Megacities are major global risk areas. Due to highest concentration of people and extreme dynamics, they are particularly prone to supply crises, social disorganization, political conflicts and natural disasters. Their vulnerability can be high.

This quote from the IGU’s MegaCity TaskForce draws a quite bleak picture of what some believe to be the future of living for humankind. The UN World Urbanisation Prospects finally saw the urban populations surpassing rural living for the first time in human history in recent years, but we must not forget that these urban populations do not all live in what is referred to as a megacity.
A megacity basically is nothing more than a very large city. Widely used is a population of 10 million, but other definitions do exist, ranging from 5 or 8 million, and some people, such as German geographer Bronger are also including the population density of 2000 p. per sq km as a defining factor). The definition of a megacity should also be seen as a rather vague delimitation for a phenomenon that – despite it’s quantitative dimension – has a very qualitative nature: What happens, when extremely large numbers of people live in a very limited amount of space, and what happens, if these areas of very high density living even continue growing.
In the year 2000 there were 39 cities with a population of more than 5 million inhabitants, 2/3 of which were in the developing countries. This was a population of 225 million people, not even 5% of the world’s population. Today we have about 400 million people living in the largest cities on the planet – still far from the majority of the now more than half of the world’s population living in cities. And perhaps the majority of people on this planet may never live in one of these megacities. Why is everyone talking about megacities then anyway? The sheer size make these cities the ultimate examples for urbanization, and provide an insight to the diverse processes in such complex urban spaces. They are like a real-life laboratory for urban geographers who try to understand the impact and implications of urbanization processes, and may contribute to solutions how the urban future of humanity can be actively created and lead to a better and perhaps more sustainable life on this planet.
A gridded population cartogram can help to understand not only the locations of these largest of cities, but provides a look into their setting within the global population patterns by giving space to people and allowing to see where many people live in these large cities, and where people are in relation to these cities. A normal map (see further down) shows the high concentration of megacities especially in Asia, but we can see from the population cartogram that these are in those anyway very densely populated regions, while the megacities in South America appear more like (relatively seen) solitary bodies.

(click for larger map)

While megacities exist on almost all continents, there are huge differences in their very characteristic nature. Few people will think of cities like Paris or London in the ‘old world’ as the prime examples for a megacity (the Rhine-Ruhr area may be see more controversial as a megacity anyway, as it consists of many independent municipalities), because it is often seen in direct relation to the rapid ongoing urbanization processes in the developing countries. The megacities of the wealthier countries in contrast have much lower growth rates or even declining populations in future (such as the population prospects for London shown in an earlier post on this website), while the megacities in other regions of the world are expected to continue growing. Therefore the dynamics of urbanization is very different in the different regions, and even the processes within the hotspot of megacities, the Asian continent, are very different due to the different economic and political conditions there.
This is how the megacities of 2015 are distributed on a normal map:

(Source: IGU MegaCity TaskForce)

Megacities may face us with very specific challenges for their further development and raise questions about how to administer such cities. But much more they are a window into the world of cities and help us to understand the implications of urbanization in their most distinct expression and can thus help to find better futures for all of the still growing shares of urban living.
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The second map shown on the page is property of the MegaCity TaskForce of the International Geographical Union.