Cities are what people do when they are not growing food

With more than half the world's population living in cities we have been told that cites are where humanity's future lies. At the same time, awareness of the future challenges humanity faces is 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 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 living within it or the land area the city covers? Where does a city end and the "countryside" begin? 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.

At the height of its power ancient Rome 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 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.

In today's fossil-fuelled world a city of 1 million is regarded as quite small. Indeed, in Adelaide 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". ("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 Adelaide must import many thousands of tonnes of food every day to survive, what of cities containing millions more inhabitants such as Sydney, Singapore 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 our only problem. 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.