Quantitative guidelines for urban sustainability

The transition from today's largely unsustainable use of resources of all kinds to a world that is becoming increasingly sustainable will be won or lost in the next few decades, and in the planet's urban areas. In order to approach the challenges associated with this transition, it is necessary to quantify the sustainable requirements for a satisfactory urban life - energy, water, food, non-renewable resources, transportation, housing, and waste management (among a number of other possibilities). We have reviewed levels of current resource use in cities in the more developed and less developed worlds, and levels that would seem adequate for the urban populations of the future, and propose quantitative 'sustainability requirements' for consideration. We suggest that these levels, or something like them, provide a basis for planning, initiating implementation, and monitoring progress toward sustainable cities in the 21st century.