By Richard Black
Environment correspondent, BBC News website
Apart from a few lower members of the animal kingdom, no-one other than human beings build cities.
Chongqing in China is growing fast - so is the pollution
They are totally artificial constructs and in them we live artificial lives. We travel differently, eat different food, receive water and energy through pipes and wires, live in different kinds of buildings, do different jobs.
All of these things come with an environmental price-tag. Given that the world's urban population is expanding at such a rate, it is worth asking what are the numbers on that price-tag, and whether they are higher or lower than the environmental cost of living a rural life.
Does a person produce more or less carbon dioxide on moving from the countryside to the city? If the answer is "less", how should that be offset against a bigger contribution to urban smog? Is trash piling up on a street corner better or worse than excess fertiliser running from farmland into the water supply?
How far does a city's environmental footprint extend beyond its boundaries - to the natural resources which feed it with water and food, or to the other side of the planet which feels its greenhouse gas emissions?
There is no simple answer.
Rush from rural
"What is needed is research that focuses on the large-scale, long-term environmental changes, not just on the immediate impact of cities," concludes the International Human Dimensions Programme on Global Environmental Change (IHDP), a global research alliance.
China has seen massive migration from areas of low agricultural productivity to coastal cities
"Until recently, this has been quite unexplored territory," it says.
It is territory which badly needs exploration if the full impacts of 21st Century urbanisation are to be fully understood.
"In China, 125 million people have moved from areas of low agricultural productivity to coastal cities - that's 25% of the workforce, and you've got another 25% waiting to move," says John Harrington from London's Overseas Development Institute (ODI).
"The rise in fossil fuel consumption in Chinese cities is now costing between three and six million life-years per year. Asia is going to see a ten-fold increase in the solid waste management burden by 2025."
Fuel use, intimately connected to urban pollution and greenhouse gas emissions, demonstrates exquisitely the problems in trying to compare the ecological footprint of the rural and urban dweller.
In 2002, the United Nations Development Programme (UNDP) focused much of its Human Development Report on China.
"Rural residents consume less than 40% of the commercial energy used by their urban counterparts," it concluded.
In Tokyo, air quality has improved over the past 50 years
"However, if biomass [principally wood-burning] is included, the average person in the countryside uses nearly one-third more energy than a city dweller."
So the rural resident apparently contributes more to global climate change than the urban citizen - but the equation hinges on how the energy is produced.
If "commercial energy" used in cities - principally electricity - is derived from renewable sources or nuclear stations, the urban dweller wins the eco-prize hands down. But if the rural citizen burns nothing but trees and always replaces them, he or she becomes "carbon neutral" and scoops the award.
"Urbanisation is a complex issue," observes Bert Metz of the Netherlands Environmental Assessment Agency (MNP), co-chair of the Intergovernmental Panel on Climate Change (IPCC) working group which looks at economic and social projections.
"Generally speaking, people in cities use more transportation and energy services than in the rural areas and rely less on biomass. Certainly that is a factor driving up greenhouse gas emissions, but there are many other drivers too."
Those other drivers include two further trends of our time, rising population and economic growth, whose imprint is difficult to distinguish from that of urbanisation.
Rules for the rich
In any case, talk of a "city environment" brings a basic question - which city?
In London and Tokyo, air quality has improved over the last 50 years. In Shanghai and Kuala Lumpur, it has gone down, though there are signs of improvement elsewhere in the developing world.
"It has happened in Delhi, for example, where there has been a huge improvement in air quality by substituting liquefied petroleum gas [LPG] for diesel in vehicles," observes John Harrington.
"Partly it is just that as cities become richer they can clean their act up, but it's also how vocal the middle classes become, which in India counts in a way it doesn't yet in China.
"But there's a paradigm question here - whether cities are being urged to move away from the traditional 'grow first and clean up later paradigm' to a 'sustainable growth paradigm'."
The "sustainable city" is a concept which has received a lot of academic attention in recent times.
An Australian group, the Halifax EcoCity Project, has developed what it calls an "ecological measuring stick".
Essential elements of sustainable urban development include, it says:
- extensive use of vegetation to filter pollution, prevent the "heat island" effect and capture carbon dioxide
- purification and recycling of all water and waste
- 100% supply of renewable energy
- a sustainable food supply which does not deplete nearby lands and grow as much as possible with city limits
The EcoCity concept, says Bert Metz, would have a major impact on greenhouse gas emissions.
"How cities are planned definitely has an impact. Are there many trees planted, which reduces the need for cooling? How are houses built - that has a huge effect on greenhouse gas emissions."
But by the Halifax yardstick, virtually every city in the developed as well as the developing world would fail the sustainability test.
Slices of life
Planning cities to allow for green spaces, wildlife, trees and watercourses can have a huge benefit on people as well as on the natural environment, says David Goode, a visiting professor at University College London and former director of the London Ecology Centre.
"When you've got more than half the world's population living in urban areas, it's crucial they have access to some kind of ecological area within a stone's throw of where they live," he argues.
"There's a lot of evidence that both physical and mental wellbeing increases with access to nature and green spaces."
Since the big clean-up began half a century ago, many British cities have become home to the fox, while birds such as the black redstart have become "urban specialists".
But is it real nature, or just a designed and planned imitation?
"It is different," says Dr Goode, "but you can encapsulate a lot of the features of natural habitat within a city environment."
In any case, much of the world's countryside is far from "natural", shaped as it has been by centuries of human agriculture, in some cases producing vast prairies of monoculture monotony where hardly a bird is heard.
But fundamentally people are not flocking to the vast cities of Asia and South America with nature and green spaces in mind.
They are coming for jobs, to improve their economic and social prospects.
It may be sobering then to consider the UNDP's judgement on China's urbanisation and its concomitant rising toll of pollution and waste: "Environmental factors are likely to constrain, or even reverse, social and economic progress."
It is the same old message: societies neglect environmental progress at their economic peril.