The continuous increase in the number and size of urban regions across the world, and the simultaneous shrinking of cities in some regions, pose great challenges for sustainable development. Local challenges include for example loss of species and habitats, social degradation of neighborhoods, and an overall erosion of resilience.
Urbanization patterns and the lifestyles of urban dwellers also affect the planet on wider scales in time and space. They contribute to shape bio-physical processes on planetary scales, and affect how humans around the world mentally connect with the Biosphere. However, through their local to global linkages, cities can play a key role in the quest to continuously and increasingly support sustainable development.