ABSTRACT
Three hundred years ago, few people lived in a city. Globally, just five cities had a population over half a million people, and even by 1800, only 3 per cent of the world population lived in settlements of more than 25,000 people (Table 6.1). Cities were mostly small with a densely populated core with intense environmental impacts, surrounded by a mid zone acting as the main resource base, followed by a distant but largely untouched peripheral zone. Today, half the global population is urban, and collectively they exert an unprecedented impact upon the environment, from local to global levels (Table 6.2). Delivering urban sustainability is thus critical to improving the lives of urban people, and the rest of the planet, people and ecosystems, impacted upon by their activities.
