Despite 1.2% of the Earth's surface is currently urbanized, Over 50% of the world's population lives in these areas.
Throughout the course of time, the human population had grown substantially. It changed from half a billion in 1600 to a billion in 1800. By the 20th century it had reached almost to 3 billion and has currently exceeds 7 billion. (purkis et al remote sensing and global environmental change)
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Urban areas are created from population explosion and the pace of migration from rural areas to cities. By the year 2025 (less than 10 years away) 60% of the world's population may live in cities. (purkis et al remote sensing and global environmental change)
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According to the 2005 revision of the UN World Urbanization prospects report, the 20th century was the "rapid urbanization of the world's population" as the global proportion of urban dramatically from 13% (220 Million ) in 1900 to 29% (732 million) in 1950 and 49% (3.2 billion) in 2005. The report also mentioned that by 2030, the population will live in urban cities equivalent to the 1987 world population (5 billion). (purkis et al remote sensing and global environmental change)
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(purkis et al remote sensing and global environmental change)