What does a city's skyline say about its sustainability?

When we think of skylines around the world, for many people the awe-inspiring buildings of New York, Hong Kong, and Dubai come to mind. These cities draw crowds of tourists eager to visit landmark buildings with observatories hundreds of meters in the air. These skylines are the result of global competition among developers and architects racing to outbuild and out-design each other to lend their city a certain aesthetic, engineering, and economic status.

But how tall and dense are these skylines really when compared to others around the world? Are other cities just mini versions of these exciting skylines, or do they take different strategies when building tall? Are skylines in developing economies different than those in the developed world? But first, and most importantly, why does this matter for sustainable development?

Understanding why city skylines vary can help inform sustainable development . More skyscrapers or vertical development tends to indicate compact urban development, which in turn correlates with lower emissions of both climate change-causing carbon dioxide and of dangerous particulate matter (known as PM2.5 as its less than 2.5 microns in diameter) that penetrates deep into the lungs, with serious health consequences. This is because residents of more compact cities tend to be less dependent on cars for transportation, leading to fewer emissions. In addition, more compact cities eat less into agricultural lands on their peripheries, which are often among the most fertile areas and thus a leading source of nutritionally important fresh fruit and vegetables for many cities.

Whether a city’s skyline is tall because of spectacular luxury condos or less architecturally lavish high-rise residential buildings that cater to the middle classes has obvious implications for the inclusiveness of urban development. At the same time, however, a more spectacular skyline can draw tourists to a city, helping to create local economic dynamism and jobs for lower-skilled workers with potential benefits for poverty reduction. Hence, a city’s skyline has important potential implications for both the sustainability and inclusiveness of its development and not just how impressive it is to the eye.

Wealthy and densely populated places like New York, Hong Kong, and Dubai naturally have more skyscrapers, however, we wanted to identify cities that still build toward the sky after controlling for, or holding constant, the key demand and supply conditions that drive tall building construction and economic development in general. To do this, in a recently published working paper, we constructed a “skyline index.” Drawing on global data on the location of all buildings above 55 meters, this index measures a city’s “skyscraperness”—that is, how much it builds up relative to other cities.

Crunching the numbers reveals that the regions around the world that rank the highest on the skyline index are not those that one might suspect if anticipating a correlation between wealth and height. South America, Eastern Europe, and South-Eastern Asia are most highly ranked for their level of economic development (see Figure 1 below). Within Asia, East Asia and West Asia (which includes the Gulf nations) are lower in the rankings for skyskraperness. South-Eastern Asia thus “overbuilds” relative to the others, given its income and population levels. East Africa and Southern Africa are, perhaps surprisingly given that Africa is not usually thought of as a region of high-rise development, highly ranked. Among more developed regions, apartment-focused Western Europe is not surprisingly higher ranked than mostly sprawling suburban North America.

More generally, the top-ranked regions make up in high-rise volume what they lack in skyscraper “bling.” Most skylines are thus less ostentatious and more functional — built to house the middle classes rather than global corporations or the elite.

To probe deeper into the regional differences, we ranked cities by their relative construction of skyscrapers (100+ m) versus high-rises (55-99 m), residential versus commercial buildings, buildings in large cities versus small ones, and construction in a city’s center versus its periphery.

Based on this analysis, we create four skyline topologies:

The skylines around the world are about evenly split between showy Capital-oriented Central Districts and less ostentatious Population-oriented Peripheral District types (see Figure 2 below). South America, South-East Asia, Eastern Europe, and Western Europe are in the latter, while East Asia, West Asia, East Africa, and South Africa are in the former.

Why do cities across regions evolve to have different types of skylines? We conjecture that there are three major reasons. First are regulations and preservation—some countries impose strict rules to reduce overcrowding or preserve their building heritage , while others are relatively laissez-faire.

Another possibility is that property owners in some regions find it harder to amass larger lots or face foggy property rights that create barriers to building tall.

A third reason might be social preferences. Living in high-rise apartments confers social status in some regions, whereas in other places, the norm is living in free-standing, single-family homes. Also, in some countries, the middle- and upper-classes prefer to live in the central city near the action, while in others, they prefer the quiet of the suburbs. Differences in the perceptions and the realities of the safety and security of living in a high-rise apartment versus a single-family home in different regions may also have a role to play.

We find that most of the differences are explained by the stringency of building height regulations and residents’ preferences for apartments versus free-standing houses.

So next time you look up at the Empire State Building or the Burj Khalifa, remember that a city’s skyline matters in more ways than you might imagine.