Bigger is not necessarily better and that is certainly the case with cities.
The UN has predicted that the global population will increase by about three billion to reach 11.2 billion by the end of the century, and all those new people will have to live somewhere.
Since the start of the Industrial Revolution, most population movement has been from the countryside to urban areas, a trend that has accelerated in recent decades and is likely to continue with migration resulting from climate change. By the end of the century, more than three quarters of the world could live in cities.
Even by 2050 what we think are the major cities of the world are likely to be very different from today.
The inexorable rise of the megacity, with populations often exceeding 40 million, will outstrip the likes of London, Paris and New York City.
Asia and Africa will become the new global population centres. Conditions in megacities will often be challenging, with massive slums, unemployment and poor infrastructure.
Other new cities are rising, packed with the latest technologies. These places will be sustainable, eco-friendly and designed to put people first.
Many of these ideas will feature at Expo 2020 Dubai in its "Cities of the Future" visitor journey.
The contrast between rich and poor may define life in 2100, and it is likely to grow. Here The National looks at both ends of the spectrum, and what city life might be like in the coming decades, based on population estimates from the Global Cities Institute.
The largest city in Africa by population is predicted to become home to a staggering 58.42 million people by 2075.
Currently home to about 15 million people, problems such as crumbling infrastructure, crime in the slums and children living on the streets could become more acute as the city grows.
Kinshasa is expected to be eclipsed by Lagos, Nigeria's commercial capital, by the end of the century. The city's population is predicted to reach 88.3 million by 2100.
Authorities in the city cannot be certain how many people live there now, with one estimate of 21 million in the greater metropolitan area. It is one of the richest cities in Africa and has one of the tallest skylines on the continent, but Lagos will nevertheless be tested by such massive growth.
Dar es Salaam
Another African city, the population in the business capital of Tanzania is predicted to grow from about seven million today to more than 73 million by 2100. The top three megacities in the world could all be in Africa by the end of the century.
The city in the American Midwest is the only emerging megacity in the developing world, according to a World Economic Forum report published in 2018. Chicago will be one of six new megacities by 2030, the report said.
With a population of about four million, it will be a minnow compared with the others – Dar es Salaam, Angolan capital Luanda, Iraqi capital Baghdad, Chennai in India and Colombian capital Bogota – except in one crucial respect.
Chicago's total gross domestic product is expected to be more than double that of he other five new megacities put together.
At the opposite end of the spectrum will be the Saudi smart city of Neom. It will be built on the Red Sea coast and the 25,000-square-kilometre site will extend to Egypt and Jordan. The project is estimated to cost £500 billion and Saudi Arabia hopes to complete the first section by 2025.
The project will include The Line, a city within a city. It will be home to one million people and will not have any cars or streets and will be free from carbon emissions.
Described as the world’s smartest city, Songdo is being built on reclaimed land about 35 kilometres south-west of the South Korean capital Seoul.
Songdo has been designed to be carbon-free, with more than 40 per cent of the city being green space. It has a seawater canal and an area inspired by Central Park in New York City. Parts of the music video for 2012 hit Gangnam Style by Psy were filmed there.
Sensors constantly monitor traffic flow and the city's rubbish is collected in underground tunnels. Songdo’s population is currently about 100,000 but is expected to triple eventually.
Nagara Rimba Nusa
Jakarta, the capital of Indonesia, was largely built on a swamp, with parts reported to be sinking by 25 centimetres a year. With sea levels predicted to rise owing to climate change, Indonesian authorities have decided to move its capital to a new location on the island of Borneo.
This city of the future is projected to cost about $33 billion and should be completed by 2045. Despite about 40 per cent of Jakarta being below sea level, it will not be abandoned and is expected to increased its population from about 10 million to nearly 16 million by 2050.
China specialises in building cities from scratch. Shenzhen was farmland on the border with Hong Kong in 1970s, but it is now a city with a population of more than 17 million. A building project in Xiongan is designed to reduce the pressure on Beijing and the new city will eventually be three times the size of New York.
Horgas, on the border with Kazakhstan, has been described as the first new city of the New Silk Road and is said to be the world’s largest dry port, meaning inland.
First settled more than 1,000 years ago, construction of the new city, which is also a major centre of robotics, began in 2017, with the population expected to increase to 200,000.
New Administrative Capital, Cairo
This summer the first civil servants are set to move to Egypt’s new capital, 45 kilometres east of overcrowded Cairo, the seat of government since the Fatimid conquest more than 1,000 years ago.
The first phase alone is to cost $25bn and will feature electric trams, a monorail and a theme park four times the size of Disneyland. If it is completed to plan, its population could reach about seven million, making it the world’s largest planned new capital.
Even for a megacity there are limits to growth. In 2021, the world’s largest city is in Japan, with the population of the greater metropolitan population recorded as more than 37 million. But Japan has an ageing population and the fertility rate is dropping, in contrast with Africa.
By the end of the century, Tokyo will have a population of about 25 million, knocking it down to 28th on the megacity league table.