Urban sustainability: Dilemmas, challenges and paradigms

With the unprecedented growth of urbanization, now encompassing half of the world population, global sustainability is increasingly an issue of urban sustainability—considering the impact of cities on the rest of the globe and of the sustainability of life in the cities themselves. The dilemmas this poses are outlined, and two paradigms for addressing them are discussed: the city as a concentrator and the city as a bio-social-machine entity.