Aridity and Urban Population

Global location of Big Cities (population > 300 000), highlighting dryland versus non-dryland distribution.
Source: United Nations Department of Economic and Social Affairs - Population Division, World Urbanization Prospects: The 2014 Revision.

Distribution of the world’s Big Cities

The Big Cities shown here are home to about 2.2 billion people or 30 % of the world’s total population. In terms of climate zones, the population distribution closely follows the big city distribution, that is, 33 % live in drylands and 67 % in non-dryland or humid zones. With projected global population growth (an estimated 2 to 3 billion new people will be added by 2050) the number of big cities will increase and many of the current 1 692 cities will become even larger. There is also the potential for shifts in the balance shown here due to increasing global aridity as rural populations increasingly move to urban centres to pursue better employment opportunities.
The trend of increasing migration to urban areas has been accelerating over the past generation. Highlighting this trend, the term “Megacities” is used for the largest urban centres of more than 10 million inhabitants. As seen in the figure, in 1990 there were 10 Megacities, which accounted for 153 million people or less than 7 % of the global urban population. Today, there 28 Megacities, with a total population of about 453 million; these now account for about 12 % of the global urban population. Some of the world’s largest Megacities include Tokyo (Japan) (the world’s largest Megacity with an agglomeration of 38 million inhabitants), Delhi (India, 25 million), Shanghai (China, 23 million) and three Megacities with approximately 21 million inhabitants each: Mexico City (Mexico), Mumbai (India) and Sao Paulo (Brazil).
Cities with 5 to 10 million inhabitants (“Large” cities) cater for a growing proportion of the global urban population. In 2014, over 300 million people lived in 43 Large cities, with 5 to 10 million inhabitants each. Examples of Large cities include Santiago (Chile), Madrid (Spain) and Singapore. Large cities now account for 8 % of the urban population of the world, and this is expected to increase by 2030. The projection is that there will be 63 Large cities in 2030, which will account for 400 million people or 9 % of the global urban population.
Here, our focus is on global cities that exceed 300 000 inhabitants, which are referred to as “Big Cities”. As of 2015, there were 1 692 Big Cities in the world with a population thatexceeded 300 000 people. The total population in Big Cities is about 2.2 billion people. The global distribution of the Big Cities is shown on the main map.

Urbanisation in different ecosystem-climate zones.
Depiction of the percentage of the world’s population that is urban by various ecosystem-climate zones, past and projected. Urbanisation in drylands is expected to increase due mainly to increased population growth in Africa and Asia.

Source: Balk D., 2008

Climate zone locations of Big Cities
Of the 1692 Big Cities, 35 % (586) are located in DRYLANDS (dry sub-humid, semi-arid, arid and hyper-arid) where the Aridity Index ≤ 0.65), while 65 % (1 106) are in non-dryland (or HUMID) regions (Aridity Index >0.65).

Source: WAD3-JRC, Reynolds, J. and Jiangshen, based on, 2016 [AP].

Urban population growth is occurring in cities of all sizes
A summary of global urban population growth in cities of different sizes.

Source: The United Nations World Urbanization Prospects (The 2014 Revision).