Ecological impacts

The more valuable and lighter resources such as fruit and vegetables, wood and metals may be drawn from areas hundreds, or thousands, of kilometres away. So may cheaper foods and other natural resources, if these are not easily produced locally. The more bulky, low value materials will usually come from close-by, as can be seen in the brick works, quarries, claypits, sand and gravel pits in and around cities, all of which have environmental impacts.

Garbage, Mathare, Nairobi. Credit: UNEP

Cities are also centres for resource degradation. Water needed for industrial processes, for supplying residential and commercial buildings and for transporting sewage is usually returned to rivers, lakes or the sea at a far lower quality than that originally supplied. Solid wastes collected from city households and businesses are usually dumped on land sites in the region around the city while much of the uncollected solid waste finds its way into water bodies, adding to the pollution. Air pollutants generated by city-based enterprises or consumers are often transferred to the surrounding region through acid rain.

Regional impacts

These arise mainly from:

• the expansion of the built-up area which is often over valuable agricultural land or land that has valuable ecological functions (such as wetlands) and the transformations this brings as valleys and swamps are filled, large volumes of clay, sand, gravel and crushed rock extracted and moved, water sources tapped and rivers and streams channelled.
  

• the demand from city-based enterprises, households and institutions for the products of forests, rangelands, farmlands, watersheds or aquatic ecosystems that are outside its boundaries. Many cities have outgrown the capacity of their locality to provide fresh water or have over-used or mismanaged local sources so these are no longer usable (see Securing Water for Cities).
  

• The 'export' of solid and liquid wastes and air-borne pollutants which can damage fisheries by untreated liquid wastes, land and ground-water pollution from inadequately designed and managed solid waste dumps and, for many of the larger and more industrial cities, acid rain.

Urban river pollution. Credit: UNEP

Global impacts

• The demands of larger and wealthier cities for food, fuel and raw materials are increasingly met by imports from distant ecosystems. This is one reason why cities such as London, Stockholm or Washington DC can maintain high environmental standards on their periphery, including preserving natural landscapes.
  

• In addition, goods that can only be made by using high levels of fossil fuel, water and other natural resources, and which involve dirty industrial processes and hazardous conditions can, conveniently, be imported. These cities' surrounds would look very different if they had to produce the natural resources used within the city and absorb the resulting wastes the city.
  

• This means that the environmental costs of the goods used in wealthy cities are borne by the people and eco-systems from which these goods are imported. Other cost transfers are into the future. For instance, air pollution may have been cut in many wealthy cities but emissions of carbon dioxide (the main greenhouse gas) remain high. Cities generate close to 80 per cent of all carbon dioxide emissions.

Mapping ecological footprints

• It is difficult to estimate the ecological costs that arise from producing all the inputs that support a city - the large and diverse range of raw materials, intermediate goods and final goods. To do so would require an accurate mapping of the scale and nature of resource inputs and also an assessment of the ecological impacts of their production.
  

• Wealthy and powerful cities have always had the capacity to draw resources from far beyond their immediate surrounds. But the scale of this capacity to draw on the productivity of distant eco-systems has been greatly increased in the last few decades as incomes have risen and transport costs declined.
  

• Particular enterprises and richer income groups contribute disproportionately to ecological footprints. High-income households in rural or suburban areas generally have larger ecological footprints than those with comparable incomes living in cities.
  
  
• Modern cities can generate environmental costs far from their boundaries. But the long term health and ecological consequences of many chemical wastes are unknown - including those arising from the accumulation of certain persistent chemicals. And it is difficult to adjust the calculations for a city's 'ecological footprint' to take account of the goods and services that its enterprises produce for those living outside its boundaries.
  
  
• Measures are needed to reduce the ecological footprint of wealthy and large cities, but this must not detract from cities' key roles within the efficient, prosperous, innovative and flexible economies that all nations want to develop. In addition, prosperous cities can also be highly efficient in their use of resources and generation of wastes.
  

• Cities provide the main market for rural produce (and farmers' incomes) while rural inhabitants and enterprises draw on urban enterprises for goods and services.

Environmental management

Good environmental management can limit the tendency for cities to transfer environmental costs to its surrounds. Here are some examples:

• enforcing pollution control to protect water quality in nearby water bodies, safeguarding those who draw water from them and also fisheries;
  
• an emphasising "waste reduction, re-use, recycle" to reduce the volume of wastes that are disposed of in the area around cities; and
  
• comprehensive storm and surface drains and garbage collection systems which reduce non-point sources of water pollution.

Cities also have many potential advantages for reducing resource use and waste:

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• the close proximity of so many water consumers gives greater scope for recycling or directly reusing waste waters.
  
• the use of motor vehicles (and the fossil fuels, air pollution and greenhouse gases that their use implies) can be reduced because cities allow many more trips can be made through walking, bicycling or public transport.

City governments are accountable to the populations living within their boundaries, not to those living in distant ecosystems on whose productivity the city producers or consumers may draw. It is also difficult for city authorities to take account of the needs and rights of future generations without a supportive national framework.

FURTHER READING:

Wackernagel, Mathis and William Rees (1995), Our Ecological Footprint: Reducing Human Impact on the Earth, New Society Publishers, Gabriola (Canada).

David Satterthwaite (editor) (1998), The Earthscan Reader on Sustainable Cities, Earthscan Publications, London.

McGranahan, G. , P. Jacobi, J. Songsore, C. Surjadi and M. Kjell�n (2001), Citizens at Risk: From Urban Sanitation to Sustainable Cities, Earthscan Publications, London.

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