Urbanization has been one of the dominant economic and social changes of the 20th century, especially in the developing world. Although cities played a significant role throughout human history, it is not until the industrial revolution that a network of large cities started to emerge in the most economically advanced parts of the world. Since 1950, the world’s urban population has more than doubled, to reach nearly 4.2 billion in 2018, about 55.2% of the global population. This transition is expected to go on well into the second half of the 21st century, a trend reflected in the growing size of cities and the increasing proportion of the urbanized population. By 2050, 70% of the global population could be urbanized, representing 6.4 billion urban residents. Cities also dominate the national economic output as they account for the bulk of the production, distribution, and consumption.
Global urbanization is the outcome of three main demographic trends:
· Natural increase. The outcome of more births than deaths in urban areas, a direct function of the fertility rate as well as the quality of healthcare systems (lower mortality rates, particularly for infants). Phases in the demographic transition are commonly linked with urbanization rates, with peak growth years corresponding to large differences between birth and death rates. Although natural increase played an essential role in the past, it is of much lesser importance today as fertility rates in many developed economies have dropped significantly. In some cases like Western Europe, Japan, and South Korea, fertility is below the replacement rate.
· Rural to urban migrations. This has been a strong urbanization factor, particularly in the developing world, where migration accounted for between 40 and 60% of urban growth. Migration endured since the beginning of the industrial revolution in the 19th century. It first took place massively in the developed world in the first half of the 20th century and then in the developing world since the second half of the 20th century. The factors behind rural to urban migrations may involve the expectation to find employment, improved agricultural productivity, which frees rural labor or even political and environmental problems where populations are constrained to leave the countryside. The industrialization of coastal China and its integration into the global trade system since the 1980s has led to the largest rural to urban migration in history. According to the United Nations Population Fund, about 18 million people migrate from rural areas to cities each year in China alone.
· International migration. The growth in international migration has been an important factor in the urbanization of major gateway cities, such as Los Angeles, Miami, New York, London, and Paris. This process tends to occur in the largest cities, but there is a trickle-down to cities of smaller size.
Through urbanization, fundamental changes in the socio-economic environment of human activities have been observed. What drives urbanization is a complex mix of economic, demographic, and technological factors. The growth in GDP per capita is a dominant driver of urbanization, but this is supported by corresponding developments in transportation systems and even the diffusion of air conditioning, allowing for settlements in high-temperature areas such as the Middle East (e.g. Dubai). Urbanization involves new forms of employment, economic activity, and lifestyle.
Urban mobility problems have increased proportionally, and in some cases, exponentially, with urbanization. This is associated with two outcomes. First is the emergence of a network of megacities that account for the most salient urban mobility challenges. Second, mobility demands tend to be concentrated over specific urban areas, such as central business districts.
Current global trends indicate a growth of about 50 million urbanites each year, roughly a million a week. More than 90% of that growth occurs in developing economies, which places intense pressures on urban infrastructures to cope, particularly transportation. What is considered urban includes a whole continuum of urban spatial structures, ranging from small towns to large urban agglomerations. This also brings the question about optimal city size since technical limitations (road, utilities) are not much of an impediment in building very large cities. Many of the world’s largest cities can be labeled as dysfunctional mainly because as city size increases, the rising operational and infrastructure complexities are not effectively coped with managerial expertise.