Category Archives: Cities

More reasons to stop putting trees on skyscrapers

Bosco Verticale

Robert Krulwich, disagreeing with me:

Two residential towers, dense with trees, will have their official opening later this year in downtown Milan, Italy, near the Porta Garibaldi railroad station. (The image is not a photograph, but an architect’s rendering. The towers are built and the trees are going in right now.) I love this. I think these towers are gorgeous. Milan is a very polluted town; these trees will cleanse the air, pumping out oxygen and greening the cityscape. I think cities one day could look like mountain vistas; I’m enthralled.

But I am not Tim De Chant, tree lover, blogger, critic, who says this won’t work. All these trees, he thinks, are about to be dead. He recently posted an essay on his Per Square Mile blog, aimed at architects. He called it, “Can we please stop drawing trees on top of skyscrapers?” He thinks builders know squat about trees. I hope he’s wrong.

I know I seem like Buzz Killington to a lot of architects—and non-architects, Krulwich included—but that wasn’t my point…entirely. To me, trees atop buildings have become an architectural crutch, a way to make your building feel sustainable without necessarily being so. And that’s a charitable assessment. Here’s how I really feel—trees on skyscrapers are a distraction from rampant development and deforestation. They’re trees for the rich and no one else. They’re the soma in architecture’s brave new world of “sustainable” development.

In reality, trees on skyscrapers will likely be anything but sustainable. Structures built to support trees need to be over-engineered compared with their abiotic equivalents—trees are heavy, so is dirt (multiply so when wet), and so are watering systems required to keep them alive. If those trees are to have a chance on these windy precipices, their planters had better be deep, which further compounds problems raised in the previous sentence. A skyscraper that’s built to support trees will require more concrete, more steel, more of anything structural. That’s a lot of carbon, not to mention other resources, spent simply hoisting vegetation dozens of stories up, probably more than will ever be recouped in the trees’ lifetimes.

Bosco Verticale, the oft, and often only, cited example of a tower to be built with trees on top, is expected to cost $85 million. Stefano Boeri, the architect, estimates adding trees to the design pushed up construction costs about 5 percent. (No word on maintenance costs.) Whether that’s true or not, we’ll have to take his word for it. If we do, that means they will spend $4.25 million to put 2.5 acres—one hectare—of forest onto the side of a building.

Now, let’s say we take that money and resuscitate the region’s natural habitat.¹ Average costs run about $500 per acre for reforestation in U.S. national forests, with a top end of about $2,000 per acre. Let’s assume the worst. That means that with $4.25 million, you could restore 2,125 acres, or about 860 hectares, of forest. That’s 860 times more forest than is plastered on the side of Bosco Verticale. At the least. If restoration costs come in at the low end, about $200 per acre, it could be as high as 8,600 times more.

Then there’s the ecological value of each. Bosco Verticale will be home to a few birds (most of which will live in the city regardless) and some invertebrates, but not much else. It’ll also require massive human inputs—water, fertilizer, tending, and replacement. I covered the first of those three in my previous essay, so I’ll just elaborate on the last point here, replacement. Let’s say trees on a skyscraper will live for an average of 20 years—a generous assumption given that more than 50 percent of street trees, which are exposed to more benign conditions, die after just 10 years—what will we have gained? A skyscraper that needs an overhaul every 20 years.

A real forest, on the other hand, can replace itself. It can also support hundreds, even thousands of species, even in the middle of the city. A survey of the 315-hectare Central Park, for example, found over 800 species. Near Milan, at Parco Regionale di Montevecchia e della Valle del Curone, there nearly on it’s 2,350 hectares. Biodiversity is just one measure. These forests also purify water, maintain nutrient cycles, and don’t require much in the way of maintenance (if any).

Here’s an alternate plan: Instead of planting trees on buildings, let’s focus on preserving and restoring places that already have, or desperately need, trees. Boeri and I agree on the importance of the latter. Bosco Verticale is the first stage of Boeri’s larger plan, one that includes preservation and restoration of existing land.² Bravo. It’s clear that Boeri understands the big picture, that to make a truly sustainable city, you have to incorporate ecosystem function on a broad scale.

We still disagree on the value of trees on skyscrapers: he, and Krulwich, see them as an inspiration; I see them as a distraction and potential liability—what if the Bosco Verticale becomes a brown eyesore, turning people off to his larger vision? I’d love it if Bosco Verticale and other proposed arboreal skyscrapers were sustainable and successful.³ Who wouldn’t want to live in a city full of tree towers? But I just can’t make a case for it. Plant physiology tells me that the trees, if they do survive, will require constant and costly maintenance throughout their short, brutal lives. Finance tells me that the money required to afforest a building would be more effectively used for restoration and preservation. And my gut tells me there are more equitable ways to give people trees, not just to those who can afford it.

  1. You could also plant street trees or reserve more land for parks, both laudable and equitable uses.
  2. Among the proposals is a greenbelt around the city. They’ll be great parks, but won’t do much to contain the city.
  3. Really, Robert, I do!


Roman, Lara. 2006. Trends in street tree survival, Philadelphia, PA. Master’s thesis.

Gorte, Ross W. 2009. U.S. Tree Planting for Carbon Sequestration. Congressional Research Service R40562.

Illustration of Bosco Verticale.

Related posts:

Can we please stop drawing trees on top of skyscrapers?

Urban trees reveal income inequality

Income inequality, as seen from space

Can you tell urban from rural?

If you were given a section of a map, could you tell if was from a city or the countryside? The answer to that question may be trickier than you expect. I pondered this a year and a half ago when I wrote, “ ‘countryside’ is inherently interpretable term, one that depends more on how the land is used than it does on population density.”

It first struck me when I was traveling around Taiwan. There, the distinction between the rural and urban areas wasn’t always apparent to my Western eyes. The same can be true with maps. Distinguishing between urban and rural depends as much on geographic and cultural contexts as it does on visual cues like road networks.

Can you tell which is which?

The following maps are road networks from a variety of locations around the globe. Guess which are cities and which are rural areas. All maps are drawn to the same scale.

1. 1. Urban or rural?

2. 2. Urban or rural?

3. 3. Urban or rural?





From top to bottom: 1. city (Denver) 2. countryside (Japan) 3. city (New York City) 4. city (Houston) 5. countryside (Taiwan) 6. city (Los Angeles) 7. countryside (Wisconsin)

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What do we mean by “rural”?

Income inequality, as seen from space

If the world’s population lived like…

Can we please stop drawing trees on top of skyscrapers?

Editt Tower

Just a couple of years ago, if you wanted to make something look trendier, you put a bird on it. Birds were everywhere. I’m not sure if Twitter was what started all the flutter, but it got so bad that Portlandia performed a skit named, you guessed it, “Put a Bird On It“.¹

It turns out architects have been doing the same thing, just with trees. Want to make a skyscraper look trendy and sustainable? Put a tree on it. Or better yet, dozens. Many high-concept skyscraper proposals are festooned with trees. On the rooftop, on terraces, in nooks and crannies, on absurdly large balconies. Basically anywhere horizontal and high off the ground. Now, I should be saying architects are drawing dozens, because I have yet to see one of these “green” skyscrapers in real life. (There’s one notable exception—BioMilano, which isn’t quite done yet.) If—and it’s a big if—any of these buildings ever get built, odds are they’ll be stripped of their foliage quicker than a developer can say “return on investment”. It’s just not realistic. I get it why architects draw them on their buildings. Really, I do. But can we please stop?

There are plenty of scientific reasons why skyscrapers don’t—and probably won’t—have trees, at least not to the heights which many architects propose. Life sucks up there. For you, for me, for trees, and just about everything else except peregrine falcons. It’s hot, cold, windy, the rain lashes at you, and the snow and sleet pelt you at high velocity. Life for city trees is hard enough on the ground. I can’t imagine what it’s like at 500 feet, where nearly every climate variable is more extreme than at street level.

Wind is perhaps the most formidable force trees face at that elevation. Ever seen trees on the top of a mountain? Their trunks bow away from the prevailing winds. That may be the most visible effect, but it’s not the most challenging. Wind also interrupts the thin layer of air between a leaf and the atmosphere, known as the boundary layer. The boundary layer is tiny by human standards—it operates on a scale small enough that normally slippery gas particles behave like viscous fluids.

For plants, the boundary layer serves to control evapotranspiration, or the loss of gas and water through the tiny pores on a leaf’s underside, known as stomata. In calm conditions, a comfortably thick boundary layer can exist on a perfectly smooth leaf. But plants that live in hot or windy places often have adaptations to deal with the harsh conditions, including tiny hairs on their leaves which expand each leaf’s surface area and thus its boundary layer. Still, plants in these environments aren’t usually tall and graceful. In other words, not the tall trees we see in architectural drawings.

Next let’s add extreme heat and cold to the mix. Extreme cold, well, we all know what that does. It can kill a plant, turning the water inside its cells into lethal, crystalline knives. At the other end, hot conditions post a different set of challenges. To cool off, plants can “sweat” by opening their stomata to release water vapor, at least as long as there’s water available. But even then, plants reach a limit. At certain temperatures, which vary from plant to plant, the photosynthetic machinery inside a leaf starts to break down. Keep in mind these are temperatures on the surface of a leaf, not ambient air temperature. The surface of leaf—especially in direct sunlight, as on the unshaded side of a skyscraper—can be many degrees hotter than the air, up to 14˚ C in some species (nearly 26˚ F).

Then there are the logistical concerns. How are these trees going to be watered and fertilized? Pruned? How will they be replaced? How often will they need to be replaced? As someone who grows bonsai, I can tell you that stressed plants require constant attention. Daily monitoring, in fact, and sometimes even more frequently. It’s not easy. Growing simple green roofs is a chore, and those plants are chosen for their hardiness and low maintenance. Trees are generally not as well adapted to the wide range of conditions likely to be experienced on the side of a skyscraper.

All of this may sound a bit ridiculous coming from someone like me, an advocate for more trees in urban spaces. It probably comes from having seen one too many sketches of a verdant vertical oasis but too few of them actually built. Plus, having studied plant physiology, I know that it’s a pipe dream in many ways. Trees just weren’t made for such conditions. Now if someone want to gin up a tree that can survive on top of a skyscraper, go ahead, I guess. But I can think of far better things we should be putting our time and effort into, like preserving places that already have trees growing on them or planting more on streets that need them.

  1. “What a sad little tote bag. I know! I’ll put a bird on it.” Etc.

Illustration of Editt Tower, a proposed 26-story building in Singapore.

Related posts:

More reasons to stop putting trees on skyscrapers

How TED and The City 2.0 took the internet for a ride

Urban trees reveal income inequality

Income inequality, as seen from space

What I learned from the blizzard

Road? What road?

Snowfall has many magical qualities, among them the ability to hush a bustling city and, in sufficient quantities, make time stand still. This past weekend’s blizzard did just that—it stopped life in its tracks. Everyone’s plans went out the window (and landed in a mounting drift, I assume). Meanwhile we all sat—contently I hope—waiting out the nor’easter.

Blizzard of 2013

As the night went on, the temperature sank and the wind howled, blowing the ever fluffier stuff into smooth scallops and pointy tufts.

whipped snow

When the sun finally rose, many of us were surprised to find that time hadn’t just stopped, it had retreated. You see, the evening before the governor had banned car travel in Massachusetts. That left the streets to snowplows and first responders and, in the morning, the people. By midmorning, the roads around my house in Cambridge resembled what they must have looked like when they were first platted hundreds of years ago. Without the horses of course. People were out in droves, shuffling down the middle of the street on their tour of the neighborhood.

Massachusetts Avenue

Cambridge has its share of pedestrians, but not like this. This doesn’t happen on a normal morning. But Saturday morning, there was plenty of novelty to draw people into the street, including the chance to literally be drawn into the street and not worry about being run over. That and the snow, piled high in magnificent masses.

marshmallows of snow

As people strolled, they stopped to chat. With nowhere to go and no schedule to keep, everyone was a bit more jovial than normal, despite the monumental amount of snow to move.

The streets were cozier, too. The plows had cleared what they could, which is to say not nearly everything. Intersections had quirky, nearly unrecognizable new boundaries. Blizzards, I’m convinced, are thinly disguised urban planning lessons. Watch where cars and plows go during and after a blizzard and you’ll see exactly how much roadway you need and not a foot more.

one-way street to one-way street

Life is slowly returning to normal. Traffic is up, even if travel speeds aren’t quite. The asphalt is showing, but many sidewalks are not. In a week or even a few days, this will be nothing more than a fond memory.


Photos by Tim De Chant.

Reading this will change the climate


It’s no secret that burning fossil fuels is changing the Earth’s climate. Emissions are bolstering the planet’s greenhouse effect, which is raising temperatures around the world. But it’s not an even warming, and neither may it all be entirely attributable to emissions alone. The heat produced from burning fossil fuels may play a role in climate change, particularly its geographic inconsistencies.

I admit I was a bit surprised to learn that in a recent study in the journal Nature Climate Change. The Earth’s atmosphere is massive—the heat we dump by burning coal, oil, and gas seems minuscule in comparison. Picking up on such a signal would be like claiming you noticed that the pool got warmer because some kid peed in the other end. Not very plausible, at least until you realize that the world has thousands of cities, each giving off a small amount of heat. It’s like thousands of kids peeing in a really big pool. Eventually, you’ll notice something. Each city (or kid) on their own may not have a noticeable effect, but combined, they can alter the distribution of warming, perhaps significantly. It’s the butterfly effect applied to the 21st century.

Global energy consumption was 509 exajoules in 2009, down slightly from 2008 (a slumping economy will do that). That sounds like a lot, but it pales in comparison—well less than 1 percent—to the amount of energy the Earth’s oceans and atmosphere move around the globe every year.

But our energy consumption—and production—isn’t evenly distributed. We make a lot of it near cities and consume nearly all of it in them. The authors of the paper sampled 86 points in the Northern Hemisphere where energy use exceeded 0.4 watts per square meter. That’s pretty sensitive, especially if you consider that Tokyo, for example, can consume up to 1,590 watts per square meter on a cold winter morning. The 86 points represent only about 1.3 percent of the Earth’s surface, but about 42 percent of our total energy consumption.

The scientists plugged that data into standard climate models and ran them for a simulated century. Without the warming data, the models returned results similar to those found before. But when the point source warming from cities was added, the warming patterns more closely hewed to real world trends. For example, the new model results predicted warmer winters over parts of Canada, China, and Siberia, which the authors note “has a high resemblance to the unexplained surface temperature trend in the second half of the twentieth century by anthropogenic radiative and aerosol forcing.” What they really mean is, “Remember some of the shortcomings in previous models? We seem to have solved them.”

While at first glance it seems improbable that the heat produced by our energy consumption could affect a system as massive as the climate, it shouldn’t come as a surprise. The climate is a complex system, and any perturbation, however small, can have unintended consequences. If you want evidence, just look around. The climate is changing, no doubt about it. The more we look, the more evidence we find of our global footprint. This is just the latest example. I’m certain there’ll be more.


Zhang G.J., Cai M. & Hu A. (2013). Energy consumption and the unexplained winter warming over northern Asia and North America, Nature Climate Change, DOI:

Photo by Señor Codo.

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Planet of the cities

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How TED and The City 2.0 took the internet for a ride

TED Global 2012

Last year, TED made a lot of noise when it announced that it was awarding its TED Prize to something called “The City 2.0”. In case you don’t know what “The City 2.0” is, it’s an idea. At least that’s what TED was telling us. They were awarding the prize to an “idea” instead of a person, sort of like when Time Magazine goes all crazy and awards the Person of the Year to a machine.

Well, TED isn’t about machines, but it is all about ideas, so it gave its award to an idea, which was really like giving an award to itself, which as you’ll see in a bit is actually a more accurate statement than you’d think. Did I mention that this idea had a website? It does. One that TED designed and built themselves. Well, half-built. See, it wasn’t exactly ready when the TED Prize was announced, which is funny because TED both built the site and gave the award.

The humdinger behind the original The City 2.0 ¹ was that people could use the site to start grass roots campaigns to change their neighborhoods and cities. The idea was that “the reach of the cloud” and “the power of the crowd” would join forces and, from that totally awesome buzzword high-five, ten winners would emerge.

Wait, ten winners? Didn’t an idea already win the prize? Turns out that even though an idea can have a website, it can’t accept $100,000. Well, it could if TED, which came up with the idea for The City 2.0, had awarded the money to itself, but that would just look tacky. So instead they split up the $100,000 with the intent of awarding it to those ten winners who would bubble up from the cloudy crowd.² I’m not sure how the cloudy crowd picks the best ideas, but at the time I guessed it would have something to do with voting, as most cloudy crowds do.

This is the awkwardly placed paragraph where I say that I reached out to The City 2.0 for comment because I felt like I had to and because I actually did. They haven’t responded, not even a “no comment” or anything.

With a whole $10,000 on the line, I’m sure TED was expecting the The City 2.0 to be a hotbed of entrepreneurial activity. It wasn’t. Two months after the prize was awarded, I checked back. The site was finished, but it didn’t seem like anybody else knew that. There were just 124 ideas, most of which only had one or two votes in favor of them. Less than two months remained until TED Global, the event where the ten winners were supposed to be announced.

To be fair, there were a few ideas that were winners, some of which would go on to be actual winners, but they still weren’t announced at TED Global. (How these winners were determined I do not know because the only thing on the site about them was a handful of lousy sentences.) Most, though, were definitely not good. Some were rehashes of city council debates:³

Livable Elgin Parkway

Eglin Pkwy in Ft Walton is a 7 lane high speed road. In order to foster development / creativity, this could be reduced to a 2 lane road. Through traffic should be redirected to a high speed location.

Others didn’t even bother mentioning cities or local issues:

Creating a Viable 3rd Party


And my favorite?

Make the public aware of City2.0 [sic]

In order for this to become a reality local residents need to be made aware of The Wish.

Umm, guys, you realize anybody can read this, right?

You know how you feel when you see someone do something embarrassing and you feel embarrassed for them? That’s how I felt for The City 2.0.

Well, the TED people aren’t idiots. They know a failed venture when they see one, so they quietly pulled the plug on “The City 2.0” and rolled out… “The City 2.0”. The, um, 2.0 version of the site scrapped the map and didn’t show people’s submissions, because, like, some people’s ideas are totally lame and we don’t want that messing up our snazzy website we dropped a lot of coin on. In it’s place were “stories” that were selected by TED along with random urbanist links. That’s, like, totally better.

Did you know that October 13 was TEDxCity2.0 Day? Neither did I, and one of the supposed TEDxCity2.0 events was in Boston, just two miles from where I live. It had people talking about leadership and Mars and poetry and financial markets and communication and aerodynamics and mind reading but it didn’t have anyone talking about cities. Huh.

To date, TED has awarded eight of its ten prizes, five of which were given to projects that had already existed before The City 2.0 got all spendy. That’s not to say the winners don’t deserve it—they’re great projects—but it’s not exactly the crowdsourced, spontaneous wonderland what TED led us to believe The City 2.0 would be about. Two more are supposed to be awarded in November and December. It’s December already.

One of the things that upsets me about The City 2.0 isn’t that it was a half-baked idea with a sucky website that dozens of other organizations were already doing way better. Or that it was a transparent marketing ploy meant to draw more attention to TED than to the issue. Or that there was absolutely zero transparency about how winners were going to be selected despite pandering to the “crowd” and its infinite wisdom. No, what gets me is that so many people bought into it. ArchDaily ran such a breathless piece that I almost mistook it for a press release. The Atlantic Cities ran a laudatory article. Inhabitat, too.

Well, not everyone was buying it. The Next American City was doubtful—at first. They wrote a bit about how The City 2.0 was a major letdown, man. But later, they heard about the relaunch of The City 2.0 somehow, wrote a nice piece about it, and ended up winning one of the ten The City 2.0 awards. I also wrote a skeptical piece about The City 2.0 way back when, but I didn’t hear about the relaunch and didn’t write a nice piece about it and didn’t win an award.

I shouldn’t be surprised by all of this. In fact, I’m not. I know TED is a marketing machine and that its only real interest is making sure that the TED name is everywhere. I mean, what else can you expect from a conference that’s feels the need to address its own elitism? But that doesn’t make me any less angry. Angry that TED exploited the rampant churnalism that’s so prevalent on the internet. Angry that so many people bought into TED’s hokey and transparently vain message. And angry that TED would so arrogantly presume to fix something as complex as the city without giving it any more thought than would a few fresh-faced marketing graduates.

  1. They obviously didn’t have any better ideas for a name. Oh, and all links in this article to the original version of the site are from a publicly viewable development server. It looks marginally worse than the actual site did at the time, but all the data stored on the dev server is legit.
  2. Apparently, TED has run out of famous wealthy people to give their prize to, so they settled on ten less famous and less wealthy people. Maybe because less wealthy people are more easily wowed by less money?
  3. Have you ever been to a city council meeting? People can get angry there. I once saw a guy punch his own dog! Not really, though. But I can totally imagine it happening.
  4. Someone should tell them how version numbers work.
  5. GASP! That’s a long acronym.
  6. Somewhere, a stenographer just got his wings.
  7. NOTE: Correlation ≠ causation. In case you didn’t know that because you haven’t taken statistics or don’t read Slashdot.
  8. Churn + journalism. Lots of news and yet somehow no actual news. Sort of like CNN.

Photo by Stefan Schäfer, Lich.

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Can crowdsourcing save the city?

In Star Wars, cities are evil

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How population density affected the 2012 presidential election

There are lots of reasons why the 2012 presidential election broke the way it did, but one that’s not often reported—but particularly germane to Per Square Mile—is the divide between cities and the country. I’ve been thinking for a while now about this split as a driving force behind the polarization of U.S. politics, and I know I’m not alone. (On election night, Adam Rogers tweeted as much.)

But I was curious. Can we actually see the divide between cities and the country in the electoral map? In short, yes, but I’ll let the maps to the rest of the talking.

Swipe back and forth to see how population density relates to each candidates’ electoral result.

Thanks to Andy Woodruff of the always interesting Bostonography for the shapefile of the election results.

Related posts:

Income inequality, as seen from space

Redrawing the United States of America

Ghosts of geography

When everyone lives in a city

The future?

Here’s a fun thought experiment. Plot the population of the world since 500 BCE. Now plot the population of the 50 largest cities over that same time. If you distill equations to describe the two trends, you’ll notice the lines cross. At some point in the future, your models predict that the population of the 50 largest cities will overtake the world’s population. Clearly that’s impossible.

What those trends are telling you is that cities are growing faster than rural areas, something we already know. But if you take that thought experiment to its mathematical extremes, you’ll see it’s possible that there comes a point when—boom—everyone lives in a city. Rural dwellers—poof—cease to exist. Suddenly, we’re all children of the concrete jungle.

That’s what Michael Batty, a well-known urban planner and geographer, noticed when he ran through those same hypotheticals. Specifically, he calculated that by 2092 all the world will be urbanized according to those trend lines. Of course that won’t happen, and he acknowledges that. The world’s population will, at minimum, be equal to the sum of its cities, and I’m 100 percent certain that at least a handful of people will still live in the country, either by choice or chance. But Batty’s idea bears consideration. What would the world look like when, as he puts it, “all the world’s a city”?

The United Nations currently estimates the world’s population will reach 10 billion by 2100, just a few years after mathematics suggests we could all be living in cities. Now, that’s not to say the Earth would be covered by one massive city. Cities may be expanding outward faster than their population growth would warrant, but 10 billion people spread across all continents but Antarctica would live at a density of about 190 people per square mile (74 per square kilometer). Hardly a city.

But what if our notion of a city changed? A single definition is already maddeningly difficult to nail down. Take New York City, for example. It has about 8.2 million people within the polity, but the greater region has over 22 million. Where does New York really end? Houston and Tokyo, on the other hand, encompass too much. Each political entity contains vast tracts of undeveloped land. It’s clear that political boundaries aren’t adequate. So instead, what if we think of a “city” as a collection of conurbations not connected by geography but by social and economic ties, as Batty suggests? In an age of plane travel and high-speed rail, physical continuity isn’t necessarily a requirement.

With this new definition, it is possible for all the world to be one city. The Earth doesn’t have to be covered in conurbation; rather, everyone simply has to live in urban areas, and those urban areas must be sufficiently connected so as to behave like a single city.

Already metros and their regional governments cross existing political boundaries. New York City is a perfect example. And at the other extreme, there are cases like Tokyo where city governments have essentially absorbed their hinterland equivalents. (We see this all the time in the United States with combined city and county governments—New York City, San Francisco, and Lexington, Kentucky, to name a few.) These mergers grew out of necessity, and it’s easy to see the same happening in a hyper-connected world. As more and more cities join the global cluster, as Batty calls it, the pressure to coordinate will rise.

It’s possible, then, that the first true world government could emerge from this collection of cities. It would be fitting. Already mayors from around the world meet to discuss common problems, and on issues like climate change where national governments have fallen flat, they have taken the lead. But it would still be a shift of epic proportions. It wouldn’t happen overnight, but at a certain point it would be inevitable. Cities could choose to sit on the sidelines, but the benefits of joining the global cluster would be too great to ignore. Eventually, nearly everyone on Earth would count themselves a resident of the One City.

The world as one city would surely be a different place. The relationship between a city and its hinterland would be tested. Indeed, what would become of the hinterland? It would certainly be smaller—although the One City wouldn’t smother the planet, it would still have an enormous footprint. The hinterland would remain inhabited by scattered few who choose to live there, perhaps living their quiet lives amongst robotized farms. A great schism between the city and the hinterland could develop. But there could also be a reconciliation. Governments could reconfigure to cope with the changing landscape, both literal and figurative. Instead tension between the city and its hinterland, there could be cooperation fostered by a sense of shared fate. So goes the city goes the hinterland, and vice versa.

Regardless of how it all plays out, a highly urbanized global population will add nearly 6 billion people to cities that only hold about 3.6 billion today. That’s growth of almost 280 percent in less than a century. Such a percentage isn’t unprecedented—between 1900 and today, the world’s urban population grew by more than 1600 percent—but the raw numbers will be. To accommodate those people, cities will have to remake themselves like never before. It’s a daunting challenge, and as I stated in my last article, we’ll need a science of the city that’s equally formidable.


Batty, M. (2011). When all the world’s a city, Environment and Planning A, 43 (4) 772. DOI: 10.1068/a43403

United Nations. 2011. “World Urbanization Prospects, the 2011 Revision.” Accessed October 8, 2012.

Photo by kevin dooley.

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If the world’s population lived in one city…

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If the world’s population lived like…

Planet of the cities

Endless city

Science fiction is littered with planet-wide cities. Star Wars had Coruscant, Isaac Asimov’s Foundation Series had Trantor, and even Star Trek, in an alternate timeline in First Contact, saw Earth paved over by the cybernetic Borg. City-planets are both a wonder and a terror—we stand in awe of our power to change the world, but also regard that ability with fear. What if we actually did it? Hundreds of years ago, it didn’t even seem possible. But the quickening pace of population growth in the 1800s changed that, and the vast environmental changes sweeping the planet today are a stern reminder of just how powerful we are.

Of course, Earth is not in any immediate danger of becoming one conurbation, but the trend isn’t promising. Urban areas around the world are expanding at double the rate of their populations. That means every person who moves to or is born in a city will, on average, take up more space than the person before them.

And soon there will be a lot more people wanting that extra space. Urban populations are expected to swell to 5 billion in the next two decades, and according to a recent study published in the Proceedings of the National Academy of Science, cities are expected to gobble up another 460,000-plus square miles (1.2 million square kilometers). Put simply, it means that cities will occupy three times more land in 2030 than they did in 2000.

That’s a staggering amount. The results are equally astounding. The east coast of China is likely to merge into one massive city. Cities in the region spanning Lake Victoria’s north shore—an arc from Burundi to Kenya—will swell 1,900 percent in area. Urban areas along the coastal strip from Côte d’Ivoire to Nigeria will grow 920 percent. Other countries like India, Mexico, and Turkey will also experience outsize urban growth. Even developed regions like North America and Japan will have to endure massive urban expansion.

There’s a lot to be concerned about with these statistics. Urban areas that grow several-fold in a few decades won’t be happy places to live—they’ll likely be dominated by slums, or at the very least snarled with traffic and choked with pollution. The situation is even more dire for the ecosystems they’ll be displacing, which are some of the most diverse in the world. That’s because we humans prefer to live in dangerous places, places dominated by disturbances like earthquakes, volcanoes, tsunamis, and floods, processes that produce rich and diverse ecosystems. As a result, the next two decades of urban expansion will imperil nearly 140 amphibian species, more than 40 mammal species, and 25 bird species.

That doesn’t mean we should halt progress and send everyone back to the farm. Cities are still more environmentally friendly on a global scale than the alternative. No, what it means is that we need to think hard—and fast—about how we’re going to cope with the booming urban areas of the next two decades. Much of that growth is going to happen in places like Africa that haven’t had large cities historically. Nigeria, for example, is projected to have more than 400 million people in 2050. That’s more that double its present population.

Success will depend on thinking big. It’ll require “more holistic policies that integrate traditional urban sectors—transport, energy, sanitation, buildings—with land use and conservation,” the study’s authors write. A “science of the city” that others have been arguing for. Developing that science is, in my mind, the only hope we have of taming these urban beasts. The cities of the future may not be the planet-consuming behemoths of our techno-futuristic tall tales, but they’ll be beyond anything we’ve ever seen before. We’ll need a science that’s equally formidable.


Seto, K.C., Guneralp, B. & Hutyra, L.R. (2012). Global forecasts of urban expansion to 2030 and direct impacts on biodiversity and carbon pools, Proceedings of the National Academy of Sciences, 109 (40) 16088. DOI: 10.1073/pnas.1211658109

Photo by Carl Lovén.

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Why neighborhood quality matters


In the summer of 2008, I found myself lost in Chicago. I was new to the sprawling city, living there for two months while working as an intern science reporter at the Chicago Tribune. To counter weekend boredom, I would explore the city by bike. I’d often find myself mildly disoriented, but this time I had really done it. I knew I was north of the Loop, but that was about it.

Finally, I happened across Halsted Street, a major north-south thoroughfare that ran a few blocks from where I was living. Relieved, I started south, coasting past nice brick apartment buildings and row houses, big box stores and tony shops. But after a few miles, the timbre of Halsted changed. Quickly. Well-kept storefronts gave way to crumbling industrial buildings and weedy sidewalks. Then came the most depressing hulk of a building you could imagine. It was a drab beige tower surrounded by acres of asphalt and dotted with boarded up windows fringed with smoke stains. I knew exactly where I was.

Cabrini-Green’s bad reputation was well-known, especially in the Midwest where I grew up. It was among the first projects built by the Chicago Housing Authority in the mid-20th century. By the 1970s, Cabrini-Green was notorious for destitution and violence. Gangs had taken over various buildings, and internecine warfare kept even the hard-nosed Chicago police at bay. Tales of horrific murders spread around the country. My aunt, who tutored elementary school students there, would recount stories she heard from residents. Cabrini-Green was at best a place to be avoided.

And here I was, blithely riding by. Though I didn’t know it at the time, Cabrini-Green’s most notorious days were past it. The tower was largely vacant, save for a few holdouts. When I moved back to Chicago a year later, the last of the high-rises were being torn down.

Housing projects are widely viewed as a failure. Their concentrated poverty seemed to descend into a negative feedback loop, where depravity fed on itself. Over the years, Chicago tried to assert control, to no avail. They muscled up the police presence; that was met with sniper fire. In 1981, then-mayor Jane Byrne moved into a ground floor apartment guarded round the clock by uniformed officers. She moved out three weeks later.

Oddly enough, Byrne’s publicity stunt was on the right track, if completely backwards. Housing projects’ needed a shock to break the cycle. But they didn’t need a mayor moving in. They needed to move people out.

That’s exactly what the United States Department of Housing and Urban Development did in the mid-1990s, when it took over the dysfunctional Chicago Housing Authority. As a part of the Moving to Opportunity program, households could enroll in a lottery which would give winners assistance in finding and vouchers to pay for housing in low poverty neighborhoods. Chicago wasn’t the only city in the trial—New York, Boston, Los Angeles, and Baltimore were also involved. The idea was that after moving to low poverty neighborhoods, families would find more and better jobs and be healthier and happier. Nearly 5,000 families were included in the experiment.

It worked, to some extent. A study published today in the journal Science details how people involved in Moving to Opportunity have fared in the 15 years since the program started. Though those who moved to low poverty neighborhoods were still as poor as those who didn’t, there were some changes. Notably, they were happier and somewhat healthier.

The study, led by Jens Ludwig of the University of Chicago, looked at a variety of factors, ranging from poverty rate to racial segregation to mental and physical health. Though poverty rates didn’t change significantly, and racial segregation nudged only slightly lower, people’s health improved. Physical health, as measured by obesity and diabetes, were slightly better in this study. (A prior study published by Ludwig and colleagues in the New England Journal of Medicine last year reported more significant improvements.) But mental health, as measured by self-reported subjective well-being, or how people felt about their lives, was significantly improved among people who lived in low poverty neighborhoods. So while people were still poor, they were happier. That says a lot about the importance of your surroundings.

Ludwig and his colleagues point out that if the objective of Moving to Opportunity was to reduce poverty, then at this point it would be considered a failure. But if the goal was to improve people’s lives in other ways, well, it’s not doing too bad.

The study only focused on adults, in part because measuring subjective well-being of youngsters is an inexact science. Because of that, they’re missing an important component. It’s difficult to break free of poverty, especially when you’ve inherited it. But children, through better education and improved health, have more potential than their parents (even if that’s only because they have more of life left to live). It’s possible that, because of a more positive home life, kids in families living in low poverty will have better opportunities than their parents, helping them break the cycle.


Ludwig, Jens, Greg J. Duncan, Lisa A. Gennetian, Lawrence F. Katz, Ronald C. Kessler, Jeffrey R. Kling Lisa Sanbonmatsu. 2012. “Neighborhood Effects on the Long-Term Well-Being of Low-Income Adults”. Science 337(21): 15051510. DOI: 10.1126/science.1224648

Photo by puroticorico.

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Marchetti’s constant, or why the 30 minute commute is here to stay

Shanghai Transrapid maglev

“How far should you live from work?”

I pondered that question a few weeks ago after perusing American Community Survey data compiled by Charlie Gardner. It showed that most people in most metro areas in the United States tended to commute about 30 minutes each way to work, give or take a few. That adds up to about an hour of commute time per day. As I looked into it further, I found a lot of research that corroborated the surprising similarity of people’s commute times.

But in all ten research papers I used to footnote that article, I was missing one. Eric Fischer pointed me toward a 1994 treatise—for lack of a better term—by Cesare Marchetti. He posited that one hour per day is as long as people have been willing to travel. Ever. Since the dawn of human society.

At least, that’s what Marchetti hypothesized. He did have some data to back it up, though it wasn’t his own. It was Yacov Zahavi’s. Zahavi was a transportation engineer who consulted for the U.S. Department of Transportation and World Bank in the 1970s and early 1980s. As part of his work for the DOT, he came up with what he called the Unified Mechanism of Travel Model, or UMOT. Zahavi produced a string of reports and papers on UMOT, which he hoped would shake up how transportation planning was done in cities.

In the process of developing UMOT, Zahavi collected many juicy tidbits of data. One observation was that as people earn more money, they spend an increasing percentage of their income on everyday travel, up to about 13 percent. That number seemed to be both a hard ceiling and firm floor once households earned above a certain amount (about $50,000 in 1976). The richer you got, the more you spent in real terms. But the percentage remained the same.

Another bit—the one that inspired Marchetti—was that people in the United States and the United Kingdom traveled about 1 hour per day. (Astute readers will notice that a 30 minute commute to work—the number most studies have settled on as the “average” commute—adds up to precisely that amount at the end of the day.) Zahavi also noticed that even though some people could travel faster—by car rather than bus, for example, they still spent the same amount of time traveling. They just traveled farther to work—a trend which more recent studies have also uncovered.

Zahavi’s data is compelling, but in Marchetti’s hands it quickly became a universal constant. If so many people in Zahavi’s study (mostly in the developed world of the 1950s and 1960s) traveled the same amount per day, Marchetti reasoned that humans must have an innate desire to travel at most and at least 1 hour per day. Boom. Marchetti’s constant.

Zahavi’s data wasn’t the only leg he had to stand on, though it certainly was his sturdiest. Cave men and Greek villages were another. Marchetti pointed out, “Walking about 5 km/hr, and coming back to the cave for the night, gives a territory radius of about 2.5 km and an area of about 20 km2. This is the definition of the territory of a village, and … this is precisely the mean area associated with Greek villages today, sedimented through centuries of history.”

Marchetti—not one to think small, apparently—then used his new universal constant as a jumping off point to explore the future of tomorrow. How fast would a transportation system need to be to serve a city of 100 billion people? An average speed of 150 km/h sounds about right. Can you turn Switzerland into one giant city? Sure, so long as you run maglev trains in sealed tunnels sucked free of atmosphere. What about if you linked Paris and Casablanca with a maglev, too? “[A] woman in Casablanca could go to work in Paris, and cook dinner for her children in the evening.”

Not a man to leave a logical extreme unreached, Marchetti realized that at some point in the future everywhere on Earth will be 30 minutes from your front door. “With mach-7 airplanes and matching Maglevs, a world city is also possible. The assimilation of the technologies in political terms, however, will take some time.”

Details. In the meantime, enjoy your 30 minute commute.


Marchetti, C. (1994). Anthropological invariants in travel behavior, Technological Forecasting and Social Change, 47 (1) 88. DOI: 10.1016/0040-1625(94)90041-8

Zahavi, Yacov. 1976. “The Unified Mechanism of Travel (UMOT) Model.”

Photo by Lars Plougmann.

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Responsive urban design


We’ve been planning cities for almost as long as they have existed. Archaeological evidence suggests the ancient Egyptians did so 5,000 years ago. Hippodamos, considered by many to be the father of urban planning, imposed street grids on every ancient Greek city that would let him. Since then, we’ve been busily drawing, revising, and otherwise fussing about how best to design our cities.

It turns out we may have it all wrong. Or at least wrong for today’s cities. Urban areas have always been in constant flux, but we’re now demanding far more of them than before. They’ve never housed, transported, or employed so many people. To cope, cities have been changing at an astonishingly rapid pace. The results can be inspiring—as they are in Seoul, Singapore, and Tokyo—or depressing—just look at anywhere with extensive slums. In some cases, it seems urban planning is up to the task. In others, it’s not.

Where it falls flat, urban planning’s failings aren’t necessarily the fault of the designers. Too often planning is focused on minutiae—ordinances, regulations, zoning, setbacks, and so on. Even when it tackles bigger problems like economic growth, it doesn’t necessarily consider the city as a whole.¹

The solution, according to Michael Batty, an urban planner and professor at University College London, is infusing planning with science. Systems science, specifically, where bright minds and complex mathematical models try to digest the entirety of a system, like a city. It’s no simple task. IBM is just one company throwing billions of dollars and tons of silicon at the problem. What they’ll get out of it is anybody’s guess, but they seem certain they’ll get something. Cities are overflowing with collectable data. It’s making sense of it that’s difficult. The possibilities it presents is what I think is going drive us to rethink city planning.

Urban planning has its origins in the design world, which is both a bonus and a handicap. Architects make natural planners—they design the buildings, why not have them design the streets, too? When those planners are enlightened designers, the results are attractive and livable cities. If they’re not? Well, we’ve all seen what happens when they’re not. But as much as good design has created great cities, I and others suspect it can’t deal with the coming challenges. Not on its own, at least. Good design can solve many problems, but it can’t solve them all. At some point, you need science.

The rate at which cities are growing and changing presents a problem for the traditional design-centric approach. Good design requires a thorough understanding of your problem. But these days, problems are appearing and evolving so quickly that we don’t have enough time to properly observe them.

Urban planning is at a crossroads, much like ecology was 50 years ago.² Planning is still largely descriptive and not very scientific, again, much like ecology was 50 years ago. Sure, cities gather hard data like traffic and sewage flows. Yes, they model projected growth and consider the social factors behind neighborhood demands. But urban planning lacks a unified, data-driven theoretical foundation.

That’s beginning to change. Michael Batty, Geoffrey West, Luis Bettencourt, and others are proposing data-driven theories and testing them, just like Robert MacArthur and Edward O. Wilson did in ecology back in the 1960s and 70s. Though these new urban theorists are trying to shake things up, they’re not trying to eliminate planning as we know it. Their science of cities won’t be a replacement for current planning, but a superset. Think of it as a grand theory to tie it all together, to make sense of why cities have evolved the way they did and how we can coax them to cope with 5 billion people.

The science of cities may be in its infancy, but we can see where it will lead. The first stage, the one we’re in right now, is descriptive. It involves gathering data, assembling huge models, and tuning them until we’re satisfied. Then we’ll apply those models, and see how the real world reacts. There will be some stumbles, but that’ll only give us more data to work with. Eventually, we’ll arrive at a theory of cities that’s universal and flexible enough that it can be applied anywhere. It will be a foundation that will underpin models that grind through piles of data and make sound, timely recommendations which designers can implement.

Getting to that last stage is important, I think. In many cities, it’s clear that we don’t know what to do with all these new urbanites. Even in cities where things appear hunky dory, cracks are beginning to show. Subways are crowded, freeways are jammed, and sewers are overflowing. Throwing money at temporary fixes will only get us so far. We need to dig deeper and develop a responsive urbanism,³ one that’s grand in scale and scientifically focused. We need to listen to what cities are telling us, decide what we want them to do, and plan accordingly.

  1. Enlightened planners out there—and there are quite a few—can take umbrage with my characterization here. But they’ll admit that there are quite a few in their profession that dabble too much in the details.
  2. Though urban planning is behind the curve relative to ecology when it comes to mathematical and theoretical rigor, data gathering is one place where it’s ahead. We already have many data sets in hand along with the infrastructure to gather more.
  3. I’m thinking bigger than pop-up parks, an oft cited example of “responsive urbanism”.


Batty, M. (2008). The Size, Scale, and Shape of Cities, Science, 319 (5864) 771. DOI: 10.1126/science.1151419

Batty, M. (2012). Building a science of cities, Cities, 29 S16. DOI: 10.1016/j.cities.2011.11.008

Bettencourt, L.M.A., Lobo, J., Helbing, D., Kuhnert, C. & West, G.B. (2007). Growth, innovation, scaling, and the pace of life in cities, Proceedings of the National Academy of Sciences, 104 (17) 7306. DOI: 10.1073/pnas.0610172104

Photo by MagnusL3D

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Thirty minutes at most, according to the wisdom of the crowds.

That comes from reams of data and piles of research that suggests commute times tend to cluster around this point. People tend to be good at weighing their options, economists think. If you live farther from work, you can usually afford a bigger house or apartment. But there’s a point where that journey becomes too onerous, and you are willing to sacrifice some of those desires to live closer to your job. That point on average seems to be between 20 and 30 minutes.

I was inspired to look into this further after seeing an article by Charlie Gardner over at his blog, The Old Urbanist. Gardner had mined the American Community Survey for average commute times in major metropolitan areas. Though there was a tight correlation between population and commute time (metros with larger populations have longer average commutes), the differences weren’t pronounced. They ranged from a low of 22.5 minutes in Kansas City to a high of 34.6 in New York City. That’s not a lot.

But before we ask why commute times hover in a tight band, perhaps we need to ask why people commute so far in the first place. Why not work next door? The answers may seem obvious, but what’s readily apparent to one person may not be to another. That’s why we examine these things scientifically. Well, in this case your hunch is probably correct. An older study by Martin Wachs and his colleagues at UCLA found, unsurprisingly, that people choose where to live not just based on commute times but also neighborhood characteristics, schools, and safety.

Now we can move on to the more curious question, why commutes tend to average 20-30 minutes. It’s not just limited to the United States, either. In the Netherlands, the average commute time in the early 2000s was about 28 minutes. Many European nations average about 35 minutes. What makes a half-hour so universal in terms of commuting?

It didn’t used to be that way. Average commute times in 1980 were around 22 minutes. Today, they’re around 25 minutes. Three minutes may not seem like much, but remember it’s an average. To increase an average by that amount, some commutes had to grow significantly to counter those that shrunk or remained the same. Now, keep in mind there is a lot of variation about those averages. Some people travel 2 minutes to work, others well over an hour. But on average, they have increased.

What’s causing that lengthening is higher job densities in major metros. Job growth is requisite to economic growth, and vice versa. As metro areas add more jobs, those jobs tend to be concentrated in business districts (after all, not everyone can work out of their homes). And as business districts fill up, commute times lengthen because the roads leading there become more congested. So when the economy booms, traffic slows to a crawl. I heard anecdotal evidence of this when I lived in San Francisco. People told me, if you think traffic is bad now, it was much worse during the tech boom of the late 1990s. When all those tech workers lost their jobs, gridlock practically evaporated, they said.

Subtle changes in urban form may also cause longer commutes. One study in the Netherlands and another in Quebec, found that polycentric metro areas—those with two or more cities, like Minneapolis-St. Paul—tend to have longer auto commute times. As cities grow and begin bumping into one another, such agglomerations are likely to become more common. It’s possible commute times may increase as well. While there may not be consensus on this point, I haven’t found any studies that claim changes in urban form will shorten commute times. That makes sense if you look at somewhere like New York City, which is both monocentric and dense. People may work a short distance from their homes, but traffic is so congested and public transit makes so many stops that commute times are still relatively long. Simply increasing density in some cities may shorten commutes for a brief period, but the honeymoon won’t last forever.

Which is a bummer, because for the most part people think their commutes are too long. A survey of 2,000 commuters in the San Francisco Bay Area reported that 52 percent of respondents said they commuted at least 5 minutes longer than they would like. Among that group, median commute times were 40 minutes, which is certainly longer than the region’s average. On the other hand, 42 percent said their commutes were just right (their median time was 15 minutes). Surprisingly, 7 percent felt their commute was too short (median of 10 minutes). But despite the fact that a majority think their commute is too long, most people said they didn’t mind it, so long as their trips were less than 100 miles.

That people don’t mind their commute may be why commute times refuse to shrink. People in the Bay Area survey who didn’t mind their commute said they agreed with statements like, “I use my commute time productively” and “My commute trip is a useful transition between home and work”, which supports anecdotal evidence I’ve heard that people enjoy the separation between work and home. Twenty to thirty minutes may be just enough time to unwind.

It’s not entirely universal, though. Tolerable commute times seem to lengthen when people switch from cars to mass transit. People may find that time more productive, or maybe the time seems shorter because driving can be stressful, while just sitting usually isn’t. Personally, I know I’m willing to commute longer by train than car. Another reason is because mass transit commutes tend to be more reliable in terms of duration (at least for trains). Not having to worry about traffic jams doubling your commute is a big advantage.

Regardless of mode, people seem to settle on an ideal commute time. And once they have settled, they don’t seem to stray from it. A study of two metro areas in Washington State discovered that commute times don’t change much when people move or switch jobs. The thinking is that if a person gets a new job that’s farther away, they are more likely to move. Plus, as people have moved to suburbia, some jobs have followed. It’s a two-way street. But that doesn’t mean employers can move to the burbs without consequences. If an employer moves and an employee doesn’t move as well, the employee is more likely to find another job. Companies looking to relocate simply to cut costs may find the high turnover that results more costly in the long run.

Commuting is a big part of our lives, so it makes perfect sense that it would affect so much of the world around us, especially the cities we live in. Take a dense city like New York that has oodles of jobs, and lots of dense housing close in. That density helps keep commute times reasonable. But somewhere like Tulsa that doesn’t have as many jobs doesn’t have as much need for density. Form follows function, and currently the freeways in Tulsa are functioning pretty well compared with New York.

That presents a real dilemma for urban planners, who have been striving to increase densities in cities across the board. One approach has been mixed-use development that blends retail, housing, and office space. That may help reduce trip times for errands and such, but it doesn’t preclude people from living in one mixed-use neighborhood and working in another. The reality is, we’re probably not going to change commute times. If we offer faster and better transportation, people will use it until it becomes overburdened. At which point they’ll just move closer to work. Attempts to influence urban form through design may not have much of an impact if jobs don’t follow.


Cervero, R. (1996). Jobs-Housing Balance Revisited: Trends and Impacts in the San Francisco Bay Area, Journal of the American Planning Association, 62 (4) 511. DOI: 10.1080/01944369608975714

Cervero, R. & Duncan, M. (2006). ‘Which Reduces Vehicle Travel More: Jobs-Housing Balance or Retail-Housing Mixing?, Journal of the American Planning Association, 72 (4) 490. DOI: 10.1080/01944360608976767

Clark, W.A.V. & Davies Withers, S. (1999). Changing Jobs and Changing Houses: Mobility Outcomes of Employment Transitions, Journal of Regional Science, 39 (4) 673. DOI: 10.1111/0022-4146.00154

Clark, W.A.V., Huang, Y. & Withers, S. (2003). Does commuting distance matter?, Regional Science and Urban Economics, 33 (2) 221. DOI: 10.1016/S0166-0462(02)00012-1

Giuliano, G. & Small, K. (1993). Is the Journey to Work Explained by Urban Structure?, Urban Studies, 30 (9) 1500. DOI: 10.1080/00420989320081461

Levinson, D.M. (1997). Job and housing tenure and the journey to work, The Annals of Regional Science, 31 (4) 471. DOI: 10.1007/s001680050058

Schwanen, T., Dieleman, F.M. & Dijst, M. (2004). The Impact of Metropolitan Structure on Commute Behavior in the Netherlands: A Multilevel Approach, Growth and Change, 35 (3) 333. DOI: 10.1111/j.1468-2257.2004.00251.x

Schwanen, T. & Dijst, M. (2002). Travel-time ratios for visits to the workplace: the relationship between commuting time and work duration, Transportation Research Part A: Policy and Practice, 36 (7) 592. DOI: 10.1016/S0965-8564(01)00023-4

Vandersmissen, M.H., Villeneuve, P. & Thériault, M. (2003). Analyzing Changes in Urban Form and Commuting Time∗, The Professional Geographer, 55 (4) 463. DOI: 10.1111/0033-0124.5504004

Wachs, M., Taylor, B., Levine, N. & Ong, P. (1993). The Changing Commute: A Case-study of the Jobs–Housing Relationship over Time, Urban Studies, 30 (10) 1729. DOI: 10.1080/00420989320081681

Photo by Jekkone.

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If the world’s population lived like…

Shortly after I started Per Square Mile, I produced an infographic that showed how big a city would have to be to house the world’s 7 billion people. There was a wrinkle, though—the city’s limits changed drastically depending on which real city it was modeled after. If we all lived like New Yorkers, for example, 7 billion people could fit into Texas. If we lived like Houstonians, though, we’d occupy much of the conterminous United States.

Here’s that infographic one more time, in case you haven’t seen it:

The world's population, concentrated

What’s missing from it is the land that it takes to support such a city. In articles and comments about my infographic, some people overlooked that aspect—either mistakenly or intentionally. They shouldn’t have. Cities’ land requirements far outstrip their immediate physical footprints. They include everything from farmland to transportation networks to forests and open space that recharge fresh water sources like rivers and aquifers. And more. Just looking at a city’s geographic extents ignores its more important ecological footprint. How much land would we really need if everyone lived like New Yorkers versus Houstonians?

It turns out that question is maddeningly difficult to answer. While some cities track resource use, most don’t. Of those that do, methodologies vary city to city, making comparisons nearly impossible. Plus, cities in most developed nations still use a shocking amount of resources, regardless of whether they are as dense as New York or as sprawling as Houston. Any comparison of the cities in my original infographic would be an exercise in futility at this point.

But what we can do is compare different countries and how many resources their people—and their lifestyles—use. For countries, the differences are far, far greater than for cities. Plus, there’s a data set that allows for reliable comparisons—the National Footprint Account from the Global Footprint Network. Their methodology is based on peer-reviewed research by Mathias Wackernagel, the organization’s founder. It’s consistent and comprehensive. Each country’s footprint is assembled from sub-footprints, ranging from cropland to carbon to urbanization to fishing grounds. For my purposes, I used only terrestrial sub-footprints. I’ll let the results speak for themselves.



Global Footprint Network. 2011. National Footprint Accounts, 2011 Edition.

Wackernagel, M., Kitzes, J., Moran, D., Goldfinger, S. & Thomas, M. (2006). The Ecological Footprint of cities and regions: comparing resource availability with resource demand, Environment and Urbanization, 18 (1) 112. DOI: 10.1177/0956247806063978

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Lone tree

David Nowak is at it again. The tireless ecologist and his frequent collaborator Eric Greenfield have given us another comprehensive snapshot of urban forests in the United States. Nowak has published numerous studies on urban trees, one of which I’ve covered previously. But that study—which discovered that some cities are leafier than their pre-urban states—takes a longer view and paints a different picture. In this new study, Nowak and Greenfield focus on the more recent past, the 2000s, and find that 17 of the 20 cities they surveyed had significantly fewer trees than just five years earlier. Sixteen became more developed, too, with an increase in impervious surfaces like roadways, parking lots, rooftops, and sidewalks. Taken together, these findings suggest that cities across the U.S. are steadily defoliating.

In this most recent study, Nowak and Greenfield turned to a favorite tool of mine for evaluating forest cover—aerial photographs. For each city, Nowak and Greenfield found a pair of images taken three to six years apart. They scattered a minimum of 1,000 points across each city’s photo pair and manually classified them into eight categories: trees and shrubs, grass and herbaceous cover, bare soil, water, and three classes of impervious surfaces (buildings, roads, and other). They were accurate to within a few percent in every class.¹ Nowak and Greenfield also compared their subset of 20 cities to other large cities in the U.S. by sampling an additional 1,000 points in metro areas across the country. The results were largely similar, meaning what they found in the 20 cities would be indicative of nationwide trends.

With the exception of Syracuse, all cities in the study had fewer trees in the later image. Overall, existing tree cover decreased 0.9 percent per year in 18 of the 20 cities (New Orleans and Detroit were excluded from most statistics—more on why later). That may not sound like much, but like any annualized percentage, it compounds. When a small amount of trees are lost from one year to the next, the change may not be that noticeable. But over 10 years, the results can be drastic. And when you remember that trees account for just 28 percent of land area in these cities, at these rates it won’t be long before trees cover the barest sliver of our urban areas.

Nowak and Greenfield excluded two of their surveyed cities, New Orleans and Detroit, from most statistics based on their extraordinary circumstances. In New Orleans, they were interested in seeing how Hurricane Katrina changed the urban forest, and in Detroit, they were looking for signs of the emerald ash borer infestation, an invasive insect that has decimated ash trees throughout New England and the Midwest. Detroit was lucky—it emerged relatively unscathed, losing only 0.7 percent of its total tree cover, in part because vacant lots make a great home for young trees. New Orleans, on the other hand, fared poorly, losing more trees than any other city in the study, nearly 10 percent.

Those cities aside, the trends don’t bode well for urban forests. Most concerning is the conversion from trees to impervious surfaces. Losing trees to grass and bare soil may be concerning, but it’s not catastrophic. Those cover types are easy to reforest. But when land becomes covered by roads, parking lots, or buildings, it very rarely reverts to vegetation of any type. At least 71 percent of the time, the move to impervious surface was because of development, not, say, a tree dying and revealing a parking lot beneath. The majority of the time, it was the more permanent type of conversion.

Which leaves us with a question—how are we going to maintain, let alone increase, tree cover in cities where buildings, streets, and parking lots are taking over the landscape, often in irreversible ways? One is for cities to take direct action, by planting trees in existing public spaces. Lots of cities have planting programs, but in many cases they’re barely able to keep pace with deaths from disease, urban stress, and old age.

Another option is to open gaps in the concrete along streets to provide new opportunities for planting. Not only does this give the city more places to plant, it sends a good message to the neighborhood, showing that the city cares. Planting trees may also be a way of leading by example, an exercise that could be followed by the offer of free trees for neighborhood residents. It’s subtle, but potentially more effective because there’s often more land behind people’s homes than in front. Cities could also add more parkland. Trees in parks aren’t exposed to the same stresses as street trees, but parkland can be expensive to procure.

That Nowak and Greenfield’s results span the last decade makes them that much more alarming. We’ve lost 1 percent of our existing tree cover per year in that time. Where will that leave us ten years from now? We don’t know, exactly. Until we have more data from earlier years, we just won’t know for certain—these sorts of changes could be cyclical. But given trends in development and population density and the rate at which trees have been converted to impervious surfaces, I’m guessing we’re headed toward fewer trees.²

  1. Just as it’s easy for even the untrained eye to see differences in income based on relative differences in tree cover, it’s a simple task for a trained photointerpreter to tell trees from grass. It’s difficult for computer algorithms to match human photointerpreters’ accuracies, but sometimes the data set is too large for manual classification. It’s always a tradeoff. In this case, the areas covered were of manageable size, so a human touch made sense.
  2. I should point out that I’m not condemning population density. Rather, we need to think long and hard about the way we accommodate high population density. Cost can’t be the only consideration.


Nowak, D.J. & Greenfield, E.J. (2012). Tree and impervious cover change in U.S. cities, Urban Forestry & Urban Greening, 11 (1) 30. DOI: 10.1016/j.ufug.2011.11.005

Photo by Franco Folini.

Related posts:

Urban trees reveal income inequality

Tree City

An ecology of gardens and yards

Your images of income inequality, seen from space

A month ago, I covered a study showing how tree cover was related to income level. A couple of weeks later, I posted images gleaned from Google Earth showing how easy it is to spot income inequality from space. I also asked you, my readers, to send me more examples of the same.

The response has been overwhelming. In fact, I’m still receiving emails and comments. I haven’t been able to go through them all, so if yours isn’t below, hang tight. I’ll get to it eventually. Consider this a post that’ll be updated continuously. As they say, check back early and often.

So without further ado, here they are. This is where you see income inequality. You may see it every day, right in your own backyard. You may have stumbled across it years ago. You may have noticed on a vacation. But collectively, you see it everywhere. Keep ’em coming.

Mexico City

From Todd Gastelum, who writes:

Although it appears that these satellite images are presented at different scales, I assure you that they were not. The houses in Lomas truly are enormous whereas San Miguel Teotongo is typical of the dense, irregular urbanization characteristic of the city’s more peripheral zones.

Lomas de Chapultepec, Miguel Hidalgo

San Miguel Teotongo

San Miguel Teotongo, Iztapalapa

Lomas de Chapultepec

Dar es Salaam, Tanzania

From Aaron Krolikowski, who writes:

I’m a PhD student (in Geography) from the University of Oxford. My work takes place in Dar es Salaam, Tanzania and I spend quite a bit of time looking at maps of the city. One of the first things I noticed was exactly what you mention in terms of the ability to see income inequality from a birds-eye view. I’ve shared some photos with you to give you an idea of what I look at (and live!) every day here.

The first – To the left you have unplanned settlements of “Mikoroshini” and “Makangira”…to the right, the highly affluent and very European “Oyster Bay”

The second – To the left is the informal settlement of Hanna Nassif and across the river is middle-class Upanga

Mikoroshini, Makangira, and Oyster Bay


Hanna Nassif and Upanga



From Sylvian Paradis.

Downtown East Side

Downtown East Side

British Properties

British Properties

Lisbon, Portugal

From João Jordão.

Cova da Moura, Amadora

Cova da Moura



Income inequality, as seen from space

Last week, I wrote about how urban trees—or the lack thereof—can reveal income inequality. After writing that article, I was curious, could I actually see income inequality from space? It turned out to be easier than I expected.

Below are satellite images from Google Earth that show two neighborhoods from a selection of cities around the world. In case it isn’t obvious, the first image is the less well-off neighborhood, the second the wealthier one.

Rio de Janeiro


Rocinha, Rio de Janeiro

Zona Sul

Zona Sul, Rio de Janeiro


West Oakland

West Oakland


Piedmont, California (enclave of Oakland)


Fourth Ward

Fourth Ward, Houston

River Oaks

River Oaks, Houston



Hyde Park

Hyde Park, Chicago



Fengtai, Beijing


Chaoyang, Beijing

Boston metro area, Massachusetts

Ball Square, Somerville

Somerville, Massachusetts

West Cambridge

Cambridge, Massachusetts

Your examples

Do you have other cities or neighborhoods in mind? I’d love to hear from you. Send me an email with photos or link to your blog post. In the next couple of weeks, I’ll put together a follow up article that features your examples.

Be sure to include the names of the cities and neighborhoods you’re highlighting and if you’d like me to mention your name.


Your examples are now posted! The response to my call for examples has been unbelievable. I’ve received hundreds of messages. I have the first batch up, and as I have time, I’ll be adding many more. Keep ’em coming.

Related posts:

Urban trees reveal income inequality

Income inequality in the Roman Empire

Ghosts of geography

Urban trees reveal income inequality

Street trees

Wealthy cities seem to have it all. Expansive, well-manicured parks. Fine dining. Renowned orchestras and theaters. More trees. Wait, trees? I’m afraid so.

Research published a few years ago shows a tight relationship between per capita income and forest cover. The study’s authors tallied total forest cover for 210 cities over 100,000 people in the contiguous United States using the U.S. Department of Agriculture’s natural resource inventory and satellite imagery. They also gathered economic data, including income, land prices, and disposable income.

They found that for every 1 percent increase in per capita income, demand for forest cover increased by 1.76 percent. But when income dropped by the same amount, demand decreased by 1.26 percent. That’s a pretty tight correlation. The researchers reason that wealthier cities can afford more trees, both on private and public property. The well-to-do can afford larger lots, which in turn can support more trees. On the public side, cities with larger tax bases can afford to plant and maintain more trees. Given the recent problems New York City has had with its aging trees dropping limbs on unsuspecting passers-by—and the lawsuits that result—it’s no surprise that poorer cities would keep lean tree inventories.

But what disturbs me is that the study’s authors say the demand curve they see for tree cover is more typical of demand for luxury goods than necessities. That’s too bad. It’s easy to see trees as a luxury when a city can barely keep its roads and sewers in working order, but that glosses over the many benefits urban trees provide. They shade houses in the summer, reducing cooling bills. They scrub the air of pollution, especially of the particulate variety, which in many poor neighborhoods is responsible for increased asthma rates and other health problems. They also reduce stress, which has its own health benefits. Large, established trees can even fight crime.

Fortunately, many cities understand the value trees bring to their cities. New York City is aiming to double the number of trees it has to 1 million. Chicago has planted over 600,000 in the last twenty years.¹ And London has been working to get 20,000 new trees in the ground before it hosts the Olympics.

But those cities are relatively wealthy. It’s the poorer ones that probably need trees the most but are the least able to plant and maintain them. The Arbor Day Foundation is a great resource in those cases, but like many non-profits, it is stretched too thin. Compounding the inequality is the fact that most tree planting programs are local. Urban forestry has sailed largely under the federal government’s radar. The U.S. Forest Service does have a urban and community forestry program, but is woefully underfunded, having only $900,000 to disperse in grants. Bolstering that program could help struggling cities plant the trees they need. After all, trees and the benefits they provide are more than just a luxury.

  1. Though like many of Chicago’s boasts that number was probably inflated by including replacement trees.


Zhu, P., & Zhang, Y. (2008). Demand for urban forests in United States cities Landscape and Urban Planning, 84 (3-4), 293-300 DOI: 10.1016/j.landurbplan.2007.09.005

Photo by Alex E. Proimos.

Related posts:

Income inequality, as seen from space

Tree City

Urban forests just aren’t the same

More than just flora and fauna

Landscape near Paris, Paul Cézanne

Peter Sigrist, writing at Polis about what he sees as ecology’s shortcomings:

Ecology “proper” is currently limited in addressing human habitation. It doesn’t usually incorporate the theory or methods of fields like economics, anthropology, political science, sociology and history. Many subfields have emerged in answer to the need for more detailed study of human-environment interaction, including human ecology, cultural ecology, political ecology, environmental sociology, historical ecology, ecological anthropology, ecological economics and ecological urbanism. But most are not closely integrated with mainstream ecology and its methods, which are primarily focused on nonhuman nature. Many ecologists portray human environmental impact as a kind of alien intervention into the natural world, and don’t attempt to understand the political, social, economic and cultural processes through which it takes place.

Sigrist isn’t entirely wrong—our understanding of the natural world must consider human impacts. The thing is, many ecologist already do that. If Sigrist had written this decades ago, I would concede his point. But times have changed. No serious ecologist draws a firm boundary between natural and anthropogenic spheres. For years, ecologists have widely acknowledged that no part of the Earth is untouched by human influences. Some may still cling to the old distinction between human and wild, but they are increasingly few and far between. Climate change has thoroughly disabused most of that notion.

Sigrist further laid out his argument in a second post, stating:

Incorporating useful elements of cultural landscape, urban political ecology and ecological urbanism can make ecology more attuned to the ways humans experience and influence cities. This is more than a shared analytical framework or conceptual lexicon (Gandy 2008: 567); it means actual integration so that ecologists are equipped to address the full complexity of human environmental relations and help make cities more just, healthy and beneficial to the planet as a whole.

To say too few ecologists study human-environment interactions isn’t just unfair, it’s incorrect. Sigrist seems to misunderstand ecology and its relation to the myriad subfields he listed above. The ecologists he describes—the ones that focus on strictly “natural” ecosystems—aren’t members of an umbrella field but a subfield. They’re just one type of ecologist. The people who study other subfields of ecology? They’re ecologists, too.

What Sigrist is proposing for ecology already exists. Perhaps he wants ecologists to avoid over-specialization.¹ Perhaps what he means to say is that there needs to be more collaboration and cross-pollination between ecologists of different subfields. He’s not wrong—there could always be more. But he’s ignoring what’s already out there. I know ecologists of all stripes—field, physiological, sociological, and so on—who collaborate with environmental historians, economists, even electrical engineers. I know ecologists who write papers about the value of ecosystem services, how to use Wall Street’s data processing techniques to understand the water cycle, or how spirituality can affect the conservation of biodiversity. Hell, I’ve been to conferences where ecologists have wrung their hands about how ecology needs more collaboration. If anyone is conscious of the need for interdisciplinary collaboration, it’s ecologists. After all, ecology is the original interdisciplinary science.

Again, that’s not to say we shouldn’t work harder to identify how humans are affecting the natural world. I’m the last to argue against that. But we need to identify our shortcomings where they actually exist, not where we imagine them to be.

  1. Not a bad idea, but good luck getting it to happen. Despite calls for interdisciplinary research, the trend for individuals is toward increasing specialization. Nipples on the surface of human knowledge and all that.

Landscape near Paris, Paul Cézanne, National Gallery of Art.