Just over a decade ago, in 2008, almost half of the world’s total population used to live in urban areas. This phenomenon has continued and is expected to gain further momentum in future. According to United Nations (UN), it is projected that the world urban population will increase by 72 percent by 2050, from 3.6 billion in 2011 to 6.3 billion in 2050 of which the majority of the population will be concentrated in the urban areas of the less developed regions.The Dark Side of Dhaka 2021.
Cities such as London or New York in developed countries have had very slow growth between 1970 and 2011, compared with cities in the developing world. India, for example, already has three megacities (Delhi, Mumbai and Kolkata) and with the addition of Bangalore, Chennai and Hyderabad, India is expected to have six by 2020. The country will have the largest concentration of megacities in the world. This gives one an idea of the pressure of population growth in cities of developing countries.In the 1970s, there were only two megacities with a population of more than ten million. At present, 9.9 percent of the world urban population lives in 23 megacities. The number is projected to increase to 37 in 2025 when they are expected to accommodate 13.6 percent of the world urban population. The number of people living in megacities has increased almost tenfold in the past 40 years, from 39.5 million in 1970 to 359.4 million in 2011, and could double again by 2025 (UN 2012). Interestingly, the largest increase in urban population between now and 2050 is expected to be concentrated in Asia and Africa.The concept of megacity evolved at the end of the twentieth century to describe the large urban agglomerations of the world. Few inconsistencies exist in literature regarding the population threshold used to define a megacity. For example, Mitchell defines a megacity as having a population exceeding one million, whilst others have used a five million or an eight million population threshold. Most concrete definition is given by the UN (2003): a megacity is a conurbation that houses ten million people or more; which is widely accepted nowadays.