Current global urbanisation is unique for a number of reasons.
What took Europe centuries to achieve, Africa and Asia is doing this in less than 50 years. We measure urbanisation as the rate of people moving to cities. Here there is a difference between net migration and circular migration. Urbanisation is defined by the movement of people from rural to urban areas and implicit in this understanding is net migration. Simply those people that move to cities permanently. This rate is faster in Africa than anywhere else in the world, and we can measure this per hour, per year per decade.
Flow is measured by people moving in and out of cities, economic geography creates flow and we are seeing new spatial forms of cities, mega-cities with different spatial configurations. Interestingly, cities not constrained by geography such as rivers, lakes, oceans and mountains are boundless. These include cities such as Johannesburg and Los Angeles that make up of clusters, networks of smaller cities or what we call city-regions.
Over the course of human history we have become more and more proficient in building and expanding cities. But with the current wave of urbanization, we face tremendous pressure to deliver service infrastructure (housing, water, sanitation, electricity) to the vast majority of urban dwellers. Here we run into limited resource constraints. Is there enough steel? Enough water? A ready supply of cement? Part of the challenge is to provide enough suitably located land, service this land with infrastructure and provide housing for informal dwellers. Cities face the great resource crunch? Can we innovate faster than the rate at which our cities are growing?