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(CNN)The coming decades will see the growth of colossal megacities as the world's population increasingly moves into urban environments, a new United Nations report predicts.
Today, 55% of the world's population is urban, a figure which is expected to grow to 68% by 2050, with the addition of 2.5 billion new city residents, according to projections by the Population Division of the UN Department of Economic and Social Affairs.
By 2030, there will be 43 megacities around the world with populations of over 10 million, up from 33 similarly sized urban centers today and just 10 in 1990.
The growth is caused by an accelerating shift from rural to urban living across the world, particularly in Asia, which despite hosting some of the world's largest cities has lagged behind other parts of the world in terms of overall urbanization.
In Asia, approximately 50% of the population live in cities, compared to 82% in North America, and 74% in Europe. Africa is the least urban populated continent, with just 43% of its population living in cities.
That is due to change, with India, China, and Nigeria accounting for 35% of the projected growth in urban populations between 2018 and 2050.
By 2050, the UN report predicts India will have added 416 million urban residents, China 255 million, and Nigeria 189 million.
Photos: The top 10 most expensive housing markets
1. Hong Kong, China – For the seventh year in a row, the 13th Annual Demographia International Housing Affordability Survey: 2017 named Hong Kong the most expensive housing market. The survey uses the "median multiple" ratio system, which divides median house price by gross annual median household income. Hong Kong clocked a median multiple of 18.1, down from 19 last year.
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Photos: The top 10 most expensive housing markets
2. Sydney, Australia – The second most expensive housing market for the second year running, Sydney has maintained a 12.2 median multiple -- the same as last year. For perspective, "severely unaffordable" housing is defined by Demographia as a market with a median multiple of 5.1 or higher.
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Photos: The top 10 most expensive housing markets
3. Vancouver, Canada – This Canadian city scored 11.8, making it the world's third most expensive housing market.
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Photos: The top 10 most expensive housing markets
4. Auckland, New Zealand – Auckland is the fourth-least affordable market, while New Zealand is the second most expensive nation for housing overall.
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Photos: The top 10 most expensive housing markets
5. San Jose, CA, US – This Silicon Valley capital has seen massive growth over the past decade. The housing market is currently among the most expensive in the US with a median multiple of 9.6.
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Photos: The top 10 most expensive housing markets
6. Melbourne, Australia – With a 9.5 median multiple score, Melbourne has become increasingly unaffordable, pricing out many domestic investors.
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Photos: The top 10 most expensive housing markets
7. Honolulu, HI, US – It might seem idyllic, but Honolulu is one of the most expensive markets in the world for housing. The island metropolis is the second least affordable in the US, with a median multiple of 9.4.
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Photos: The top 10 most expensive housing markets
8. Los Angeles, CA, US – In Los Angeles -- which scored 9.3 -- housing prices rose the equivalent of 14 months in household income in only 12 months, the report said.
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Photos: The top 10 most expensive housing markets
9. San Francisco, CA, US – This West Coast city is among the most expensive in the US, with a median multiple of 9.2.
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Photos: The top 10 most expensive housing markets
10. Bournemouth & Dorset, UK – About an hour southwest of London, this housing market is the most expensive in England.
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Rise of the megacity
Delhi is expected to overtake Tokyo as the world's largest metropolitan area around 2028, as Japan's population declines and India's continues to grow. Currently, 37 million people live in greater Tokyo, compared to 29 million in New Delhi, they are followed by Shanghai's 26 million, and Mexico City and Sao Paulo, each with around 22 million inhabitants.
Cairo, Mumbai, Beijing, and Dhaka all have close to 20 million inhabitants.
Not all cities will see growth, the report highlights Nagasaki and Busan, in Japan and South Korea respectively, as having undergone population decline since 2000, as have several cities in eastern Europe.
Urban growth is not necessarily sustainable or even all that effective. In Africa, several countries are experimenting with so-called "smart cities" in an attempt to alleviate the problems related with rapid urbanization such as traffic, pollution, inadequate housing and public spaces.
Elsewhere, countries are considering slicing up their larger cities into more manageable parts. Sydney, Australia's most populous urban center, has announced plans to split into three smaller cities by 2040 to improve housing, quality of life and governance.