Megacities

A megacity is usually defined as a metropolitan area with a total population in excess of ten million people. A megacity can be a single metropolitan area or two or more metropolitan areas that converge. 

In 1800, only 3% of the world's population lived in cities, a figure that rose 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 population as a whole will double every 45 years.

Rank	Megacity	Country	Continent	Population
1	Tokyo	Japan	Asia	34,800,000
2	Guangzhou	China	Asia	31,700,000
3	Shanghai	China	Asia	28,900,000
4	Jakarta	Indonesia	Asia	26,400,000
5	Seoul	South Korea	Asia	25,800,000
6	Delhi	India	Asia	24,000,000
7	Mexico City	Mexico	North America	23,800,000
8	Karachi	Pakistan	Asia	23,500,000
9	Manila	Philippines	Asia	22,200,000
10	New York City	United States	North America	21,600,000
11	São Paulo	Brazil	South America	21,600,000
12	Mumbai	India	Asia	21,400,000
13	Beijing	China	Asia	19,300,000
14	Los Angeles	United States	North America	17,200,000
15	Osaka	Japan	Asia	16,800,000
16	Dhaka	Bangladesh	Asia	16,300,000
17	Cairo	Egypt	Africa	16,100,000
18	Kolkata	India	Asia	16,000,000
19	London	United Kingdom	Europe	15,500,000
20	Buenos Aires	Argentina	South America	14,500,000

1975		2000		2015 Projected
Tokyo New York Shanghai Mexico City São Paolo	19.8 15.9 11.4 11.2 10	Tokyo Mexico City Mumbai São Paolo Shanghai New York Lagos Los Angeles Kolkuta Buenos Aires Dhaka Karachi Delhi Jakarta Osaka Metro Manila Beijing Rio de Janeiro Cairo	26.4 18.1 18.1 17.8 17 16.6 13.4 13.1 12.9 12.6 12.3 11.8 11.7 11 11 10.9 10.8 10.6 10.6	Tokyo Mumbai Lagos Dhaka São Paolo Karachi Mexico City New York Jakarta Kolkuta Delhi Metro Manila Shanghai Los Angeles Buenos Aires Cairo Istanbul Beijing Rio de Janeiro Osaka Tianjin Hyderabad Bangkok	26.4 26.1 23.2 21.1 20.4 19.2 19.2 17.4 17.3 17.3 16.8 14.8 14.6 14.1 14.1 13.8 12.5 12.3 11.9 11 10.7 10.5 10.1

 Source: State of the World Population 200,1 Chapter 3,  UNFPA

Challenges in megacities:

Megacities will have to face different challenges of social, economic and ecologic dimensions, and should develop new solutions and actions, to answer to citizens to all their needs.

• Buuildings, housing, slums
• Homelessness
• Traffic congestion
• Waste
• Urban sprawl (also known as suburban sprawl)
• Gentrification
• Environmental problems (air pollution)
• Energy
• Water