Defining the 'urban critical zone' for global sustainable development

With urbanization, cities are becoming new landscapes, significantly altering the properties, processes and pathways of previous natural environments. The natural critical zones that have existed for millennia are rapidly getting modified by the superimposition of the urban components to give rise to a new critical zone system, viz. the urban critical zone. The clogging and bypassing of the natural process pathways and the increasing demands of urban populations for ecosystem services put the native critical zone and the adjoining zones under stress. To elucidate this point, we present a case study on Delhi, the capital city of India, to demonstrate how the urban critical zone is unsustainable. We exemplify the increasing demand and supply gap of basic ecosystem services, such as clean air and water, that are essential to sustain life. In doing so, we redefine the limits of the critical zone in urban areas, recognizing that significant parts of cities are beyond the presently defined critical zone.