Over the last 50 years, more and more cities have reached sizes that are historically unprecedented. While there are examples in history of cities that had populations of one million or more inhabitants (including imperial Rome at the height of its powers and Edo, the precursor of Tokyo in the 13th century), the city with several million inhabitants is a relatively new phenomenon.
London was the first city to have several million inhabitants, reaching this size in the second half of the 19th century, reflecting its status as the economic and political centre of the British Empire. By 2005, there were 50 cities with more than 5 million people, including 20 'mega-cities' with more than 10 million people.
Many other cities have grown very considerably. In 1990, the average size of the world's 100 largest cities was around 5.1 million inhabitants, compared to 2.1 million in 1950, around 700,000 in 1900 and just under 200,000 in 1800. (See graph below).
During the 1980s, many had more people moving out than moving in (since their population growth rate was lower than their rate of natural increase). These included Mexico City, Sao Paulo, Calcutta, Buenos Aires and Rio de Janeiro. However, the graph above shows that very large cities with relatively modest population growth rates can still have large annual increments in their populations. In 2003, there were 20 megacities with over 10 million inhabitants. It is projected that by 2015, 22 cities will be this large, all but five of them in the developing world. The population of these 22 cities in 2015 will be about 358 million - 75 million more than today, but still only about 5 per cent of the expected global population of over 7 billion. 
Regional patterns
Historically Asia has always had a high proportion of the world's largest cities - and South and Central America and North Africa have also long had large cities. In 2005, 28 of the world's 50 cities with over 5 million inhabitants were in Asia and this proportion is likely to increase, reflecting its increasing weight within the world economy.
Among the world's regions, North America and sub-Saharan Africa stand out as having most 'new' large cities - settlements that had not been founded or did not exist as urban centres by 1800. In 1900, Europe had more than half the world's 100 largest cities. Today it only has 53 of the world's 414 cities with a population of over 1 million.
Fast-growing cities
Although cities such as Lagos, Karachi and Dhaka are often said to be among the world's fastest growing cities, there are hundreds of smaller cities with higher population growth rates. Indeed census data shows that some relatively small urban centres have been growing by 10 or more per cent per year - far above even the fastest growing city listed above.
The current population of most of the world's largest urban areas including Tokyo, London, Los Angeles, Shanghai, Beijing, Jakarta, Dhaka and Bombay/Mumbai can go up or down by many million inhabitants, depending on which boundaries are used. This is why any international list of 'the world's largest cities' risks considerable inaccuracy as some city populations are for large urbanized regions with thousands of square kilometres while others are for older 'city boundaries' with a few hundred square kilometres.
Although it is often assumed that the world's most rapidly growing cities are concentrated in Latin America, Asia and Africa, this is not so. The table below shows how several cities in the United States rank among the most rapidly growing cities in the world during this century. For instance, Nairobi is often held up as one of the world's most rapidly growing cities - but both Miami and Phoenix in the United States had larger populations than Nairobi in 1990, although all were small settlements in 1900. FACTSHEETSUse this LINK to access key data sheets for your NOTES on GLOBAL URBAN GROWTHBACKGOUNDUse this LINK for useful comparisons of major cities around the world. The URBAN GEOGRAPHY OF LEEDS is a document you need and should read
