What Is a Megacity? Ten Million People and the Problems That Follow
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A megacity is conventionally defined as an urban area with more than ten million inhabitants. There were two in 1950 and there are more than thirty now, with most of the growth in Asia and Africa and almost none in Europe, which makes the category a useful shorthand for where the twenty-first century's urban population actually lives.
The definition problem
The ten million threshold is arbitrary and the harder question is what counts as the city. Three boundaries are commonly used and they give very different figures for the same place. The city proper is the administrative unit, which frequently covers a small fraction of the built-up area and produces absurdly low figures for cities whose government boundaries were fixed long ago. The urban agglomeration is the contiguous built-up area regardless of administrative lines, which is usually the most meaningful and is what most rankings use. The metropolitan area includes the surrounding zone economically integrated with the core through commuting, which produces the largest figures. Tokyo, Delhi, Shanghai, Dhaka, Sao Paulo, Cairo, Mexico City, Beijing, Mumbai and Osaka appear near the top of most lists, and their relative order depends substantially on which definition is applied, which is why published rankings disagree and why comparing a figure from one source with a figure from another is frequently meaningless.
How they grow
Three distinct processes contribute and their relative importance has shifted:
- •Natural increase, meaning births exceeding deaths among the existing population, which is now the largest contributor in many rapidly growing cities rather than migration
- •Rural to urban migration, driven by wage differentials, mechanisation of agriculture, land pressure and access to services
- •Reclassification, where surrounding settlements are absorbed as the built-up area expands, which adds population without anyone moving
- •International migration, significant in some cities and minor in most
- •Displacement from conflict and environmental pressure, including drought and flooding, which is expected to increase
- •Growth rates now exceed the capacity of formal housing and infrastructure in many cases, which is why a large share of megacity residents live in informal settlements built without planning permission or secure tenure
The recurring problems
Very large cities share a set of difficulties that arise from scale rather than from any particular country's failings. Housing supply rarely keeps pace, so informal settlement becomes the default, with residents facing insecure tenure, limited services and vulnerability to eviction. Water and sanitation are the most consequential shortfalls, since providing piped water and sewerage to a rapidly expanding periphery is expensive and slow, and the health consequences of not doing so are severe. Transport congestion rises faster than population because trip distances lengthen. Air quality deteriorates, with several megacities recording pollution levels associated with substantial excess mortality. Governance is fragmented, since a functional urban area typically spans many local authorities with no body responsible for the whole, which makes coordinated transport and land use planning difficult. And many of the largest cities are coastal or on deltas, which places them directly in the path of sea level rise and storm surge, with Jakarta subsiding fast enough that the Indonesian government has begun relocating its capital.
Why they persist anyway
Cities keep growing despite those problems because concentration produces real advantages that are measurable. Agglomeration economies arise when firms locate near each other, sharing suppliers, labour pools and infrastructure, and when workers can find better matches for their particular skills in a larger market. Knowledge spillovers occur because people in the same place exchange ideas informally, and patent and productivity data show measurable effects. Consumption variety, meaning access to specialised goods, services, healthcare and culture, is a genuine draw. Research on urban scaling has found that many quantities follow consistent mathematical relationships with population size: infrastructure requirements grow more slowly than population, making large cities more efficient per person in road length and energy use, while economic output and innovation grow faster than population, so productivity per person rises with city size. Crime and disease transmission also scale superlinearly, which is the cost side of the same relationship.
The takeaway
A megacity holds more than ten million people, a threshold crossed by two cities in 1950 and by more than thirty now, almost entirely in Asia and Africa. Figures differ wildly depending on whether the administrative city, the built-up agglomeration or the commuting region is measured. Natural increase now outweighs migration in many cases, and growth outpaces formal housing, which is why informal settlement is the default. Infrastructure needs grow more slowly than population while output grows faster.