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Are you interested in urban expansion shifts across 3 decades?

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Our summary today works with the article titled Global urban structure growth shows a profound shift from spreading out to building up from 2024, by Steve Frolking, Richa Mahtta, Tom Milliman, Thomas Esch, and Karen C. Seto, published in the Nature Cities journal.

This is a great preparation to our next interview with Keller Easterling in episode 342 talking about spatial politics and urban infrastructure.

Since we are investigating the future of cities, I thought it would be interesting to see global urban structural growth across time and its implications. This article introduces the shift from horizontal to vertical expansion and its effects on resource use, climate and urban living.

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Welcome to today’s What is The Future For Cities podcast and its Research episode; my name is Fanni, and today we will introduce a research by summarising it. The episode really is just a short summary of the original investigation, and, in case it is interesting enough, I would encourage everyone to check out the whole documentation. This conversation was produced and generated with Notebook LM as two hosts dissecting the whole research.

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Speaker 1: That feeling you get sometimes looking at a city how it seems familiar. But also like it’s changing subtly all the time.

Speaker 2: Yeah. Like it’s constantly evolving under our feet or maybe over our heads.

Speaker 1: Exactly. It turns out that feeling is spot on. Our cities globally. They’ve been going through some really quite dramatic changes over the last 30 years or so,

Speaker 2: and often in ways we don’t really consciously notice day to day.

But the scale is huge.

Speaker 1: Right. And what’s amazing is we can actually see this scale now thanks to this study we’re looking at today. It’s

Speaker 2: fascinating. Yeah. Researchers used a whole bunch of satellite data. We’re talking scatter, TERs, LANsat data, pulling it all together

Speaker 1: to map how cities grew across the entire planet,

Speaker 2: across the whole planet from the 1990s, right through to the 2010s.

So three decades of urban evolution seen from space.

Speaker 1: Okay, so today we’re getting this bird’s eye view of 30 years of city growth,

Speaker 2: pretty much.

Speaker 1: And our mission really is to figure out the biggest shifts in how cities have expanded and maybe more importantly. What those changes actually mean for us.

Speaker 2: It’s a shortcut in a way to understanding this really complex, global thing that’s happening.

Speaker 1: Definitely. And let’s just put the main finding out there right away because it’s the core of it all. Go for it. Globally, there’s been this major shift, cities moving away from just spreading outwards horizontally to building upwards. Vertically.

Speaker 2: That’s the headline. Absolutely. The study really digs into these two main ways

Speaker 1: cities grow. Okay, so break those down for us

Speaker 2: first, you’ve got lateral spreading out. That’s pretty intuitive. Building on land that wasn’t urban before. Expanding the city limits

Speaker 1: sprawl.

Speaker 2: Yeah. Then there’s vertical growth that’s about taller buildings, increasing the density upwards.

Speaker 1: And there was a third one too, right?

Infilling.

Speaker 2: Yeah, infilling. That’s building on vacant lots or underused land within the existing city Boundaries. Important. But the big story here is that outwards versus upwards dynamic.

Speaker 1: So how on earth do you measure that stuff from? Space? Satellites aren’t exactly holding, measuring tapes down here.

Speaker 2: No, not quite. They use two really smart metrics that give different pieces of the puzzle. The first is called building fraction, BF for short. It comes from LANsat data. Think of it as measuring the horizontal footprint. How much ground is covered by buildings,

Speaker 1: so that captures the spreading out and maybe some of the infill too.

Speaker 2: Exactly. The horizontal expansion. Then the second metric is microwave back, scatter or pr. This comes from different satellites. Scatter ter.

Speaker 1: Oh, what does that tell us?

Speaker 2: This is where it gets really interesting for the vertical part. PR is sensitive to the physical volume of buildings. The microwave signal bounces differently off taller,

Speaker 1: denser structures. So it’s not just area, it’s like mass height.

Speaker 2: It correlates strongly with building volume, which includes height. So yeah, think of it as an indicator of how built up, how tall and dense an area is becoming.

Speaker 1: And these are measured independently,

Speaker 2: totally independent data sources. Yeah,

Speaker 1: that’s key. Why is looking at both together so important then?

Speaker 2: Because it gives you that three dimensional picture. BF shows the spread PR hints at the height and volume. Suddenly you’re not just seeing a flat map expanding. You’re seeing how the structure of the city is changing,

Speaker 1: moving beyond just 2D. Most studies before were limited that way.

Speaker 2: A lot of’em were, yeah, it was harder to get consistent data on that vertical dimension globally. Over time, this study really cracked up.

Speaker 1: Okay, got the tools BF for spread PR for height and volume. Let’s get back to that big shift. You mentioned outwards to upwards. What did the numbers actually show?

Speaker 2: This is the crux of it. They saw this clear divergence. Over the three decades, the rate of growth in building fraction, the spreading out actually decreased in most regions in big cities.

Speaker 1: So cities were still spreading, but more slowly.

Speaker 2: Yeah. The pace of outward expansion generally slowed down, but at the same time,

Speaker 1: let me guess, PR went up

Speaker 2: exactly. The growth rates in microwave back scatter, our heightened volume indicator increased in almost all regions and cities.

Speaker 1: Wow. So less outward sprawl, more upward growth.

That’s a pretty clear global trend.

Speaker 2: It really is. This difference, this divergence between the two metrics strongly points towards this global tendency for cities to develop more vertically.

Speaker 1: But you said it wasn’t the same everywhere.

Speaker 2: No, definitely not. The super important, this transition, this shift, it happened at different times and to different degrees all across the world.

Speaker 1: Makes sense. Different places, different development stages. Where was this vertical push strongest?

Speaker 2: The biggest increases in that PR growth rate. The vertical indicator were really concentrated in Asian cities.

Speaker 1: Okay. Like where specifically,

Speaker 2: China is a huge example. They saw a big drop in areas that were growing slowly, but also these massive areas of rapid growth in both BF and pr, what the study called Up and Out

Speaker 1: building. Up and out at the same time.

Speaker 2: Yeah. Intense growth in both dimensions. But what’s also telling is that in the cities, like Beijing and Shanghai. The rate of the outward spread, the BF increase started slowing down compared to the PR increase later on,

Speaker 1: suggesting a stronger push upwards in those massive centres. Maybe land getting really expensive.

Speaker 2: That could definitely be part of it. Rapid urbanization, rising land values. It pushes development upwards.

Speaker 1: What about other big Asian countries like India?

Speaker 2: In India, especially in the 2010s, the biggest city is over 5 million people. We’re mostly showing either that up and out pattern or just strong outward growth. Still a lot of expansion.

Speaker 1: Say Europe.

Speaker 2: Europe was really different over the whole study period. There was actually very little shift towards vertical growth overall, maybe more stable planning, different economic drivers.

Speaker 1: Interesting. And Latin America.

Speaker 2: The study noted that building growth generally slowed down there over the period.

Speaker 1: What about Africa and other parts of Asia?

Speaker 2: There you saw a clearer shift. Places like West Africa, central Asia, and the big cities in South Asia. They tended to move from mostly outward growth in the two thousands towards more upward growth in the 2010s.

Speaker 1: So that outward to upward pattern showed up there too. Just maybe a bit later

Speaker 2: though South Africa was a bit different, saw a slowing of rapid growth overall.

Speaker 1: The study links this timing to economic development.

Speaker 2: It hints strongly at that. Yeah, it suggests there’s a connection between a region’s economic progress and when it starts making that switch towards vertical development as economies, mature land values rise,

Speaker 1: and building tall becomes more viable, even necessary.

Make sense? Did they look at different city sizes within regions? Did mega cities grow differently than smaller towns?

Speaker 2: They did. I. They found that this shift from lateral to vertical often seemed to start right in the core of the central areas of the biggest cities. The mega cities, over 10 million people,

Speaker 1: like the downtown areas intensify first.

Speaker 2: Yeah. And then that trend sometimes appeared to spread outwards from the core over time. DACA in Bangladesh was a notable exception apparently.

Speaker 1: Okay. So big city centers go vertical first. Mm-hmm. Where did they see the slower growth?

Speaker 2: Slower growth was more common out on the edges, the peripheries of these big cities, and also just generally in smaller cities or towns that were developing more slowly overall.

Speaker 1: So the economic engine in the core drives the first.

Speaker 2: Now the study also mentioned these urban growth typologies.

Speaker 1: It’s actually a really neat way. They classify different growth patterns. They use a statistical method. Carry means clustering. Basically, they let a computer group areas together based on how similar their growth was, grouping them based on their starting point, how built up they were initially in terms of BF and pr, and then how those measures changed, how fast they grew over the three decades. It identifies different like styles of urban growth.

Speaker 2: Okay. Like personalities for city growth, what were some key styles?

Speaker 1: One big one was budding. These were areas that started small, mostly low-rise and just grew slowly. This was actually the most common type globally at first, but it share decreased over time. Makes sense. Things starting small.

Speaker 2: What

Speaker 1: else? Then you had stabilized areas. These were already larger, maybe low to mid-rise. Their growth in both spread and height had really slowed down. This type actually increased its share globally. So some areas that were growing fast maybe matured into this stabilized state

Speaker 2: exactly. Often areas that were previously classified as growing outward

Speaker 1: and was outward.

Speaker 2: Outward was medium sized areas. Mostly low rise, but expanding horizontally really fast. Saw a lot of this in Asia and Africa in the nineties and two thousands, but many transitioned to other types by the 2010s,

Speaker 1: okay? And the vertical ones, there was

Speaker 2: upward, medium to very large areas, often high rise, growing fast vertically, but slower horizontally. Found more in East Asia and Europe initially, but then saw more pop up in China and Southeast Asia later on.

Speaker 1: And finally that up and out one you mentioned for China?

Speaker 2: Yes, up and out, moderate to large areas, mix of mid and high rise, growing rapidly in both directions, spreading and getting taller, denser, increasingly seen in China, but also popping up in parts of Africa and Southeast Asia by the 2010s.

Speaker 1: So a real mix of patterns and their dynamic changing over time. Which brings us to the big, so what. Why does this shift, this global trend towards building up actually matter?

Speaker 2: Oh, it matters hugely. It touches on so many things, resource and energy use. For starters, how think about constructing and operating very tall buildings, they can have a pretty significant carbon footprint. Both in the materials used the embodied carbon and the energy needed to run them elevators. Climate control.

Speaker 1: Right? And it probably affects the local climate too.

Speaker 2: Definitely dense tall buildings create urban heat. Islands change wind patterns affects the whole microclimate of the city.

Speaker 1: What about just living in these cities? How does more verticality change urban life?

Speaker 2: It has huge implications for how we live. On the one hand, higher population density, and importantly, job density can support better public transport.

Speaker 1: Makes sense. More people in one place makes buses and trains more viable,

Speaker 2: right? And it can lead to more walkable neighbourhoods, which is often seen as a positive. Plus building up instead of out can in theory, save land for agriculture or nature.

Speaker 1: But there are downsides too.

Speaker 2: Yeah. Simply cramming more people in isn’t automatically sustainable. You need good urban design, enough green space infrastructure, and as we said, those very tall buildings come with their own environmental costs. It’s a trade-off.

Speaker 1: So density alone isn’t the answer.

Speaker 2: Not necessarily. No. Job density needs to keep pace with population density. Otherwise, you just get dense residential areas with long commutes. The study kind of ties into these broader theories of urbanization showing this city building process linked to economic development.

Speaker 1: You highlighted China earlier as having that really intense up and outgrowth. Why was it so pronounced there?

Speaker 2: China’s case is remarkable. It was massively driven by this enormous real estate boom and they experienced during the study period,

Speaker 1: just how big are we talking?

Speaker 2: The numbers are staggering. Like between 2003 and 2014, the amount of residential floor space built in China. Was more than the entire commercial real estate stock in the US in 2018.

Speaker 1: That’s hard even picture.

Speaker 2: It really is. And they were building a huge percentage of the world’s new skyscrapers during that time too. So that fast up and out typology really reflects this specific economic context

Speaker 1: and all that construction drove up prices

Speaker 2: massively. Land and real estate prices shot up, which led some economists to talk about a potential real estate bubble there.

Speaker 1: Okay, so China, as a kind of supercharged example of these trends, now every study has its limits. What should we keep in mind about this one?

Speaker 2: The main thing with the back scattered data, the PR metric is its resolution. It’s about five to 10 kilometres, which means it’s great for seeing regional trends and changes in large cities. It makes it hard to zoom right into specific neighbourhoods or analyse smaller cities and find detail.

Speaker 1: Okay. So broad strokes, not fine. Brush work

Speaker 2: pretty much. Also, using data from different satellites over 30 years means a lot of careful work went into making sure the measurements were consistent calibration. That’s always a challenge. Sure. And finally, just interpreting that back scatter signal. An increase in a grid cell could be from a few new tall buildings or lots of new shorter ones or placing short with tall. It indicates increased volume, but the exact form can vary.

Speaker 1: So if there’s one big takeaway from this deep dive into three decades of global urban growth, what is it?

Speaker 2: I think the key message is really this profound global shift in how cities are growing. I. We’re fundamentally moving from a pattern dominated by outward expansion to one where vertical growth is increasingly important, even dominant in many places.

Speaker 1: A major transition.

Speaker 2: A major transition, yeah. And while it plays out differently everywhere, it’s a fundamental change with really wide-ranging consequences for resources, the environment, how we actually live our lives in cities.

Speaker 1: And this study gives us this new data-driven way to see that happen. Independent of just looking at economic stats.

Speaker 2: Exactly. It provides this powerful objective view of the physical evolution of cities over time, which is incredibly valuable for understanding our world. I.

Speaker 1: It really is fascinating stuff. So maybe a final thought for everyone listening as you go about your day. Maybe think about this. What does this increasing verticality, this shift upwards mean for your future?

Speaker 2: How might it change the way we live, work, connect with nature in the coming decades? How does it look in your own city or region?

Speaker 1: Does your city feel like it’s growing more outwards or upwards?

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Episode and transcript generated with ⁠⁠Descript⁠⁠ assistance (⁠⁠affiliate link⁠⁠).