Amongst discussions about sustainable or regenerative cities the subject of food supply is often forgotten, and yet food comprises 23% of the average ecological footprint of a person in the developed world.

We are in the age of Petropolis, of globalisation, where the carbon and other impacts of food supply are magnified as a result of the distance it travels, the mode of travel, and the heavy use of phosphates, nitrates and other aspects of intensive agriculture.

In the age of Agropolis, cities supplied much of their own food from their hinterlands, which also were fed by the 'night soil' bought out during the daytime to fertilise the land. (Terms borrowed from Herbert Girardet's Creating Regenerating Cities.)

Due to the massive growth in cities in the last hundred years and projected growth into the future this is no longer possible.

So something has to change. Cities must once again produce much more of their own food. New research finds that, surprisingly, cities are already producing much more food than was previously thought.

A study published in the November issue of the journal Environmental Research Letters establishes that an area greater than the size of the European Union is being used already around and within cities worldwide to grow food.

It finds in urban and peri-urban areas an estimated "total area of urban croplands of up to 67 Mha with 24 Mha irrigated and 44 Mha rainfed. Including peri-urban areas within 20 km of urban extents, we found up to 456 Mha of total croplands; of which, 130 Mha are irrigated and 327 Mha are rainfed croplands."

The area of the European Union's member states is 442.3Mha.

Map of the percentage of urban land given over to growing food around the world.

"This is the first study to document the global scale of food production in and around urban settings," explained co-author Pay Drechsel, a researcher for the International Water Management Institute (IWMI).

The team acknowledges that the study could actually be conservative, as it focused on urban areas with populations of 50,000 or greater.

The danger is, though, that with rapid urbanisation much of this land will be built over. Its value needs to be recognised and the practice of growing food needs to be preserved. When citizens have forgotten how to grow food, as with many second and third generation citydwellers, it is much harder for them to learn again.

The study remarks that: "Understanding the role of urban and peri-urban crop production in urban food security at scale remains a major knowledge gap in the field of urban agriculture."

Nevertheless, one conclusion of the above study is as follows:

"Using a more conservative estimate of urban extent boundaries, Martellozzo et al estimate that a mean of thirty percent of the global urban extent area would need to be allocated to meet actual urban vegetable consumption. However, this value varied substantially between countries ranging from 1.2 to 397.4 percent or urban extent land area. In contrast, this study found countries allocating an average of 4.8 percent (SD 8.3) and 13.4 percent (SD 13.3) of urban extent areas towards urban irrigated and rainfed croplands, respectively."

In both studies the role of small and medium urban extents was substantial. While these two studies are not directly comparable due to differences in the urban extent boundaries used and the differing metrics of urban vegetable demand and urban cropland extent, these findings nonetheless suggest that urban and peri-urban croplands are playing a non-negligible role in meeting urban food demands at the global scale.

Bar chart showing the location of urban and peri-urban agriculture around the world.

So how much of our food does this supply? More than you might think, since in some cases "multiple crop rotations occur over the period of one year", but as yet there is a dearth of research on this.

What kind of crops are being grown? Mostly staples, including vegetables, wheat, rice, fodder grasses, maize, sugarcane and even cotton. The study does not take into account animal stock.

So the Martellozzo study published in the same journal suggests that around one third of the ground area covered by every city needs to be given over to growing food. This is an increase of between six and three times as much area as at present. But this assumes traditional agricultural practices and not the use of indoor farming – which can be multi-story – nor the use of rooftop farming.

Future cities must consider the multiple ways in which food can be provided locally, which include:

• Indoor and vertical farming;
• Rooftop farming;
• Hydroponics and aquaponics;
• Community supported agriculture;
• Community gardens;
• Allotments and dachas;
• Gardens.

This would have the following benefits:

• More productive land use with far fewer negative environmental impacts;
• More employment
• Greater physical and mental health and well-being, reducing the burden on the welfare state and health service
• Requiring little or no taxpayer subsidies, unlike much conventional farming
• Improvement of the local economy, resilience and food security
• Reduction in greenhouse gas emissions
• Increase in biodiversity.

This is particularly true if combined with closed loop recycling of organic waste to produce compost.

A city that is already practising this is Adelaide. Here, 180,000 tonnes of compost is produced at a central unit just outside the city from organic waste produced in the city, and returned to the soil in the hinterlands for growing food every year.

The future of food must be increasingly local. Read more here.