With more than half the world’s population living in cities we have been told that cites are where humanities future lies. At the same time, awareness of the future challenges humanity faces are growing. The climate is warming and critical resources such as energy and fresh water are, or soon will be, in decline. The cry has gone out for “sustainable cities” and urban planners the world over are responding. In most people’s (and urban planners) minds cities primarily consist of people to accommodate and methods to transport them. Shall we have urban sprawl or people stacked into multistorey apartments along transport routes in “transport oriented developments” (TODs) or some other arrangement? The problem with answering this question is that urban planners have forgotten the fundamental reason thing that allows cities to exist and that will determine their existence in future.

How big is a city? Are we talking about the number of people that live within it or the land area that the city covers? Where does a city end and the “countryside” begin? As we travel outwards from a city we usually pass a zone where housing becomes less dense and fields open up. When the number of buildings is sufficiently few we think we have left the city behind. In the minds of urban planners and most city inhabitants each city is an island connected to other “city islands” by air, roads, rail lines and sea lanes.

The boundaries of cities in medieval Europe were defined by walls built to protect against aggressors. However, conquering armies showed the falsity of this idea. By laying siege to the cities the inhabitants could only survive so long as their food stores lasted. A city simply cannot exist without its surrounding “foodshed”. Today cheap and rapid transport fueled by oil brings food from all over the world to large cities inhabited by millions of people. Today, a large city’s foodshed is the entire world.

A typical (responsible) daily diet for an average human might consist of 50 grams of protein, 300 grams of carbohydrate and 60 grams of fat, i.e. just over 400g of food. (If you live in the “industrial” world you probably eat considerably more.) This means that, at an absolute theoretical minimum, a city of 1 million people would require over 400 tonnes of food per day. However, this weight does not include fibre, the water content of food or the large amount of waste generated during preparation and left after eating. It does not include packaging. Thus, a city of 1 million inhabitants consumes many thousands of tonnes of food items per day. This food is grown in vast agricultural areas outside of what we usually regard as the city. But if a city cannot survive without its foodshed can we truly regard farms as separate from a city? Where exactly does a city end and farming begin?

Before cities existed humans were nomadic and harvested food from the landscape though which they walked. Only when agriculture developed could people “settle” in one place. However, agricultural settlement itself is not sufficient to produce a city. It is only when farmers produce food surplus to their own needs that some people can give up farming and do something else. Cities are what people do when they are not growing food (or wandering around trying to find it). Put another way, the most fundamental constraint on cities is their food supply. Without a foodshed, and the energy to transport food to the city from it, a city cannot exit.

When was the last time you heard an urban planner talk about the food supply? For most urban planners food supply is an “externality”. Is can simply be assumed to happen independent of what happens within a city. But this will not be true in a future, resource-constrained world.

At the height of its power ancient Rome is said to have supported 1 million inhabitants. To feed these people grain was imported from all over the Mediterranean. Every year over 400,000 tonnes of grain arrived at Rome on ships! That is around 1,150 tonnes per day brought to Rome from as far away as Egypt without the use of fossil fuels. However, since navigation of the Mediterranean Sea was suspended during the four winter months this weight of grain needed to arrive at an even greater rate during the 8 months when transport was possible. Rome relied on stored grain to survive the winter.

As described in Merchant vessels and maritime commerce in Roman times:

In Nero’s time, the arrival of the grain fleet from Alexandria during the month of June was welcomed as an event of great importance. The merchant ships were escorted by warships and preceded by tabellariae ships, which announced the arrival of the fleet which would release the populace from hunger. Seneca has left us a dramatic description of the excitement that would overcome the crowds in the port of Pozzuoli, in the Campania region.

In today’s fossil-fuelled world a city of 1 million is regarded as quite small. Indeed, in the city of Adelaide in Australia where I live, urban planners and developers (and the politicians they sponsor) continually cajole us that our population of 1.3 million is too few and that we are insufficiently “vibrant”. (Greater “vibrancy” is the most commonly heard excuse for expanding Adelaide’s population but vibrancy is surprisingly difficult to define and even harder to measure.) Apparently, self respect and the vital respect of other cities can only be earned by possession of several million inhabitants. (Interestingly – and inconveniently – these views contrast with the results of a survey by the Property Council of Australia showing that Adelaide’s citizens are happier with the “liveability” of their city than any of their bigger-city cousins in the rest of Australia.)

If a city like Adelaide must import many thousands of tonnes of food per day to survive, what of cities containing more than ten million inhabitants such as London, New York or Mexico City? More importantly, how will these tens of thousands of tonnes of food be delivered and distributed every day without cheap and abundant fossil fuels? The cities of 100 years ago had far smaller populations and much closer foodsheds.Distribution of food could exploit the muscle power of animals. In our sprawling modern cities that is no longer possible….

In the last few years, the rising price of oil has led to dramatic increases in the price of food. (The two are intimately coupled.) This led to food riots, export bans and political unrest in poorer parts of the world such as some Asian and Middle Eastern nations where the need to purchase food already claims most of people’s incomes. However, in the wealthier industrialized nations where incomes are higher the main concern over fuel prices has been the cost of transport. In particular, rising fuel prices have undermined the household budgets of the residents of the “outer suburban asteroid belts” of sprawling American and Australian cities who have found driving to work increasingly expensive.

The rising price of fuel and the idea that supplies of oil could soon decline has focused the minds of developers and their politicians who can no longer promote urban sprawl as a viable future vision. What to do? In Adelaide the developers sponsored a round-the-world junket for their politicians and senior “public” servants to promote the new vision of TODs where cities will now grow upwards along existing transport routes instead of outwards. In the past years in Adelaide we have seen this new paradigm dramatically enforced as permission for multistorey developments has been rammed through against the wishes of many local councils and the loud (but pointless) protests of our city’s disenfranchised inhabitants.

If we ignore the higher energy costs of multistorey buildings and the social failure of previous multistorey urban developments then TODs can appear to be a rational response to higher transport costs due to scarcer fuel. However, there are two fatal flaws in the TOD concept that urban planners have missed.

First, TODs are being used as an excuse to allow further expansion of urban populations. Thus the outer suburbs, although poverty-stricken, will remain in the future while more and more people are crowded into the city centre. There will be no decrease in total fuel or energy use, only a slower rate of increase and the problem of supplying cities with ever increasing tonnages of food from great distances will not be solved. There will be an inevitable conflict between TOD-facilitated population growth and declining fuel and food supplies (with TODs inevitably the loser).

Second, declining fuel supplies are not the only threat to our future capacity to grow food. Other critical and irreplaceable resources – such as phosphate fertilisers that are essential to modern agriculture – are also in decline. Fortunately, while oil can only be burned once, essential nutrients such as phosphate can be recycled as long as we capture our human wastes rather than flushing them down the toilet and out to sea as we do currently. But even if you could capture wastes from TODs, how could you then transport large volumes of this heavy material back to the distant agricultural foodshed when transport fuels are in short supply? The energy to do this simply will not exist.

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? We cannot blithely assume that a solution exists since for many cities the scale of the problem may already be too large to remedy. However, for the sake of discussion let us assume so. 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 minimize 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. We should be opening up areas upon which to grow food, not building on them.

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. According to cubaagriculture.com:

In Havana, 90% of the city’s fresh produce came from local urban farms and gardens, all organic. In 2003, more than 200,000 Cubans worked in the expanding urban agriculture sector.

In 2003, the Cuban Ministry of Agriculture was using less than 50% of the diesel fuel it used in 1989, less than 10% of the chemical fertilisers and less than 7% of the synthetic insecticides. A chain of 220 bio-pesticide centres provided safe alternatives for pest control.

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.

In the time remaining before the limitations imposed on us by declining resources become obvious in the western world (they are already undeniable in many developing nations) urban planners need to 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.