For the last week I have been posting articles describing highlights from the Vertical Farming and Urban Agriculture conference held at Nottingham University last week. In this article I will draw together all the threads and try to present a picture of how I now see that cities will feed themselves in the future.

Why cities must feed their people

There are three dominant trends to which cities and national governments must respond to secure food supplies to their people.

1. Firstly, between 1980 and 2011 the global population not dependent on agriculture doubled (to 4.4 billion), also growing at a rate about five times more than that of agricultural population growth (according to the FAO).
2. Secondly, the amount of agricultural land available for growing food is declining and will be severely affected by climate change.
3. Thirdly, agricultural subsidies totalled an estimated $486 billion in the top 21 food-producing countries in the world in 2011. At the same time, agriculture and livestock remain a major source of greenhouse gas emissions – 4.69billion tonnes of carbon dioxide-equivalent in 2010, an increase of 13% over 1990 (IPCC). While efforts are ongoing to reduce emissions from energy production and transport, food consumption is being ignored.

The obvious point is that subsidies need to be redirected from unsustainable practices to sustainable ones. But what are they?

The options for growing food in cities

The growing of food in cities will never mean that conventional agriculture will disappear. At the moment we cannot grow staple foods such as grains indoors or at a small scale. We might be able to keep chickens and produce eggs in urban environments, but our consumption of meat protein is either going to have to decline considerably due to the greenhouse gas emissions associated with animal production and the inefficient use of land, or perhaps we will find a way to grow meat in factories without the need for animals (as is the mycoprotein Quorn).

But for the remainder of our diet we are able to choose now from the following range of options:

• community growing
• community supported agriculture
• allotments
• rooftop growing
• rooftop greenhouses
• growing on the vertical walls of buildings
• growing indoors, with: 
• hydroponics
• aquaponics
• aquaculture.

All of these are being practised now, and, if supported by a switch of agricultural subsidies, can produce perhaps as much as 30% of the nutritional requirements of city-dwellers. In many cases they do not even need support.

Add to this the use of hinterlands around cities, as used to be the case (Paris was able to feed itself in the 19th century from its hinterlands), and this would have the following benefits:

1. it would drastically reduce the carbon footprint associated with food miles
2. growing in a controlled environment means reduced food waste due to crop failure
3. linking people more closely to the origin of their food
4. improving health through fitness (where people participate in growing)
5. improving health through better diet
6. much reduced water pollution due to nutrient recycling
7. organic waste recycling will drastically reduce the need for artificial fertilisers (the Global Phosphorus Research Initiative says we could hit "peak phosphorus" production by around 2030, and nitrates cause pollution and global warming)
8. organic food (in a controlled environment there is no need for pesticides and herbicides)
9. greater water efficiency due to water recycling
10. it is safe and hygienic.

The corporate option

The vast majority of this local food will, I'm convinced, come from giant warehouse-type buildings. Growing outdoors, whether in vacant lots or on rooftops, without the use of polytunnels and greenhouses, leaves you open to the vagaries of the weather and the strictures of the seasons.

In a controlled environment you can have several growing cycles the year: for quick-growing leafy vegetables such as spinach perhaps as many as 20. Root vegetables can be continually harvested if grown in a soilless environment without the need to dig them up and kill the plant.

So there will be one or more of these giant sheds (with or without glazing) in every city, owned by the likes of Amazon, or perhaps in the form of franchises like McDonald's, Starbucks and Costa. They may include in their portfolios high street cafes, hotels and restaurants where you can pick your tomatoes and greens and eat them fresh because they are grown in the same building. Topager-Pullman restaurant in Paris has a rooftop garden supplying its kitchens (below).

But in these warehouses there will be tiers of rack after rack of vegetables being grown in precisely controlled conditions, with or without fish to provide compost (most commonly without). Amazon already has the technology to monitor what is happening in remote, upper corners of its climate-controlled warehouses and automatically bring down products on demand for shipping.

From a consumer point of view you will be able to order your vegetables using an app, and they will be harvested and delivered to your door within hours: fresh, local and organic.

These warehouses will be fine-tuned to minimise energy use and produce the maximum amount of nutrition and produce for the minimum amount of inputs. Far more efficient than what happens in an open field.

You might find this a horrifying prospect, but the choice is stark: either this or starvation for many of the 10 billion people who will walk the earth in 2050 and beyond.

The technologies to produce and operate these warehouses is already here. They are on the cusp of being commercially viable. A marketing push will be required to persuade people to buy the products – or perhaps they won't care where their food comes from.

Small scale options

In Detroit, Farmed Here's CTYO and Head of Development Paul Hardej runs a profitable indoor vertical farm, the first and only USDA Organic Certified and HACCp certified and audited aquaponic VF in the USA. He says that it has a greater than 95% success rate on plants compared with 70% for traditional farming in the United States.

The Mid-West of America has been furnished with 90% of its fresh vegetables from California up to now, but the ongoing terrible drought throws the sustainability of this into question. The region must grow its own food.

Green Spirit Farms near New Buffalo targets underused urban space to create its vertical farms and has designed a system that is simple to operate and harvest. it has a stacked indoor growing area that yields 12 harvests per year compared to 45-50 days in California, or traditional farming in Michigan. It takes up to 5 days to ship produce from California but Green Spirit can deliver locally in hours.

During the London Olympics at the sustainable agriculture summit, Green Spirit grew salad for 125 participants – carrots, strawberries, radish, raspberries, peppers, spinach, romaine from a specially built pop-up farm, demonstrating how quick they are to erect and get producing.

Green Spirit's rotary growing unit.

Green Spirit and Farmed Here are examples of pioneering urban growing enterprises that support the local community, source everything locally, and train unemployed teenagers. They are highly ethical.

This part of the industry is similar to the state of the World Wide Web at the beginning, until it became big enough to be of interest to giant corporations.

The same will happen to indoor growing.

City administration

City administrations need to change as well. There are no established, widely used standards, no building codes, no proper regulations that let city authorities issue permits to developers who want to grow food or livestock such as fish in cities. Right now it's problematic to get permission.

In Chicago Hardej had to construct an undercover test farm as a pilot to develop his technology before he could persuade the Mayor that his idea worked.

R&D

In the UK, CambridgeHOK, Phillips Lighting and Stockbridge Technology Centre are validating work on plant responses, measuring the number of grams per mole of nutrients and working out how they can scale up. Tim Haworth of CambridgeHOK says that the economic models "need to be worked out, factoring in how the model is affected by local situations, outside climate and expected prices. Automated processes also need to be refined to cut labour costs further and maximize the use of space to maximize the productivity per cubic meter".

The Netherlands already boasts many experts in the technology required. Dr. ir. Jan Westra, a strategic business developer for Priva, which manufactures control technology for indoor growing, travels the world looking for business opportunities. "We can completely controlled indoor climates. In Holland we use underground heat storage to moderate internal temperatures, storing solar heat in the summer and releasing it in the winter. It's very efficient. We control light levels and humidity also," he told me.

Elsewhere, phase change materials or passive cooling might be used to control indoor temperatures, or heat pumps for refrigeration. Technology for controlling CO₂ levels and air change frequencies is also being developed. "The idea is to make a modular system that comply can play anywhere in the world," says Haworth. "We will also need to provide training for future operators."

These buildings might even be able to satisfy their own energy requirements using heat pumps, dye-coated solar surfaces, geothermal energy, anaerobic digestion and so on.

A rooftop growing experiment in Paris is testing the results of using different substrates from urban organic waste. Given a choice of compost, vermiculture, coffee grounds with mycelium and compost or crushed wood, different root and leaf vegetables will thrive on different substrates. The same experimenters, from the Agronomic Research School, have tested leaves of plants grown in cities for lead concentration and, fortunately, found them to be well below the danger threshold. Anne-Cécile Daniel said that they are even producing honey and cereals in their pilot programs.

It's not just food, but pharmaceuticals that will be going this way. Caliber is growing tobacco to extract chemicals for medicines. Microsoft and Twitter boast GSky-made living green food walls at their HQs, an aesthetic installation that also provides food.

Community initiatives

Community growing fuses communities, generates links, makes people happy, but not everybody wants to do it. For many people growing their own food is a drag. It's not for everybody.

But when people actively participating in sourcing their own food, as in Vetch Veg, Swansea, or foraging workshops in Nottingham or Agrocité in Parisian suburbs at the foot of high rise aprtment blocks with a community garden and sales to residents, it galvanises individuals, creates new respect for nature which needs to be brought into cities to regenerate them, and reconnects urbanised humanity to the soil with which it was once intimately familiar.

For this reason community growing initiatives are vital in whatever form they take.

Cities that adopt these practices will thrive and achieve greater food security. The pioneering cities may even be held to export their expertise. Every area will find their own patchwork of solutions. There is a smorgasbord of options, but it seems that local, organic food will be more on the table than it is now.

This is the last of a series of articles on matters arising from the International Conference on Vertical Farming and Urban Agriculture. The others are:

• Urban Agriculture – A Next Big Thing for Cities
• The Nine Challenges to Food Security That Threaten Our Ability to Feed the Cities
• Launch of the international Centre for Urban Agriculture in Nottingham
• How Singapore and Japan are Feeding Cities with Low Carbon Indoor Farms
• China's Indoor Farming Research to Feed Cities Leads the World
• Cityfood: Encouraging Urban Agriculture and Forestry in Developing Countries