The world’s population grows by about 83 million people annually, and it stands to reason that the population of most cities grows along with it. Natural factors have something to do with the increase: Longer life spans in much of the world and increased accessibility to health care raise the birth rate and lower the rate of deaths. Economic opportunities that encourage immigration, from both within a country and without, also contribute.
As cities grow, so does our definition of impressive size. Back around 7000 B.C., Jericho — now a Palestinian city in the West Bank — was thought to be the world’s largest city. It had a population of about 2,000. Baghdad may have been the first city to reach a population of a million, around 900 A.D.
Today, nine urban agglomerations around the globe are home to more than 20 million people. Six of these are in Asia — with two each in the world’s most populous countries, China and India. Two more are in Latin America and one is on the African continent. These are just the leaders among the world’s 33 mega-cities.
In 2018, the United Nations published a list of the world’s 30 largest urban agglomerations ranked by population in five-year increments from 1950 through 2035 (the post 2020 numbers obviously being estimates).
According to the UN, “The term ‘urban agglomeration’ refers to the population contained within the contours of a contiguous territory inhabited at urban density levels without regard to administrative boundaries.” This typically includes the population of a core city along with that of adjacent suburban areas. Some countries produce data based on the city alone or its metropolitan area. In those cases, the UN attempts to adjust the numbers to conform to the urban agglomeration concept
Based on the UN’s statistics, the independent California-based data gathering and publishing organization World Population Review (WSR) has computed population estimates for 2021. According to those numbers, these are the 20 cities — or urban agglomerations — that currently have populations of more than 20 million. It’s probably not a surprise that most of these are among the world’s most stressed-out cities.