At the start of the twenty-first century, cities emerged as the source of the greatest challenges that the planet has faced since humans became social. Although they have proven to be humanity's engines of creativity, wealth creation and economic growth, cities have also been the source of much pollution and disease. Rapid urbanization and accelerating socio-economic development have generated global problems from climate change and its environmental impacts to incipient crises in food, energy and water availability, public health, financial markets and the global economy