
In 2007 the United Nations estimated that, for the first time in recorded history, more than half of the world's population lived in urban areas rather than rural ones. The shift was driven by decades of rural-to-urban migration and natural growth concentrated in the rapidly expanding cities of Asia, Africa and Latin America. It marked a demographic threshold humanity had never previously crossed. Projections at the time anticipated the urban share rising toward two-thirds by the middle of the century.
The crossing made the city, rather than the countryside, the default setting of human life and thus of human-environment management. It reframed urban planning from a specialist concern into a central instrument for housing, health, climate and economic policy worldwide.