Temps de lecture :4 minutes
In 2005, there were twenty agglomerations of more than 10 million inhabitants whereas there were only two in 1950 and three in 1975. Fifteen of these cities are in developing countries. These cities alone account for 9 % of the world’s population. The largest is Tokyo with 35 million inhabitants and the 9 other largest megacities are Mumbai (or Bombay) and Mexico with 21 million inhabitants, Sao Paulo -20 million, New York -19 million, Delhi -18 million, Shanghai -17 million and finally, Calcutta, Dhaka and Jakarta with 16 million inhabitants. (1)
The fabric of society of large cities tends to merge to create huge « conurbations ». Hence, the Tokaido corridor around Tokyo and Osaka could represent over 80 million inhabitants whilst the North-East corridor, between Washington, New York and Boston has 55 million inhabitants. It is often hard to draw the precise line between a city and an agglomeration.
The dynamics of urbanisation
The world’s population rapidly became urbanized in the 20th century. Whilst the population in towns in 1900 was barely over 13%, it went up to 50% in 2007. We went from 220 million people living in towns in 1900 to 3.3 billion in 2007. The populations of Europe, America and Oceania are already more than 70% urban. Africa and Asia’s populations are less so (about 40% urban), but should be more than 50% urban by 2030.
This transformation was in large part due to the rural exodus. In countries in the North, the migration of inhabitants from the countrysides to the towns started in the 18th century. Today, it mainly affects countries in the South and will increase in the next few decades. According to the United Nations, 50% of growth in cities in developing countries is linked to the rural exodus.
This phenomenon coincides with a characteristic shared by developing countries : a demographic explosion (Africa went from 302 millions inhabitants in 1975 to 922 million in 2005). Certain cities’ extremely rapid growth obviously causes considerable organisation problems. According to the United Nations Population Fund, the city of Dhaka, for example, went from 300 000 inhabitants in 1950 to 10 million in 2004.
Conversely, in certain developed countries, those living in cities are leaving urban areas to live in small towns. They make up the neo-rural group. In France, over the past five years, 2 million people living in urbanized areas have moved to towns with less than 2000 inhabitants. (2).
Towns can take very different forms.
As opposed to the European town, the North American town has a town centre conceived according to a Hippodamian plan (the roads cross each other at a right angle). The towns are made up of three levels : the first is the CBD (central business district) which is the business district, the second is a very impoverished zone (ghettos) whilst the homes of those who earn a comfortable income live on the outskirts which are very spread out and take up a lot of space.
In Africa, town formation is linked to colonial heritage with the exception of certain old North African towns and the Nile valley.
These towns attract new populations thus taking up a little more space and speeding up urban fragmentation. The African town is indeed often spread out, with shanty town development on the outskirts. For example, Cairo’s agglomeration spreads over 46 kilometres from North to South and 35 kilometres from East to West.
The Latin American town develops in almost the same way. The outskirts, especially valleys, are characterised by precarious housing (shanty towns) which developed from a fabric of society strongly marked by the colonial period (the grid plan, for example). Individual spaced out dwellings also tend to spread close to these humble ones.
In Europe, the urban spread can go hand in hand with a form of exclusion and cultural distancing but one must differentiate the periurban of the poorer classes from the middle and even upper class periurban made up of individual housing.
In other places, urbanisation causes an increase of informal settlements, favelas, barrios, slums, shanty towns etc. characterised by severe poverty, poor access to drinking water, toilets, education, health services etc.
In 2005, 37 % of the urban population in developing countries lived in shanty towns.
On a world scale, the average density is 906 hab/km2 . It is higher in developing countries where it can go up to 1392 hab/km2 and even 2547 hab/km2 in the least developed countries. (3) In the large major cities, concentration can be very high. Hence, Paris has a density of 20 437 hab/km2 whilst the national density is 109 hab/km2 ; London has almost 5000 hab/km2 whilst the national density is 400 hab/km2.
In terms of surface, those living in towns are concentrated on 2.7% of land surface. But, paradoxically, if the populations of cities are increasing their density is decreasing : towns are spreading out. The phenomenon takes on very different forms depending on the regions and the periods but it has been estimated that the surface of large agglomerations increases by about 1% on average in Europe every year. According to a European Environment Agency report, during the 1990-2000 period, urban areas and their associated infrastructures grew by about 800 000 hectares (a 5.4% increase over the period) which is the equivalent of Luxembourg’s surface. (4)
Moreover, high buildings do not necessarily mean concentration. Indeed, many suburbs with their large empty neighbouring esplanades are half as dense as Haussmannian areas.
Density and the environment
The dense town is often associated with hazards, pollution and even a lack of hygiene. However, a spread-out city devours space and plays a part in degrading certain areas and landscapes.
Moreover, a spread-out city generates perpendicular urban movements, and therefore , more greenhouse gas emissions than a dense city. However, the high use of public transport in the compact town limits and replaces the traffic of private vehicles even if the strong pressure put on the few free areas can have a negative impact on landscapes.