A megacity is usually defined as a metropolitan area with a total population in excess of 10 million people. Some definitions also set a minimum level for population density (at least 2,000 persons/square km). A megacity can be a single metropolitan area or two or more metropolitan areas that converge. The terms conurbation, metropolis and metroplex are also applied to the latter. The terms "megapolis'" and megalopolis are sometimes used synonymously with megacity," though those terms denote a semi-continuous chain of large metropolitan cities.
As of 2011, there are 21 megacities in existence, which is the official figure despite the list below containing 27 megacities  – with conurbations such as Mumbai, Tokyo, New York City, Dhaka, and Mexico City having populations in excess of 20 million inhabitants each. New Megacities like Johannesburg have population of over 10 million.
In 1800, only 3% of the world's population population lived in cities, a figure that has risen to 47% by the end of the twentieth century. In 1950, there were 83 cities with populations exceeding one million; by 2007, this number had risen to 468. If the trend continues, the world's urban population will double every 38 years. The UN forecasts that today's urban population of 3.2 billion will rise to nearly 5 billion by 2030, when three out of five people will live in cities.
This increase will be most dramatic on the least-urbanized continents, Asia and Africa. Surveys and projections indicate that all urban growth over the next 25 years will be in developing countries. One billion people, almost one-seventh of the world's population, now live in shanty towns. In many poor countries overpopulated slums exhibit high rates of disease due to unsanitary conditions, malnutrition, and lack of basic health care. By 2030, over 2 billion people in the world will be living in slums. Over 90% of the urban population of Ethiopia, Malawi and Uganda, three of the world's most rural countries, already live in slums.
By 2025, according to the Far Eastern Economic Review, Asia alone will have at least 10 megacities, including Mumbai, India (33 million), Shanghai, China (27 million), Karachi, Pakistan (26.5 million), Dhaka, Bangladesh (26 million) and Jakarta, Indonesia (24.9 million people). Lagos, Nigeria has grown from 300,000 in 1950 to an estimated 12.5 million today, and the Nigerian government estimates that the city will have expanded to 25 million residents by 2015.
For almost a thousand years, Rome was the largest, wealthiest, and most politically important city in Europe. Its population passed a million people by the end of the 1st century BC. Rome's population started dropping in 402 AD when Flavius Honorius moved the government to Ravenna and Rome's population declined to a mere 20,000 during the Early Middle Ages, reducing the sprawling city to groups of inhabited buildings interspersed among large areas of ruins and vegetation.
Chinese capital cities Chang'an, Kaifeng also experienced huge population booms during prosperous empires. According to the census in the year 742 recorded in the New Book of Tang, 362,921 families with 1,960,188 persons were counted in Jingzhao Fu (京兆府), the metropolitan area including small cities in the vicinity.
The medieval settlement surrounding Angkor, the one-time capital of the Khmer Empire which flourished between the 9th and 15th centuries, could have supported a population of up to one million people.
In 1950, New York City was the only urban area with a population of over 10 million. Geographers had identified 25 such areas as of October 2005, as compared with 19 megacities in 2004 and only nine in 1985. This increase has happened as the world's population moves towards the high (75–85%) urbanization levels of North America and Western Europe. The 1990 census marked the first time the majority of US citizens lived in cities with over 1 million inhabitants.
In the 2000s, the largest megacity is the Greater Tokyo Area. The population of this urban agglomeration includes areas such as Yokohama and Kawasaki, and is estimated to be between 35 and 36 million. This variation in estimates can be accounted for by different definitions of what the area encompasses. While the prefectures of Tokyo, Chiba, Kanagawa, and Saitama are commonly included in statistical information, the Japan Statistics Bureau only includes the area within 50 kilometers of the Tokyo Metropolitan Government Offices in Shinjuku, thus arriving at a smaller population estimate. A characteristic issue of megacities is the difficulty in defining their outer limits and accurately estimating the populations.
Based on the population criteria, the world's 27 megacities are, in rank of population:
Rank Megacity Country Continent Population Annual Growth 1 Tokyo Japan Asia 34,300,000 0.60% 2 Guangzhou China Asia 25,200,000 4.00% 3 Seoul South Korea Asia 25,100,000 1.40% 4 Shanghai China Asia 24,800,000 2.20% 5 Delhi India Asia 23,300,000 4.60% 6 Mumbai India Asia 23,000,000 2.90% 7 Mexico City Mexico North America 22,900,000 2.00% 8 New York City USA North America 22,000,000 0.30% 9 São Paulo Brazil South America 20,900,000 1.40% 10 Manila  Philippines Asia 20,300,000 2.50% 11 Jakarta Indonesia Asia 18,900,000 2.00% 12 Los Angeles USA North America 18,100,000 1.10% 13 Karachi Pakistan Asia 17,000,000 4.90% 14 Osaka Japan Asia 16,700,000 0.15% 15 Kolkata India Asia 16,600,000 2.00% 16 Cairo Egypt Africa 15,300,000 2.60% 17 Buenos Aires Argentina South America 14,800,000 1.00% 18 Moscow Russia Europe 14,800,000 0.20% 19 Dhaka Bangladesh Asia 14,000,000 4.10% 20 Beijing China Asia 13,900,000 2.70% 21 Tehran Iran Asia 13,100,000 2.60% 22 Istanbul Turkey Europe & Asia 13,000,000 2.80% 23 London United Kingdom Europe 12,500,000 0.70% 24 Rio de Janeiro Brazil South America 12,500,000 1.00% 25 Lagos Nigeria Africa 12,100,000 3.20% 26 Paris France Europe 10,197,678 1.00%
Another list defines megacities as urban agglomerations instead of metropolitan areas. As of 2010, there are 25 megacities by this definition.
According to the United Nations, the proportion of urban dwellers living in slums decreased from 47 percent to 37 percent in the developing world between 1990 and 2005. However, due to rising population, the absolute number of slum dwellers is rising. The majority of these come from the fringes of urban margins, located in legal and illegal settlements with insufficient housing and sanitation. This has been caused by massive migration, both internal and transnational, into cities, which has caused growth rates of urban populations and spatial concentrations not seen before in history. These issues raise problems in the political, social, and economic arenas. Slum dwellers often have minimal or no access to education, healthcare, or the urban economy.
Megacities often have significant numbers of homeless people. The actual legal definition of homelessness varies from country to country, or among different entities or institutions in the same country or region.
Urban sprawl, also known as suburban sprawl, is a multifaceted concept, which includes the spreading outwards of a city and its suburbs to its outskirts to low-density, auto-dependent development on rural land, with associated design features that encourage car dependency. As a result, some critics argue that sprawl has certain disadvantages, including, longer transport distances to work, high car dependence, inadequate facilities e.g.: health, cultural. etc. and higher per-person infrastructure costs. Discussions and debates about sprawl are often obfuscated by the ambiguity associated with the phrase. For example, some commentators measure sprawl only with the average number of residential units per acre in a given area. But others associate it with decentralization (spread of population without a well-defined center), discontinuity (leapfrog development), segregation of uses, etc.
Gentrification and urban gentrification denote the socio-cultural changes in an area resulting from wealthier people buying housing property in a less prosperous community. Consequent to gentrification, the average income increases and average family size decreases in the community, which may result in the informal economic eviction of the lower-income residents, because of increased rents, house prices, and property taxes. This type of population change reduces industrial land use when it is redeveloped for commerce and housing. In addition, new businesses, catering to a more affluent base of consumers, tend to move into formerly blighted areas, further increasing the appeal to more affluent migrants and decreasing the accessibility to less wealthy natives.
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment, into the atmosphere. Many urban areas have significant problems with smog, a type of air pollution derived from vehicular emission from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form photochemical smog.
Smog is also caused by large amounts of coal burning, which creates a mixture of smoke and sulfur dioxide. World coal consumption was about 6,743,786,000 short tons in 2006 and is expected to increase 48% to 9.98 billion short tons by 2030. China produced 2.38 billion tons in 2006. India produced about 447.3 million tons in 2006. 68.7% of China's electricity comes from coal. The USA consumes about 14% of the world total, using 90% of it for generation of electricity.
- Many of the following fictional cities were inspired by Fritz Lang's 1927 film, Metropolis and the influential depiction of Los Angeles in Ridley Scott's 1982 film, Blade Runner.
- Megacities are a common backdrop in dystopian science fiction, with examples such as the Sprawl in William Gibson's Neuromancer, and Mega-City One, a megalopolis of over 400 million people across the east coast of the United States, in the Judge Dredd comic. In Demolition Man a megacity called "San Angeles" was formed from the joining of Los Angeles, Santa Barbara, San Diego and the surrounding metropolitan regions following a massive earthquake in 2010.
- Fictional planet-wide megacities (ecumenopoleis) include Trantor in Isaac Asimov's Foundation series of books and Coruscant in the Star Wars universe. Other examples are 'City Europe' in David Wingrove's Chung Kuo series of books, Holy Terra and the hive cities of Necromunda in Warhammer 40,000 and Ravnica in the eponymous Magic: The Gathering expansion.
- The futuristic comic seriesTransmetropolitan is based in a megacity simply referred to as The City which seems to be an amalgamation of many cities along the East coast of America.
- The Fifth Element features a parody of New York City set in 2263. Buildings are so high that people use flying cars to get around and the ground level of the Earth is obscured by pollution.
- The 1973 film Soylent Green, based on Harry Harrison's novel Make Room, Make Room, depicted New York City in 2022 with a population of 40 million. It is not said how large the city is, but the main character does make the comment that a wanted criminal is "over the city line in Philadelphia" implying that it has sprawled that far.
- The sprawling metropolis featured in The Matrix series of films can be considered a megacity. It is based on Sydney, Chicago and Oakland, California. While the city is never referenced by name in the films, in the MMORPG The Matrix Online, the city itself is called the Mega City.
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- United Nations Department of Economic and Social Affairs, Population Division
- e-Geopolis project Research Group, University Paris-Diderot
- Lagos la Vida Loca by Mariana van Zeller on Current TV Nov. 2007
- Megacities Task Force
- Maps of US Megacities from radicalcartography.net
- IEEE Spectrum report on "Engineering the Megacity"
- URBAN PLANET: Collective Identities, Governance and Empowerment in Megacities
World's twenty most populous metropolitan areas
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