since more than half of the world s population has been living in urban areas and the figure is estimated to exceed per cent by this is a hallmark of the transformation of humans economic base and social structure inasmuch as previously populations lived and worked primarily in rural areas by concentrating people investment and resources a process known as agglomeration cities heighten the possibilities for economic development innovation and social interaction more specifically cities also make it possible to lower unit costs so as to provide public services such as water and sanitation health care education electricity emergency services and public recreational areas polese satterthwaite along these lines cities also face challenges that threaten their efforts to achieve sustainability for example through improvement of access to and efficiency in the use of public services as well as reduction of their ecological footprint and financial fragility and the building of resilience against the adverse impact of natural hazards