Why We Need a Better 'Science of Cities'

Q&A with Planet of Cities author Shlomo Angel.

Angel’s latest book was released in

Building more resilient and prosperous cities is perhaps the grandest of the grand challenges facing the world today.

By now, virtually all urbanists know that half the world's population lives in urban areas, a figure than will likely rise to 75 percent by 2100, when 7.5 to 8.8 billion people will live in cities.

In his just-released Planet of CitiesShlomo Angel argues that urban policy-makers and planners must do more to meet the challenge of urbanization. Angel, who is a member of the Urbanization Project at New York University and who conducted his research as a visiting fellow at the Lincoln Institute of Land Policy, provides a detailed, data-driven analysis filled with maps of world urbanization patterns, as well as charts and tables documenting the challenges facing global cities. He took time out from his busy schedule to talk to Atlantic Cities about the key challenges facing our increasingly urban world.

RF: You open the book by saying we need a better "science of cities" to guide our policies for global urban problems. What would this look like, and what would cities gain from it?

SA: There are some 4,000 cities in the world today with populations of 100,000 or more. Simply studying some U.S. cities and finding that they all possess a certain trait does not mean that all cities in the world, or in the U.S. for that matter, have that feature. [W]hat I hope that a science of cities can bring forth are truths about the norms — what is expected, but not necessarily ideal — in the universe of cities as a whole, not just a subset of cities that may be special and unique in one way or another. ... The value for city officials or citizens of a particular city is obvious: they can take these global norms more seriously if they apply to the universe of cities as a whole rather than to a subset of cities that may be entirely different than their city.

Author Shlomo Angel

RF: We live in an expanding urban world. How much and what kind of expansion can we anticipate? What parts of the world will see the most of it, and how can we best cope?

SA: In the coming decades, say between 2010 and 2050, cities in industrialized countries will add 170 million to their populations while developing countries will add 2.5 billion, or 15 times that. The largest shares of this growth, 25 percent each, will be in Sub-Saharan Africa and the Indian subcontinent, and an additional 15 percent will be in China.

[W]e must make minimal preparations for it, preparations that involve, at the very least, correct projections of the amount of land cities will need for their expansion, enshrining new city limits that can accommodate this expansion in law, the aggressive protection of a hierarchy of open spaces from occupation by formal and informal developers, and the acquisition of the right-of-way for an arterial infrastructure grid now — before lands on the urban fringe is subdivided and land prices there become exorbitant.

RF: You highlight three great ages of urbanization and economic development. Where we are today and where we are headed?

SA: We now know that the movement of people to cities everywhere is accompanied by increased life expectancy and lower fertility — people live longer and have fewer children. Urbanization goes hand-in-hand with lower rates of population growth. By 2100, the world population growth rate will be close to 0, the world will have 75 to 80 percent of its 9 to 11 billion people living in cities, and that percentage is likely to be quite stable — most people that want to live in cities will have moved there. ... By that time, the urbanization project will largely be over.

RF: Based on your research, do you think our global system today will be dominated by bigger and more powerful cities?

SA: We are only at the beginning of studying the global system of cities...It is quite clear already that cities come in all sizes; that the shares of cities of every size are rather stable; and that cities of different size tend to grow at approximately the same average rates. This means that megacities, for example, are not growing any faster than intermediate-size cities or smaller cities. It may also mean that, all in all, there may be no inherent advantage to city size.

Map of the 3,646 cities with populations of 100,000 or more in 2000. (Courtesy of Planet of Cities)

RF: You write that our cities have not only expanded faster than their growth in population, but that they are also quite fragmented. How is this playing out?

SA: [W]hen we measure the amount of open spaces within the built-up areas of cities, we find that, on average, cities are half-empty, namely that they contain open spaces that are equivalent in area to their built-up area. This is quite surprising because we tend to think of cities as fully built-up. They are not, especially at their fringes, and they are not likely to be. We do find that fragmentation the world over is on the decline, a sow but perceptible decline, but that need not mean that we can expect cities to fill in their open spaces any time soon. As open spaces close to their centers fill in, new open spaces are created along their fringes. This realization should moderate our expectation that future urban growth can simply fill in the existing open spaces, eliminating the need to plan for urban expansion.

Urban expansion in Bangkok, Thailand. (Courtesy of
Planet of Cities)

RF: I am intrigued by your "sustainable densities proposition," which states that the density of some cities is too high and should decrease, some are sustainable, and some are too low and should increase. Thus, a denser city is not necessarily better. Can you explain how this proposition works?

SA: Dhaka, the capital of Bangladesh, is an example ... It is overcrowded and there is not enough living and working space per person. It needs to expand and suburbanize so that its densities can be reduced.

Cities in most developing countries (and in some European countries) are in the sustainable range. They allow for adequate living and working space, and yet are dense enough to support public transport, so as to limit energy use and carbon emissions.

A large number of cities in the U.S., Australia, and Canada contain vast areas where densities are too low to support public transport ... These densities can be said to be unsustainable: they use more than a fair share of energy and they generate more than a fair share of carbon emissions. I use the expression ‘fair share’ here in the sense that if everyone used that much energy and generated that much carbon, the planet would not be sustainable.

RF: Another intriguing idea, your "decent housing proposition," provided the main motivation for writing the book.

SA: [T]he housing problem in the cities of developing countries is fundamentally a land problem. Namely, when the poor can get access to an affordable plot of land, they can build their own houses by themselves using their savings and their sweat equity, with financial help from family, employers and acquaintances... [T]he density of development accumulates only slowly as communities get built up, and that in most cities in developing countries — with the exception of China where capital is now available — you cannot expect high-rise apartment blocks to sprout out from day one. My conclusion in both cases, however, is essentially the same: housing will remain affordable — both in the formal and informal housing sectors — as long as land on the urban fringe is plentiful and affordable. When land use regulations, administrative delays, or the absence of trunk infrastructure create land supply bottlenecks, land prices rise, and housing becomes unaffordable both for the poor and for the not so poor.

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