting cannot be done at the same scale, the effect of which is minimal. In fact, greenhouses can save energy because they allow land which would otherwise not be able to support some crop to stay warm enough to grow it. This reduces shipping, which is now a significant portion of the cost and energy consumption of agriculture's many types of produce in the United States travel an average of more than 2,000 miles to their destination.  A more conservative weighted average of all produce purchased in the States across all seasons is about 1,400 miles; at 160g CO2 per ton per mile on average for trucks, this means that shipping a ton of produce on average generates .25 tons of CO2 alone.  Although there are other environmental effects of outdoor farms, shipping and other vehicle costs are a large environmental energy costs.
Many crops can be grown in non-endemic places with "supplemental" lighting, i.e. some percentage of their lighting can come from the sun and some from artificial lights. Generally, the lights that are most efficient (in terms of cost, energy, and effectiveness) are LEDs.  LED technology has improved steadily, but there are many physical limits to its long-term improvement. We take lettuce as an example because it has a very high water weight (meaning that it needs relatively little input energy per unit weight) and because it has been extensively studied (see Fig. 1). Based on the daily light intake of lettuce, the current efficacy of LED lights, and the average CO2 cost of power in the US,  we arrive at an overall energy cost of 8.0 kg of CO2 per kg of lettuce (where the average head of lettuce is about 150g). Although there are other costs associated with outdoor farming than those mentioned above, we see that we are already working from a large deficit in trying to make indoor farming produce less CO2 than traditional farming.
Depending on the crop and the growing strategy, various other energy costs could be incurred by a partially or fully indoor farm for climate control (which includes heating, cooling, humidification, CO2 control, air circulation, water control, etc.).  Note that a given farm does not always optimize for minimal energy or environmental footprint - energy efficiency could always be sacrificed if, for example, production per unit area is the primary purpose of the farm--which means that much depends on the purpose and mission of the farm and on the relative costs of the options. Furthermore, environmental factors greatly affect the cost of many of the climate control cost (e.g. hot outside temperature necessitates more cooling).
There are many additional environmental and energy benefits, and some other detriments, to various types of indoor farming. For example, there is no consensus on how to account for the energy and environmental costs of constructing the indoor farms (this is a fixed cost at the outset, but it is typically high - should it be averaged over the lifetime production of the farm? How much will that be? Etc.).
On the other hand, if we are truly worried about meeting the food needs of a population which will soon outstrip current technology's ability to feed it, energy is hardly our biggest concern. The necessities of arable land, potable water, and easy consumer access could all outweigh concerns about energy efficiency. For this reason, innovation and technological advancement in indoor farming should be prioritized even though they may not be more energy efficient than traditional farming in the short (or even the long) term.
© Zack Swafford. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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