Achieving more sustainable cities
Traditional settlements were delineated and structured by transport and production systems based on human or animal power.
A major effect of fossil fuel-based technology has been that the high density of traditional cities has given way to urban sprawl. Fossil fuel-powered transport, starting with railways, steam ships and more latterly cars and aircraft, has also caused many cities to stop relying on resources from their local regions and to become dependent on an ever-widening area, creating an ecological footprint.
As cities draw resources from increasing distances, they also accumulate large amounts of inert and toxic materials within themselves – that is to say, pollution. Waste gases and water expand the negative impact of cities at a regional and increasingly global scales.
A large part of the increasing amount of carbon dioxide in the atmosphere is attributable to combustion in the world's cities. In addition, most rail, road and aircraft traffic occurs between cities. Virtually all the world's climatologists agree on their concern about climate change, resulting mainly from fossil fuel burning enhancing the natural greenhouse effect.
In nature, a circular metabolism is developed whereby every input is also able to renew and sustain the living environment by recycling the outputs. In the past medieval cities had something approaching this relationship, with the following closely linked nearby: market gardens, orchards, arable and grazing land, local water supply, forest products and so on.