Food from 458 m2—calculation for a sustainable, circular, and local land-based and landless food production system

Four hundred fifty-eight meter square is the available cropland per person throughout Africa, if the population will increase 4 to 5 times towards 4.3 to 5.9 billion people in 2100, the maximum estimation of the UN 2019 (95% confidence interval). This space is not enough for food sovereignty, if the low African yields remain. Even with the global average yields, nearly 3 times higher than African yields, will not allow food sovereignty. Hunger, wars, diseases, and mass migration can be the consequences already long time before 2100. Nevertheless, food sovereignty is possible, but not in the way as it is done up to today by governments and development projects. In the future, intensification of (yields) and/or expansion (grassland, forest: LULUCF) of agriculture will not be able to produce enough, nutritious, and affordable food for everyone. But clever combining of land-based and landless food production can be a solution for a local, sustainable, and circular food security. Maize and soybeans are best for WFP minimum diets and have the best yields. Using insects and earthworms as protein source can deliver enough and nutritious protein, and local photoreactors can produce oil/and/or starch for food energy. Later can be large industrial and very small household scaled. This “out-of-the-box” system approach needs research and development. Every good research needs good questions and a concept with some simple calculations to assess the strengths, weaknesses, opportunities, and threats. Socio-economic aspects are often not considered enough in technical focused and far ahead R&D.

adaptive in diets and food sources. As omnivores, we can digest a wide range of plants, fungi, animals, and others, at least after processing and/or cooking (Gibbons 2007). Despite or because of this fact, it is difficult to find a "typical ration" for a "typical human" in kilogram of food per day. The need is defined on the basis of nutritional demand.
Life needs food for energy (calories, joule) and structural material for body growth and rebuilding, delivered as macro and micro-nutrients (carbohydrates, fats, fiber, minerals, proteins, vitamins) and water. Carbohydrates and protein deliver about 17 kJ (4 kcal) and fat 37 kJ (9 kcal) energy per gram DM. Vitamins, minerals, fiber, and water do not deliver energy, but are required as structural material, health components, and digestion. All food has at least some of the nutrients mentioned above. Not all food can be digested; therefore, the feces carry energy and structural material. Because it is so difficult to measure the food quantity per person per day, the energy and structural material (protein, etc.) is used for calculations.
The human food energy requirement is measured in kilocalories and Joule (1 cal is 4184 J), about 1 kcal per kg liveweight and hour as minimum energy without any  (FAO 2001). Energy and ingredients density are different between all the different foods. Due to availability, the food baskets and food cultures are very different throughout the world. Despite the high variability, it can be assumed, that about 2 kg food as fresh matter (0.75 kg dry matter) are the daily need for an "adult average human" (30 years, normal activity, healthy, temperate climate) (without losses, usually 25% extra). The stomach of such an "adult average human" has a capacity of 1 to 1.5 l and can digest about 1 to 1.5 times filling a day (depends on digestibility of food). Therefore, the stomach can digest about 1 to 2.25 kg fresh matter food a day. Food must have a digestible nutrient density, that fits with the capacity of the stomach. The EAT-Lancet commission does recommend 2.500 kcal/p/year, but is focusing on global food production for 10 billion in 2050 (EAT-Lancet commission 2019). This is not a realistic scenario for challenging regions like Africa.
The World Food Programme (WFP 2019) does offer a food basket for emergencies and refugees with 2100 kcal (10-12% from protein and 17% from fat). A recommended WFP-standard ration is composed by wheat, maize or rice, lentils, soybeans, or other pulses, vegetable oil (fortified with vitamin A and D), sugar, and iodized salt. Additionally, 1 to 1.3 g crude protein xP per person and day should be available (WHO 2017). Of course, this minimum ration is not enough for adult and hard-working man or lactating woman, but much more than a young child or an elder person needs. Nevertheless, in a society, this minimum ration should be fine, if people share it in context to the individual demand (elder people, adults, children; hard or less hard working) ( Table 2).
If we assume, that the WFP daily ration has 2100 kcal energy and 85 g protein, the minimum annual need per person is 767,000 kcal and 24 kg protein. 1 This has to be produced on available cropland or imported, if other option like landless food production is not considered.

Food production
Seventy thousand years was the collection of wild plant and hunting, the basis of food security. Till 10,000 BC, a maximum of 2 people per km 2 (50 ha per person) could find enough food and survive and only estimated 1 to 15 million pre-historic humans lived on the earth. With the invention of agriculture, about 12,000 years ago in Mesopotamia and adjacent areas (Bellwood 2005), humans have been able to produce more food per ha for increasing population densities (Puleston and Tuljapurkar 2008). In the year 1400, 500 mio humans used extensively 7% of the global land surface which have been used for farming (1.1 billion ha crop and grassland), respectively 2.2 ha per person. The year 1804 is seen as the first time when 1 billion humans lived on the earth. Today, 7.6 billion humans use 4.8 billion ha crop and grassland intensively (0.6 ha per person). The global population density has raised towards 57 people per km 2 (USCB 2019). Most of our recent food comes from landlocked plants and livestock, only 10% from fishing and aquaculture. Thirty-six percent of the land surface (13.5 billion ha, excluding Antarctica) is used for crop and livestock production. Further encroachment into deserts, forests, mountains, and frosty areas is difficult and/or costly.
A "good" diet is a balance between different food to meet the demand, and there are many different staple foods creating a food basket of local diets and food cultures all over the world. In the last decades, a harmonization of food cultures took place. Today, only 3 food plants (wheat, maize, rice) contribute about 60% of human food intake, direct as plant food or indirect as meat, eggs, or milk (FAO 2019b).
Compared to rice and wheat, maize is the most important food product of the world (Table 3). This crop covers already 14% of the total global arable land. A lot of the global maize production is for animal feed. Additionally, to maize, soybean is best choice as pulse for a WFP minimum ration. This legume is more valuable for a WFP ration than others, due to high protein and fat content (Table 2). Today, 8.6% share of the global arable land is cultivated with soybeans, like maize mainly for livestock feed.
Maize and soybeans have not only high nutritional and production yield advantages. These two plants grow in a wide climatic/weather range; high performance varieties for most of the earth are available. This does include GMO seeds, which are pest or herbicide resistant, and in the future, probably with higher water efficiency and salinity tolerance and, last but not least, high nutritional values (vitamins, amino acids, etc.). Because GMO are under ethical discussion (private business, patents, ecological and health risks), not invented for poor farmers but as expensive commodity, and the production costs with GMO are high (high input -high output systems) and therefore difficult for poor and remote small scale farmers, the ecological and socio-economic assessments of GM maize and soybeans have not been finalized yet.

Food insecurity can increase
The future food security is a global challenge and, for example, defined the United Nations Sustainable Development Goal SDG No. 2 (Zero Hunger) till 2030. Today, 240 (29%) of the global hungry 820 mio people live in Africa (FAO 2019a, b). It should not be forgotten, that never have been more people feed sufficiently on the earth (more than 6.5 billion), and obesity is a contradictory problem of hunger issues (1.2 billion people with BMI > 25). This is also the case for Africa. Nearly 1 billion people have enough food and 150 mio of them face obesity. Hunger is a problem of poverty, and a fair distribution of food does still not happening, despite enough food would be available for everyone and the UN has declared several programmes in the last decades (millennium and sustainable development goals).
The real challenge will appear after 2030. Most predictable, whole Sub-Saharan Africa and some countries in Asia (e.g., India, Pakistan, Bangladesh) will have severe food security problems in recent future, due to many factors. Ecological and soil degradation, water shortage, population growth, climate change, and socio-economic difficulties are already today observable, and the conditions for sufficient food production becomes worse in those regions, despite all efforts and developments of farming systems and food chains.
But, the main change and challenge will appear after 2050, particularly in dense populated and less developed areas of the world. For example, in Africa will be only 458 to 629 m 2 cropland per person available (Table 4). Increasing yields (very low) is difficult due to lack of knowledge and markets (farm inputs and outputs). Encroaching cropping on grassland and nature areas is limited and difficult due to lack of water, infrastructure, capital, and land rights. Food import is also limited due to lack of money and/or competitive products for the world market. Food aid seems to be the only option for most of the countries. Despite there will be not only food security problems in Africa, the following calculation will use this continent as an example to extrapolate the development. Comparable scenarios could be made for other countries and regions as well.
Till 2100, in 80 years, the African population will increase from 1.2 billion towards 4.3 or even 5.9 billion, and the continent will host about 50 to 60% of the global population. Already today, Africa is the continent of hunger and malnutrition (Rahmann et al. 2019). Land degradation, water shortage, and lack in agricultural infrastructures are obvious throughout the continent.
Food sovereignty for Africa is only possible, if the population will not raise above 5.3 billion (Fig. 1), and that (a) not more than the WFP ration is consumed, (b) no post-harvest losses, (c) the global crop production yields are achieved, and (d) only maize and soybeans are produced on all 269 mio ha African cropland. If the production yields will remain low like today, only 2 billion can be fed, even 0.8 billion people more than today. That even this can be achieved can be doubted, because already today 240 mio (20%) of the people face hunger and there is already food import to Africa. The continent has imported 2016 about 20 mio tons of maize for 4.1 billion US$ (205 US$/ton) and 2 mio tons of soybeans for 812 million US$ (406 US$/ton) (FAOstat 2019).
It can be doubted that an increase of African crop production towards global yields (280% for maize, 200% for soybeans) throughout the continent (Table 6). To increase the production from today's low external input-low output production towards today's global medium input-medium output yields, the  (2019).
If the production yields in Africa will remain low, 53 to 66% of the required minimum food has to be imported (Fig. 1). That would be between 413 and 737 mio tons of maize and 136 to 233 mio tons of soybeans every year (Table 5). If the food is produced for example in the Americas (highest continental yields with 8 tons per ha maize and 3.2 tons per ha soybeans), between 82 and 169 mio ha arable land would be needed to produce for food export to Africa. This would be about 5% and 10% of the global arable land today (2017).
Using the prizes of 2016 ("cost include fright" -cif-Africa: 205 US$ per ton maize and 406 US$ per ton soybeans; FAOstat 2019), the value of the African minimum food import in 2100 would be between 144 and 246 billion US$, every year. That would be between 34 and 42 US$ per person and year for food import to Africa (Table 5). The question will be: who will pay this? Probably, Africa will not have enough money to afford it, and food aid would be necessary.
Not only the production, but also the food distribution will be challenged. While rural people can produce their food on their own (subsistence), urban people have to buy food (market). Already today more than 50% of the African population lives in urban areas, and Africa's cities will grow very fast in the future. Today, none of the global biggest 10 cities is in Africa, but in 2100, five of them and all are meta-cities with more than 20 mio inhabitants: Lagos (88 mio; biggest city of the world), Kinshasa (83 mio, no. 2), Dar Es Salaam (74 mio, no. 3), Khartoum and Niamey (56 mio each, nos. 6 and 7) (Hoornweg and Pope 2017).

Solutions to reduce food insecurity
In context to the above worst-case scenarios for Africa, that not all needed food can be produced on limited local cropland, other options are needed: & Global: increasing of global food trade (from high productive towards high demand areas) and/or & Local: to produce it locally land-based (e.g., intensive gardening) and landless (balcony, indoor, roof, vertical, container, reactor food, etc.).
Both options have advantages and disadvantages, and both will remain in the future and need to be developed into the direction to solve future food challenges. Let us have a short view on the global option. Despite all the impacts and effects, the globalized food chain has brought, significant problems and risks cannot be ignored. The main problems are private-and profit-related global food chains, market difficulties (transport and processing disruptions, food demanding areas are not able to pay for imported food, and aid is needed), degradation, and contamination (pesticides, nutrients in water, drug resistant germs) of natural resources (soil, water, biodiversity, air, landscape). On the other side, the global land-based food is not free from risks like natural calamities (more frequent and damaging storms, droughts, floods) and last but not the least, political risks (e.g., wars, protection, embargos, terrorism). Therefore, global food chains have done a good job, but the impact has negative impacts as well. Several food system changes try to reduce the impacts, for example, organic agriculture with globally already 1.6% farmland share (IFOAM 2019), but this is not scaled-up enough, probably not good enough for the real future challenges, because the production yields are not high and the ecological impacts are not low enough (Rahmann et al. 2008(Rahmann et al. , 2009(Rahmann et al. , 2017 (Table 6).
Let us spend some thoughts regarding local option. Food sovereignty in very densely populated and low developed areas/regions is becoming less secure and safe in very dense populated and low developed areas/ regions. Not only productive farmland is becoming more and more scarce, but also enough and clean water, necessary nutrients, productive and healthy seeds, renewable energy and-very important-better knowledge of all actors in improving food systems sustainable (from production to consumption). For such conditions, we proposed a combination of land-based and landless food production for a local, circular, and sustainable food chain (Rahmann et al. 2019).

Space efficient food production
Maize and soybean are the most space efficient crops to produce a WFP ration. In the case of Africa, 489 m 2 / person would be necessary (Table 7), if global average yields can be achieved in Africa in the next decades, no food losses and only maize and soybeans are produced. 2 If the production yields will remain low like today, 1216 m 2 /person crop land would be needed, and-visa-versa-with the highest continental yields of Americas, 22% could be saved (382 m 2 /person). This shows, there would be a chance to achieve food sovereignty, but only in the case of very high yields.
With the assumptions in Table 5, we can calculate, that 28 to 36 US$ per person and year are the threshold for the landless production to substitute 118 to 210 kg imported food. This would cost roughly 0.20 US$/kg (maize import -cif-Africa: 205 US$/ton) and would be very low, compared to production costs of recent photo-reactors, which produce high quality products for cosmetics and food additives for 10 to 50 US$/kg dry matter.
Of course, these model calculations of minimum diet and minimum cropland space (Table 1) do not consider all aspects. Some other crops (e.g., potatoes, white beans) do have comparable high yields and product qualities; some areas allow more than one harvest a year; maize and soybean cannot produced everywhere. On the other side, food chain losses and nutritional and food culture needs are not considered. For this paper, these factors are not considered.

Local, circular, and sustainable food chains
If food is insecure and import and aid not possible, local food systems have to be developed. A local, circular, and sustainable nutrient, energy and food chain was designed (Rahmann et al. 2019). As shown in Average African production yields 2017: maize 2.1, soybean = 1.9 tons/ha/ year (FAOstat 2019). Assumptions: only maize and soybeans are produced with a land use relation of 71% maize and 29% soybeans, no post-harvest losses, no LULUCF, only WFP ration (2100 kcal/p/day): 190 kg maize and 60 kg soybeans per person and year.
In Fig. 2, the "green chain" displays the traditional nutrient and food chain: from cropping to human and livestock. Because this chain is not able to produce enough food, the "blue chain" has been added. Biomass from the "green chain," sewage, and waste-water from households are used for energy production and become homogenizing for a reactor-based and landless food production. Both chains together have to produce enough, healthy and affordable food for people in high populated regions and low development conditions. Let us have some look to the two chains. The crop production of the "green chain" is the core of the system. If only 360 m 2 per person is available, an intensive production for a maximum of needed food has to be carried out. Maize and soybean are the best crops to meet WFP ration demands, plus cabbage as an example for vegetables with high production yields and valuable nutrients and taste. A cultivation scenario of these three crops shall show the potential and limitations of the "green chain" (Table 8).
The nutrient flow in Average African production yields 2017: maize 2.1, soybean = 1.9 tons/ha/year (FAOstat 2019). Assumptions: only maize and soybeans are produced with a land use relation of 71% maize and 29% soybeans, no post-harvest losses, no LULUCF, only WFP ration (2100 kcal/p/day): 190 kg maize and 60 kg soybeans per person and year (Table 9). Figure 2 shows that it is not possible to produce enough food with low African production yields. Not protein, but production of food energy (kcal) is the main deficit. Less than one-third can be harvested. Protein is also a challenge, but it can be enough produced, if mushrooms are cultivated on the 70-kg non-food biomass from cropping (0.1 kg mushroom/kg biomass DM with 2.5% protein) and earthworms and insects are used for animal protein (insects with 50% and earthworms with 60% protein).
Average African production yields 2017: maize 2.1, soybean = 1.9 tons/ha/year (FAOstat 2019). Assumptions: only maize and soybeans are produced with a land use relation of 71% maize and 29% soybeans, no post-harvest losses, no LULUCF, only WFP ration (2100 kcal/p/day): 190 kg maize and 60 kg soybeans per person and year.
If global yields would be achieved on 458 m 2 cropland, not all the space would be necessary for maize (50% of total cropland) and soybeans (25%) (Average Global production yields 2017: maize 5.8, soybean 2.8 = 4.9 tons/ha/year (FAOstat 2019). Assumptions: only maize and soybeans are produced with a land use relation of 71% maize and 29% soybeans, no post-   Fig. 3). Up to 25% could be planted with vegetable and/or fruits. The system would be much more productive and efficient. Even chicken could be kept, fed with protein from mushrooms and crop production.
Average Global production yields 2017: maize 5.8, soybean 2.8 = 4.9 tons/ha/year (FAOstat 2019). Assumptions: only maize and soybeans are produced with a land use relation of 71% maize and 29% soybeans, no post-harvest losses, no LULUCF, only WFP ration (2100 kcal/p/day): 190 kg maize and 60 kg soybeans per person and year.
Average African production yields 2017: maize 2.1, soybean = 1.9 tons/ha/y (FAOstat 2019). Assumptions: only maize and soybeans are produced with a land use relation of 71% maize and 29% soybeans, no post-harvest losses, no LULUCF, only WFP ration (2,100 kcal/p/d): 190 kg maize and 60 kg soybeans per person and year.

Landless food production
Landless food production is just scratched by some pioneers and inventors. Recent research is going for, e.g., artificial meat (Ireland 2019), roof/vertical gardens (Southey 2019), and container hydroponics (Sustainia 2019). They are all producing with highly sophisticated in technology, infrastructure, knowledge, and hygiene  as well as capital intensive. Additionally, they are not linked to the land-and water-based food production nor the nutrient chains (e.g., human feces). Potential products targeting high price food chains. Therefore, all of them lacking the ability for solution of food insecurity in less developed areas/regions with very high population densities. Landless food production, using contaminated nutrients and produce for low/no price staple food for poor and fragile markets is an open research area. We initiated the project "LandLessFood" (https://www. t h u e n e n . d e / e n / o l / b y -s p e c i a l i s t -d i s c i p l i n e s /biodiversity/landlessfood/) as a conceptional model to cope with these defined pre-conditions and published it in Rahmann et al. (2019). To ensure food sovere ignty i n Africa, photoreactor-based food production would have to deliver 475,000 kcal/person/year, if only 458 m 2 cropland per person are available and the production yields remain low like today. This could be 53 kg oil (9000 kcal/kg) or 286 kg starch (3700 kcal/kg), produced by algae or bacteria.
The reactor-based production costs of food energy must be low, e.g., 0.06 US$/1000 kcal, if the food import costs (cheapest is maize with 205 US$/ ton cif Africa) is considered as economic benchmark. If the reactor food would be more expensive, the import of maize would be the better alternative (including the protein in maize), if available and payable. The market price for 1 kg of oil could not exceed 0.54 US$ and 1 kg of starch 0.22 US$, respectively. This would be 32 US$ per person and year, a very low price, but about 188 billion US$ for 5.9 billion people. Reactor food technology can be high-tech and industrial-like, but it would be much more suitable to have it low-tech, homebased, and robust. If algae or bacteria can be produced at home by people, some of the main costs in production can be ignored: buildings and labor. Simple and cheap technology for reactor-based production of oil or starch is a R&D challenge.
Funding Information Open Access funding provided by Projekt DEAL.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons. org/licenses/by/4.0/.