Mick Pearce was one of the speakers during the 'African Perspectives' event held December 6, 2007 in Delft in the Netherlands, a joint initiative of the Faculty of Architecture at Delft University of Technology and the ArchiAfrika Foundation. He delivered a lecture about the consequences of the upcoming energy crisis for the development of cities: 'From Eastgate to CH2: building on the energy watershed'.

From Eastgate to CH2: building on the energy watershed

december 2007 -

Figure 1 depicts an installation made of discarded tins by Congolese artist Bodys Isek Kingelez, of Kinshasa. His vision of utopia is a high-energy megacity, unlike the reality of Kinshasa which, according to locals, is a city composed of ‘swept up villages’. The expectations people have of the need for development is still so very much alive – in Africa, Europe, Asia, the Americas and Europe – despite the energy crisis we face, that I thought, like Kingelez, I would try to make up my own city for the new age, not in tins but in words. But, first I need to remind myself what it means when we say that fossil fuels are fast running out.

To do this I need to describe how two recent buildings that I have been associated with respond to this energy imperative. These are Eastgate in Harare, Zimbabwe, and CH2, which is presently being built in Melbourne. CH2 will be followed by the retrofitting of the existing council house, called CH1. I also need to explore the science of ecology that lies behind the design of these buildings. After a short biographical note which relates to the development of my thinking on architecture and building, I outline the development and predicted fall of the fossil-fuel age. I then consider aspects of the scientific theories and laws. I ask in what ways cities are like forests and termitaries, and people are like elks and beavers. I conclude with a brief consideration of the importance of collaborative architecture and of developing fruitful metaphors for the urban architecture we need today.

The oil is running out

The city today is almost without exception a successful enterprise, in that 80 per cent of the planet’s assets are owned by 20 per cent of the people, all of whom live in cities (Carley and Spapens 1998). Their exuberant lifestyles impose impossible demands on the hinterland through the city’s eco­footprint. This lifestyle also imposes unattainable expectations on the remaining 80 per cent of the planet’s people. It is a way of life that, we are told, is attained with ‘democracy and development’ and it carries with it, consciously or unconsciously, a lie – a lie because, in order for all six billion people on the planet to achieve this lifestyle, the natural resources of four more planets would be required. Here is the dilemma. There is no question of the rich 20 per cent giving up anything, because it would mean a lowering of their living standards, and there is no question of the 80 per cent giving up the expectations so graphically presented to them every hour from cyberspace.

Yet, the city still functions quite well: possibly it is a bit dysfunctional at the edges but, as the city is where every wants to live, the majority supports the enterprise. The problem is that the lifeblood or energy that is necessary to power cities is not only disastrously harming the environment and social order but it is also running out. The world isn’t about to run out of oil, but demand is now at 80 million barrels a day and as this grows, the production of easily extracted oil will peak and then start to decline. The view of when demand will come to a peak varies from 2006 to 2016 for the world outside the Middle East and, for the world including the Middle East, from 2023 to 2040 respectively. There is a growing body of opinion that even these estimates are on the optimistic side (National Geographic, 2004).
Such predictions are notoriously bad and depend on the bias of the prophet, but as city designers we have to assume that there will be a shift of the energy base during the life of the developments we are now building. The price of fossil fuels will rise and rise, never to return to previous levels as soon as the peak is reached – and that could be very soon. And where will energy to fuel the city come from unaware of our complete reliance on fossil fuel until the pump runs dry.

Living by sunlight in 2050

In the year 2050 humans have begun to relearn to live by sunlight just as the rest of the natural world had been doing for the previous three billion years. What Fredrick Soddy had predicted in the 1920s has come true. He saw the fossil fuel age as a flamboyant period of history, during which the natural capital stock of entrapped solar energy was used up. He saw the fossil fuel age as a passing phase, after which humanity would return to live by the sun. This vision has now become true, because the limiting factor to growth was the rate of flow of energy supplied each day by the sun. For as long as there had been enough resources to exploit growth, mainstream economics made sense. When oil reserves peaked in 2010, followed by the peak of natural gas reserves in 2020, the price of fossil fuels rose. World poverty and conflict could no longer be solved by development that was driven by economic growth. There were not enough natural resources left to support a high energy-consumptive lifestyle to fulfil the expectations of the 11 billion people on the planet. The price of fossil fuel made burning it in your car like burning diamonds. Three international laws now govern the use of natural resources and, therefore, world economies. These are the laws of scale, allocation and distribution which formed the basis of our taxation system.

The law of scale limits the rate of use of resources. Every 60 minutes the sun delivers enough solar energy to run all human activity on the planet for one year. The problem in the past has been how to harvest it, how store it and, finally, how to use it at a rate, within the capacity of the natural ecosystem, to absorb the resultant entropy. The law of entropy (based on the second law of thermodynamics) has allowed us to explore the great complexities of natural systems. Every living organism has been given a metabolic rating which measured the organism’s capacity to transform energy both within its living boundaries (let us call this its skin) and, where applicable, within the inanimate extensions to the organism. Thus, in the case of human individuals, the car, the house and all associated appendages are included as an added extension to their overall metabolic rate, based on their total consumption of energy and matter. In extreme cases this ends up rating a well-equipped human as being equivalent to a 30-tonne mammal.

The law of allocation encourages the efficient use of resources. Ever since this law was passed there has been a burst of innovation unequalled since the early days of the Industrial Revolution. The technologies invented during the machine age continue to be found useful only if they can consume far less energy than they did then. Fossil fuel, which used to be burned, is now used for extremely high-performance materials. Industrial designers and architects use biological forms and the processes of nature for their inspiration. The design teams of the built environment are composed of new disciplines that lean more towards the physiologist than the mechanical engineer, because the human is now seen as the participant in a living system; the building itself. The building has is seen now more as an extension of human physiology than the ancient idea that is a machine for living in.
The law of distribution controls access to resources. This issue used to be the greatest challenge to world peace. International conferences were now focused on two main global crises, security and the survival of the planet. The idea of living by the sun had already begun to drive the energy industries of the post-industrial nations. Most people in the formerly underdeveloped world had not lost the knowledge of living by sunlight and have therefore in many respects led the way out of the survival crisis by their example. They also benefited from this shift in the energy base because there is more sunshine in the south than in the north. However, while this has helped to redress the balance of power between north and south, the expectations of the wealth produced in the city lingered and urban terrorism and its counterpart, the war on terror, persists. On the corner of every street in every city in the old developed world, a camera recorded every movement, and surrounding the earth beyond the ionosphere, satellites monitored all life below.

In the south the cities grew in a totally different way but ended up with the same form. The self-building traditions, which had begun several decades earlier, had improved. They had started originally using recycled materials and as confidence and population pressure had grown, more permanent materials had replaced plastic and corrugated iron. The form of these cities was based on the old traditional rural home but clustered into densely packed villages that were surrounded by intensive agriculture. These villages were networked together with roads for buses and bicycles, and cars for the rich ran on homegrown bio-diesel fuel or biogas. The cities had become vast networks of linked sub-centres. In the developed world the cities had also changed. The central business districts had diminished in size and importance as they became far less viable without access to fossil fuel. The sub­linked the former suburban centres, which have also now grown in importance. The private car is now powered with a fuel cell that uses liquid hydrogen (produced from renewable power) as fuel. The car has become a mini mobile power plant when parked by the house, either feeding the power web or the house directly. The model city is a polycentric, fully networked city, which resembled the ecosystem.

Web City

By 2050 the information age will have moved completely into a new biological age. The fully motorized garden city image will be history. At this time cities will have begun to look more different from each other; reflecting more and more their historical, social, economic and natural context.

In Europe the walled city-state of the pre-industrial era was limited in size to a day’s journey by horse. This all changed with the coal-driven machine age. The railways fed the dark, satanic mills and an expanding hinterland of industrialized farming. This all changed with the information age. The dark, satanic mills were replaced with the central business district, where the towers grew higher and higher as the endless network of motorways linked the garden suburbs and vast open fields of the ever-expanding urban footprints. This all changed when the oil and gas began to run out and the eco­footprints overlapped.

So that now by 2050 a new biological age is well under way. Web City is becoming a vast network of small centres linked by webs. An energy web is increasingly fed by the sun, and the information web links the energy web to the cyber sphere that now surrounds the planet. The business of growing food without oil has become possible with the introduction of highly intensive solar-powered agriculture, producing food close to the home. The habitat and farming is now becoming culturally linked. The house reflects now much more than ever the business of harvesting solar energy to produce shelter, food, mobility and communication. People meet these days in the centres for social exchange less often than they used to, but with greater celebration.

When I began writing this description I began to think that those who are the losers today could be the winners in the new solar age, or at least, their ability to cope with living by sunshine would place them in a much stronger position. I also began to think of how my life and work can be brought more in line structural integrity and adaptability of natural ecosystems.

Born white in a black world

Being born white in Zimbabwe was to be privileged from the start. My parents were liberal and I lived a charmed life. I was called Ishe, which means ‘prince’. I was sent farther and father away from home as I grew up, going to schools and universities in South Africa, Canada and the UK so that, I returned to Africa, even more privileged than when I had left. In the meanwhile, things at home had changed. Black power was alive. My father died suddenly, leaving my mother vulnerable to white power. She had become increasingly pro-black (against white power). I decided to stay around in neighbouring Zambia, rather than pursue a career as an architect in London, where I had been trained at the Architectural Association. Zambia had achieved black power and had embarked on the fast track to development and I was soon very busy there.

I also became politically involved with the struggle for freedom in Zimbabwe. It was the late 1960s and the world was clearly divided. Because, like my mother, I was pro-black I became increasingly involved in fast-tracking black power at home in Zimbabwe. By the early 1970s my mother had caught the brunt of our efforts and was arrested by white power. She managed eventually to leave and go to the UK. Soon after this my family and I also moved there. We lived in a working-class mining and shipbuilding town called Sunderland in the northeast of England (called by an academic friend the ‘armpit of England’). Here, for the next nine years, I helped to start and manage a radical building co­operative called Sunderlandia.

In 1982 we returned to a Zimbabwe freed from colonialism, and became totally absorbed with the development which is tied to western aid and promoted by the west. In 1985 my first seeds of doubt about the idea of development were sown when I heard Bill Mollinson give a lecture in Harare on permaculture. This way of thinking provided a logical connection between ecology and architecture. Fast tracking development in Zimbabwe had caused a foreign currency crisis, due mainly to the demand for imports of plant consuming high levels of energy. I designed and built three major commercial office blocks in Harare during the following six years, each one making a small step in the direction of low energy consumption. By 1991 my low-energy consuming buildings became very Eastgate building in Harare, a very large mixed-development building in the central business district. Eastgate was also ecologically based. Like a termitary it was ‘air-conditioned’ naturally. Cool night air was harvested and stored within the concrete structure of the building for dispersal the following day to cool the building passively. A number of smaller buildings followed in which we experimented with using rock stores and wind turbines.

Eastgate in Harare Zimbabwe 1992-6 based on a termitary

In 1999 Mugabe’s fast-tracking ran out of steam. Mugabe was clever enough to hang onto power by replaying the black/white power game. I moved in an opposite direction, to support the opposition to ZANU-PF, his party. As a result I was placed in prison for a short spell. This has led me to leave Africa until things cooled down and I gratefully accepted a job by City of Melbourne to help build CH2, the new low-energy council offices, in August 2002.

CH2 (Council House Two)

CH2 (Council House two) like Eastgate is a mixed use development comprising 1100m2 of offices and shops. Also, like Eastgate, it is ecologically based. Like a tree, the building exploits the natural environment of Melbourne and will be seen to do so. The ten-storey Swanston Street façade will appear like a timber box formed of slatted recycled timber shutters which will open and close as the sun moves. At night and each morning when the shutters are open, the building’s interior will be revealed.
Five 14 m high transparent tubes will be suspended from the Little Collins Street façade. These will enclose water showers, in which water will rain down onto glass deflectors where the evaporatively down to the basement for storing in the phase change materials (PCM) batteries (described below) in the basement. The air is directed into the shops for cooling. These shower towers will express the natural process of evaporative cooling that is achieved by adding water to the dry desert winds that characterize Melbourne’s climate.

Six wind-driven turbines top the towers rising up the northern façade. Here, solar energy in the form of wind will be harvested to aid power-driven fans that are needed to remove exhaust air from the offices.

Unseen from the outside, in the basement two new technologies will operate. One will mine the sewer in Little Collins Street. In this case 100 000 l of water will be harvested each day by filtration processes to produce A grade water, while returning the solids to the sewer. The other will store cold energy in PCMs stored in stainless steel balls suspended in water in four large pressure tanks. PCMs freeze at plus 150C. In this case the latent energy is exploited to store cold energy when the outside air is cold, usually at night. This is released to cold radiators suspended under the office ceilings to help maintain comfort conditions during working hours.

Both Eastgate and CH2 are forerunners of the city buildings of the near future solar age. The aesthetic expression locates the architecture within its natural, social, and economic environment. They do not look like anything that has been copied and pasted from cyberspace. More like a tree, they grow their look from their site in a rain forest.

CH2 being built in Melbourne at present

The city and nature

To call architecture ‘ecological’ implies a new relationship between the city and nature. The idea that the city can be seen in the same way as an ecosystem is at least as new as the word ‘ecosystem’. In western culture the city has generally been perceived as something that is separate from nature. As James Hillman (1995) reminds us ancient Athens kept wild, unruly nature outside the city walls. Nature, to the ancient Greeks, was dangerous and disorderly. It was also the home of Dionysus, the god of wine, intoxication and creative ecstasy, while Apollo, the sun god of male perfection, high art and science, the god of the order created out of disorder, presided inside the walls. The pristine city stood clearly apart from nature.

The idea of creating order from disorder is embodied in the laws of thermodynamics, which state that everything (in nature) tends towards disorder. Energy/matter flows from order to disorder, while life modifies energy/matter to create order from disorder. Today in the developed world wild, untamed nature is usually regarded with nostalgia, a place where beauty and peace reside. The suburban garden and the park represent nature that is preserved, often fenced in and protected. The car is used (too often) to convey urban dwellers back to these peaceful preserves of wild nature. It is also the urban dweller that campaigns most for nature conservation reservations. On the other hand, it is the urban dweller that demands that the city be kept clean. City pigeons are poisoned, in case they spread disease.

This, according to Hillman, is an Apollonian ideal, part of the inherited memory of the west. Thus we still have a city that is pristine but we also have wild nature that is contained either in the park, the suburban garden or the nature conservation area. Therefore, although there are so few scraps of land left on the planet that have not been affected in some way by human intervention and while we want to create order on a vast scale, at the same time we want to fence in and to preserve disorder.

To see the city as an ecosystem is to let Dionysus in. It is not only to see the city as a living system within nature but it is also to apply the sciences of ecology, physiology and psychology into the design of city buildings. I think that there is a need for us to understand the complexities of city in the same way in which scientists are continuously reconstructing nature For instance, the large-scale demolition in a city, such as what happens in China today, may be no different from clear felling in a natural rain forest. And the regrowth forest any different from the original mature rain forest?

In the case of the forest the difference is palpable, at least to a botanist. The process of regrowth involves the succession of plant species, starting with pioneers and ending, over a period of time with a diverse, mature forest: a process that can take 200 years. If you take a walk through the large district of a newly built city, it feels very different from a district that is more than a hundred years old. In the latter case there is a presence of historical diversity of style and also the richness of disorder. Biologists call a forest a climax forest when it achieves what they call ‘steady state’. When does a city become a climax city? A steady state is a physical state in which checks and balances hold the energy flows through the forest as steady as a spinning top, or as a spinning vortex. This state can be seen as an almost fixed structure, although it is within itself in a state or dynamic movement. This can also be described as state of negative feedback, the way the spinning balls of a governor control a steam engine or a thermostat controls an iron from overheating.

If I am right about the similarity between the city and an ecosystem, are there laws which apply to both? Are there ways of seeing the city which may help us to relate the processes of city building with processes in nature? Is ecological architecture nothing more than the demand for more by-laws that have been invented to protect some human construct of wild nature behind a fence, or is it a new way to see our cities as part of the process of evolution on the planet? In arguing for the latter, I have been influenced by Gaia theory and the laws of thermodynamics.

The Gaia theory is a model of nature, which according to James Lovelock, its originator, sees the evolution of all forms of life as so closely coupled with the evolution of the physical and chemical environment that they constitute a single evolutionary self-regulating process like the steady state described above. As Lovelock (2000, p. 25) says:

… the climate, the composition of the rocks, the air and the oceans, are not just given by geology; they are also the consequences of the presence of life. Through the ceaseless activity of living organisms, conditions on the planet have been kept favourable for life for the past 3.8 billion years. Any species that adversely affects the environment, making it less favourable for its progeny, will ultimately be cast out, just as will those members of a species who will fail to pass the fitness test.

The basis of James Lovelock’s contribution to the theory of Gaia is the laws of thermodynamics. The laws of thermodynamics are concerned with the relationships between energy, heat and work. The first states that the total amount of energy in the universe is constant. The second states that some fraction of energy is lost to random molecular motion, or entropy (or disorder), whenever energy transformation occurs. Another way of putting the first law is to say that energy/matter cannot be created or destroyed within the closed system of the universe: it can be transformed only from one state to another. Another way to put the second law is to say that in any transformation of energy some amount of energy becomes non-returnable or is no longer able to perform work. Entropy is the measure of this energy.

Figure 7 is called Georgescu-Roegen’s entropy hourglass.

It illustrate these two laws and relates them to the use by humans of time-trapped terrestrial energy/matter, which is usually fossil fuel and is here called terrestrial stock.

The first law: that total energy/matter in the universe is constant; is represented by the sand trapped in the hourglass. The sand in the top chamber, which has the potential to fall and thereby to do work, represents the second law: that entropy increases in a closed system. This top sand is referred to as low entropy matter/energy. The sand in the bottom chamber is high entropy (or used up) matter/energy. But, note that this hourglass cannot be overturned, confirming that the process is irreversible. The narrowing waist of the hourglass causes the rate of flow to be constant, in the same way as for instance, the daily delivery of solar energy to the earth.
Terrestrial, or virtually nonrenewable, low entropy energy (fossil fuels) is stuck to the side of the lower chamber. This time-trapped bio-solar energy called fossil fuel was formed during the last 150 to 250 million years. It was originally derived from the remains of living systems that captured solar energy through the process of photosynthesis and were buried in the form of low entropy energy/matter. This process is continuing all the time. Humans have found ways to transform this energy at whatever rate they chose, unlike solar energy. At present we are burning it up at the rate of one million years’ worth in one year – far faster than it is being formed – so that we have to consider it a non-renewable resource. The disastrous consequences of this high-entropy consumption are deal with later.

The laws of the ecosystem

How do these basic laws of physics apply to life? Organisms, which are highly ordered systems, are like eddies of low entropy (being high in embodied energy). They exist only by disordering the universe in which they exist. They are involved in transferring energy from disorder to order. By structurally modifying their environment, organisms manipulate and adaptively modify the ways energy and matter flow through the environment. In so doing, they modify the ways energy and matter flow through them. There are three principle groups of energy transformers in all ecosystems: the photosynthesizers (the plants), the consumers (animals) and the fermenters. The photosynthesizers use the energy of sunlight directly to fix carbon dioxide and water to make sugar and release oxygen to the air:

Sunlight + carbon dioxide + water = organic matter + oxygen

The consumers capture energy by consuming the organic matter and oxygen produced by the photosynthesizers.

Organic matter + oxygen = carbon dioxide + water + body energy

Some of the organic matter escapes oxidation from the above process and sinks into sediments. Here the fermenters digest it and convert it into carbon dioxide and methane.

(Waste) organic matter = carbon dioxide + methane (CH4) + body energy

This is an extreme over-simplification of the process of Gaia. However it gives a picture of a system in balance, in equilibrium, in a steady state.

The law of metabolic rate

The consumption of energy is the common basis against which to measure both the city and nature. Any living system exists within a dynamic state of flow of energy called the metabolic rate. Metabolism supplies the power that organisms need for growth, maintenance and reproduction. Specifically, metabolism is the measure of the transformation of energy undertaken by every life form. For example, a large mammal, such as an elk weighing 200 kg is 10 000 times heavier than a mouse weighing 20 g. However, an elk eats only 1000 times (not 10 000 times) more calories than a mouse. This, for the mathematically minded, means that the metabolic rate per gram of body tissue is proportional to body mass raised to the power of –¼. The metabolic rates of mouse to elk is 0.75 and 7500 respectively. Other biological rates show the same pattern. Lifespans increase with biomass to the power of -¼, meaning that each gram of mouse tissue uses energy at 10 times the rate of a gram of elk tissue and the mouse’s heart beats 10 times faster. But the elk lives ten times longer than the mouse and its gestation period is also 10 times longer. The same -¼ power rate applies to plants. The area of leaves relates to the rate of photosynthesis scale as the ¾ power of mass and the lifespan as the -¼ power. Looking beyond the individual, we can see that this provides a method of measuring the geometric structure of an ecosystem and the physical constraints on resource distribution networks transformation.
The ultimate niche constructers are humans. As Brown and West of the University of New Mexico says,

a woman in one of the most developed nations uses as much energy and has the same reproductive rate as a hypothetical primate weighing 30 tonnes. (Brown and West, 2004, p. 41)

This finding has important implications for about city construction. Try defining ways in which one might model the sustainable city on the ecosystem and you may find yourself faced with this ‘Queen Kong’. Imagine her in a rainforest without the SUV 4x4, or any of her first world trappings, and you have a modern Eve with a metabolic rate of 100 watts. Imagine her in a poor country today and her rate would rise to 300 watts; 200 watts more than the modern Eve because the rural peasant has developed agriculture methods and tools in ways which greatly extend her ability to harvest natural resources. Finally, imagine her back as ‘Queen Kong’ and her rate jumps up to 11 000 watts. Every watt above the original 100 (that is, 10 900 watts) is related to her external physiology which is powered mostly by fossil fuels. Metabolic rate is therefore a method of measuring the consumption of energy/matter by different organisms in a living system. How organisms grow is proportional to their body mass and hence also to their metabolic rate. Thus, the law of metabolic rate is a useful tool to understand the complexity of the ecosystem and the efficiency of the transfer of solar energy from one form to another. We know that it will take decades of further work before we understand just how far the pace of life controls the ecology of life on Earth. However, there are other aspects of this study, which link it to the design of the human built environment.
We cannot assume, however, that the forest itself in this scenario is a constant. It, too, has been affected by the increasing CO2 and temperature levels of the atmosphere, an increase due to human activity, including the introduction of fossil fuel-derived energy leading to global warming.

The extended organism

The story about Queen Kong is a clue to a link between the metabolic law of the ecosystem and Scott Turner’s idea of the extended organism.

Are animal-built structures properly things external to the animals that built them, or are they properly parts of the animals themselves?… By structurally modifying the environment … organisms manipulate and adaptively modify the ways energy and matter flow through the environment. In so doing, they modify the ways energy and matter flow through them. Thus, an animal’s physiological function is comprised really of two physiologies; the ‘internal physiology’… [within the skin] and an external physiology, which results from adaptive modification of the environment. (Turner, 2000, p.3, pp. 6–7)

In this book Turner argues that animals extend their physiology by modifying their immediate environment in order to increase their capacity.

Niche construction

I have recently come across the idea of ‘niche construction’ developed by Odling-Smee, Laland and Feldman (2003). These authors claim that niche construction, which has been ignored by biologists, is part of the process of evolution. Niche construction is about the extension of an organism (or super­organism). The nest, the burrow or the beaver’s dam and, in the human case, the hunter’s bow, the farmer’s hoe and the megacity are all niches. They are modifications to the surrounding natural environment, which have become embodied into the animal’s total physiology. It is as though there are two physiologies, one focused inside the animal’s skin and one occupying the space between the animal’s skin and the boundaries of the nest. The standard view of evolution is that organisms adapt to a given environment. The new view is that organisms modify their environment for survival. This is a two-way process, in which the environment itself adapts to the niche constructor. By altering the environment to which it is adapting, the organism has a short-term involvement in the evolutionary process. This theory has important implications for the role of human city building.

The termitary is important for me, not only because it helped to inspire the design of the Eastgate development in Harare, but also because it is a clear example from the nonhuman animal world of architecture. The termite tower can be seen as performing the same function as a lung, and the fungi garden as performing the same function as a stomach. Air is harvested through a mucus-glued earth membrane, probably by using the diffusion of gases by osmosis. A mixing process driven by buoyancy forces and powered by the heat of the metabolism balances the oxygen/carbon dioxide levels. The air and the termites themselves circulate the energy. Water is mined, not only for their own consumption, but also for cooling the air and for their elaborate building process. The queen termite performs the function of the mind and the communication system is the nervous system. In my search for an architecture that goes beyond the age of the machine, this is a perfect model. The building, no longer a machine for living in, has become the building as a living system. Unfortunately, not all niche constructions are functionally integrated: some contribute to disorder on a runaway scale. That is, the rate at which they create disorder is faster than the total system can cope with. As we know, this applies to the process of human niche constructions in our cities, our farms and our methods of travel, which are all related to our use of time-unlocked fossil fuel energy.

Economics of limits

Today we are faced with a world population of 6 billion people to be fed, kept warm, transported and entertained in cities whose increase in size is made possible by fossil fuel. The concept of sustainable development must reflect a shift in our perception of how human economic activities relate to the natural world. The ecosystem is finite within its boundaries and it is therefore materially closed. The demands of human activities on the ecosystem for the regeneration of raw material inputs and the absorption of waste outputs must be kept at ecologically sustainable levels. This means we must start to replace the accepted economic norm of growth with that of a qualitative improvement of the ecosystem resulting from creatively managing the way we construct our niches. As Daly (1996) argued in Beyond Growth, there are three problems to be recognized in connection with the distribution and use of natural resources – allocation (the efficient use of energy/matter), the just distribution of energy/matter amongst all humanity, and scale (the rate of use in time of energy/matter relative to the capacity of the ecosystem to absorb the resultant waste).

Conclusion

Rome was the last solar city. Before the fossil-fuel age the size of a city was limited to the capacity of the surrounding ecosystem, the transportation systems and the population’s capacity to exploit natural resources as well as its power to exploit surrounding communities. The population of ancient Rome peaked at one million and declined to 30 000 as the empire collapsed and the ecosystems of North Africa (where most of the wheat came from) also declined.
Or was it Tenochtitlan? The Aztec city of Tenochtitlan, on islands in a now-drained lake upon which Mexico City lies today, may also have reached a population of a million but this may have been due to the development of an extensive market-gardening system fertilized by human excrement, rather than an empire. This is a picture of the city seen by Cortés before he razed it to the ground. It was a truly solar city, modelled on an ecosystem. The market garden system, called chinampas, produced seven crops a year. The secret (which has only recently been discovered,) of this highly intensive urban agriculture was the existence of a heat-loving microbe similar to those found in hot springs. This bacterium binds nitrogen from human waste, neutralizes dangerous pathogens in sewage and speeds up the decomposition of organic waste. The gardens were made on narrow promontories built out into the lake and they were watered and fertilized by skin buckets, which scooped up water and sludge from the canals between promontories. This sludge was rich in human waste. If this is a true description of what happened, this is a model of sustainability.

References
Brown, J. and West, G. (2004). One rate to rule them all. New Scientist, 182, 2445, 38–41
Carley, M. and Spapens, P. (1998). Sharing the World: Sustainable Living and Global Equity in the 21st Century. Earthscan.
Daly, H. E. (1996). Beyond Growth. Beacon Press.
Hillman, J. (1995). Beauty Without Nature. Audio Cassette: Sound Horizons Audio-Video.
Lovelock, J. (2000). Gaia, the Practical Science of Planetary Medicine. Gaia Books.
National Geographic (2004). Issue 90.
Scott Turner, J. (2000). The Extended Organism: The Physiology of Animal-Built Structures. Harvard University Press.