Cities are what people do when they are not growing food

Cities cannot continue to exist if declining oil and phosphate make feeding them impossible. Is there any solution to the problem of multiple simultaneously declining essential resources? For the sake of discussion let us assume a solution is possible. There are a number of things we must do. (Urban planners can regard these as design constraints):

1) We need to stop increasing the demand for food. This requires that we stop promoting population growth.

2) We must minimise the distance between food producers and food consumers both to reduce the energy required to transport food and to allow nutrients to be recycled back to food growing land.

3) We must encourage replacement of mechanized agriculture with physical labour.

Obviously these constraints mean that we must grow as much food as possible within cities. Composting toilets will allow us to recapture nutrients and recycle them back to the food growing areas. (Composting toilets also greatly reduce water use and save large amounts of electricity otherwise used for pumping and purifying water.) Alternatively, human wastes can be collected (locally) for anaerobic digestion to produce methane gas (for e.g. cooking fuel) and then the nutrient-rich residue of the process can be used as fertiliser. Food growing both within and close to cities can also provide much needed employment for people whose jobs disappear as the economy contracts due to declining energy supplies.

Cities have one pre-existing advantage over rural landscapes. A city represents a large area with a reticulated water supply that can be used to support intensive urban agriculture such as vegetable growing. But food plants also need sunlight and land to grow on. So open space without shadowing from surrounding structures is essential. Tragically, the city of Adelaide in which I live has very little in the way of park space (~5% of the metropolitan area is parks). Adelaide is a city of private gardens rather than public parks – and those private gardens are rapidly disappearing under subdivisions that leave no space for bare earth. To exacerbate the situation by now stacking people in TODs smacks of insanity.

The city of Havana, Cuba with over 2 million inhabitants is often held up as the poster child of urban agriculture. Over 40% of the city's 721 km² area is devoted to agriculture. In 2003, "90% of the city's fresh produce came from local urban farms and gardens, all organic [and] more than 200,000 Cubans worked in the expanding urban agriculture sector".

Cuban agriculture has the advantage of a tropical climate and fertile soils but the inefficient state agricultural sector means that this nation is still not self-sufficient in food.

If we are to plan for growing food within cities how much earth must be left uncovered to allow us to feed each inhabitant? The agricultural productivity of land varies greatly according to soil type, climate, water availability, labour input and many other factors so the land required to grow enough food for one person will be different in every city. However, claims have been made of intensive vegetable growing feeding one person from 400 square metres in a climate not so dissimilar to that of many southern Australian cities. When one considers that it is now common in Adelaide to divide suburban blocks down to 300 square metres and then to cover almost all of that space with a house and driveway we can see how far from long term sustainability we have come. (For food and shelter a family of four would need nearly 2000 square metres or two traditional "quarter acre" blocks.) If world oil supplies decline rapidly we may need to disperse a fraction of our urban populations into rural areas both to supply agriculture with labour and to put people closer to available food-growing land.

Urban planners must revise their ideas of what constitutes "sustainable development". Declining oil supplies mean that cities and their foodsheds must be united. Provision of food is the greatest future constraint on city structure.