Urban Resilience

Frontier by Maywa Montenegro / February 16, 2010

Merging complex systems science and ecology, resilience scientists have broken new ground on understanding—and preserving—natural ecosystems. Now, as more and more people move into urban hubs, they are bringing this novel science to the city.

Illustration by Joe Kloc

Four-and-a-half years ago, Hurricane Katrina plowed into the coast of Louisiana, pummeling New Orleans for eight hours straight with high-speed winds and storm surges reaching 15 feet. Swollen beyond capacity, Lake Pontchartrain spilled into the northern part of the city, and the federal flood protection system, built to protect NOLA from a repeat of Hurricane Andrew, failed in more than 50 places. One day later, nearly every levee in the metro district had been breached, leaving 80 percent of the city underwater.

In the aftermath, Americans watched in disbelief as thousands of newly homeless poured into the Superdome for shelter and TV cameras captured those left behind clinging to rooftops, wading through the streets, and looting empty storefronts. Scenes of destruction, desperation, and poverty, made only more poignant by the overwhelming evidence of official negligence. New Orleanians themselves, as the New York Times put it, were left “terrified, stunned, gasping, speechless.”

But to some scientists, what happened in New Orleans, while devastating, wasn’t very surprising or unexpected. They see a system that was insufficiently robust to handle the blow it was dealt. They see a highly ordered, complex state—commercial districts and neighborhoods, social networks and infrastructure networks, cycles of water, energy, and food consumption—reduced to a state of chaos and disorder. From this perspective, the problem wasn’t merely an incompetent leadership and not enough FEMA trailers. It was a fundamental question of resilience.

Resilience theory, first introduced by Canadian ecologist C.S. “Buzz” Holling in 1973, begins with two radical premises. The first is that humans and nature are strongly coupled and co-evolving, and should therefore be conceived of as one “social-ecological” system. The second is that the long-held assumption that systems respond to change in a linear, predictable fashion is simply wrong. According to resilience thinking, systems are in constant flux; they are highly unpredictable and self-organizing, with feedbacks across time and space. In the jargon of theorists, they are complex adaptive systems, exhibiting the hallmarks of complexity.

A key feature of complex adaptive systems is that they can settle into a number of different equilibria. A lake, for example, will stabilize in either an oxygen-rich, clear state or algae-dominated, murky one. A financial market can float on a housing bubble or settle into a basin of recession. Historically, we’ve tended to view the transition between such states as gradual. But there is increasing evidence that systems often don’t respond to change that way: The clear lake seems hardly affected by fertilizer runoff until a critical threshold is passed, at which point the water abruptly goes turbid.

Resilience science focuses on these sorts of tipping points. It looks at gradual stresses, such as climate change, as well as chance events—things like storms, fires, even stock market crashes—that can tip a system into another equilibrium state from which it is difficult, if not impossible, to recover. How much shock can a system absorb before it transforms into something fundamentally different? That, in a nutshell, is the essence of resilience.

The concept of resilience upends old ideas about “sustainability”: Instead of embracing stasis, resilience emphasizes volatility, flexibility, and de-centralization. Change, from a resilience perspective, has the potential to create opportunity for development, novelty, and innovation. As Holling himself once put it, there is “no sacred balance” in nature. “That is a very dangerous idea.”

Over the past decade, resilience science has expanded beyond the founding group of ecologists to include economists, political scientists, mathematicians, social scientists, and archaeologists. And they have made remarkable progress in studying how habitats—including coral reefs, lakes, wetlands, forests, and irrigation systems, among others—absorb disturbance while continuing to function.

New Orleans, however, presents an interesting example to resilience scientists. If a lake can shift from clear to murky, could a city shift to a dramatically different stable state too? If biodiversity in ecosystems makes them resilient to disturbance, could diversity in urban systems serve a similar purpose? “Cities aren’t dominated by nature to the same extent as things like lakes and wetlands and coral reefs,” says Australian ecologist Brian Walker, “But we wondered, could we look at them in the same way?”

Metro Planet

In 2008, a historic milestone was crossed, with more than half of the world’s population now living in cities. The UN estimates that by 2030, the planet’s current 2.9 billion urban residents will rise to a staggering 5 billion. By 2050, humanity may well be 80 percent urban.

Urban centers have always been hubs of innovation, creativity, and wealth, but they are also hubs of crime, disease, and environmental pollution. Cities can be models of resource efficiency—the average Manhattanite uses only 29 percent of the energy an average American uses in a year—but they also concentrate the need for huge amounts of power, water, food, and other resources. In the developing world, cities are changing faster than scientists can understand the diverse factors driving those changes, and to complicate matters further, many of those forces operate in contradictory directions and at differing scales.

In short, cities are the quintessential complex adaptive system. Which makes them, in many ways, the perfect place to explore resilience.
Brian Walker is former program director and chair of the Resilience Alliance, a loose international coalition of natural and social scientists who, in their own words, “collaborate to explore the dynamics of social-ecological systems.” In 2005, recognizing the growing impact of urbanization, the Alliance held a series of brainstorming sessions, laying the groundwork for the “Urban Network,” based out of the Stockholm Resilience Center, an interdisciplinary research group that formed at Stockholm University in 2008.

The Urban Network has research sites in 12 cities: Bangalore, New Dehli, Cape Town, Johannesburg, Chicago, New York City, Phoenix, Canberra, Helsinki, Istanbul, and Stockholm. These cities span the globe and differ vastly in terms of culture, history, and economic development. The ultimate goal, according to Thomas Elmqvist, lead researcher of the Network, is to do a comparative analysis of these cities. How are they similar or different with respect to handling development? How do they compare it comes to withstanding shocks and surprises?

“As humans, we should try to understand how to manage systems in order to avoid passing thresholds,” says Elmqvist. But this is especially difficult in urban contexts, which have already been so transformed by humans that they’ve breached most of the thresholds ecologists are familiar with. When great expanses of concrete and steel now exist where trees and streams once did, new tipping points must be defined for places that are, as Elmqvist puts it, “already tipped.”

Case studies are now underway in each of the Network’s 12 participating cities. But in deciding what kind of data to gather, researchers have had to ask themselves:  What would a city look like through the lens of resilience?


A city’s lifeblood is a continuous flow of stuff—fuel, consumer products, people, and services that enter it either actively, through human effort, or passively through natural processes like solar radiation, atmospheric currents, and precipitation. Ecologists often talk about these resource flows in terms of inputs and outputs. They’ve developed several budgetary models of accounting for them, including the well-known “ecological footprint.”

The resilience approach, according to ecologist Guy Barnett of the Urban Network’s Canberra research team, focuses less on the resources that cities consume and more on the interdependencies along the chain of supply and demand. Dependence on a single type of fuel as an energy source, for instance, creates a highly vulnerable system—especially if fuel prices are volatile or if the supply is prone to disruption.
Consider what happened just outside of Melbourne in 1998. Several explosions at Esso Australia’s natural gas plant there killed two people and halted power supply to the city for nearly two weeks. As a result, the regional dairy industry, which relies on natural gas to power its milk pasteurization, was forced to shut down several of its plants. Some 25 million liters of raw milk went to waste.

So what went wrong? From a resilience perspective, it was partly the drive for efficiency. If the dairies had hedged their risk with backup fuel supplies, building more resilience into the system, milk pasteurization would not have ground to a complete halt. The number of supervisors at the gas plant had been reduced from four to one, and all the engineers had been relocated to the head office in Melbourne, leaving just one person at the controls. Simply having more people could have helped safeguard against catastrophe.

Efficiency per se isn’t the problem, says Barnett. But the way efficiency is conceived, and pursued, is often too narrow. Society strives for efficiency by trying to eliminate apparent redundancies, but things that seemed redundant in a stable climate turn out to be valuable when conditions change. “The quest for increasing efficiency tends to result in systems optimized towards single rather than multiple solutions, centralized rather than distributed organizational responses, all of which are counter to the fundamental concepts of resilience thinking—‘redundancy,’ ‘diversity,’ and ‘modularity,’” says Barnett.

Of course, building in such attributes—contracting with an array of energy suppliers, hiring more engineers—almost always means more money, both in terms of upfront capital investment and long-term management costs. Decisions therefore involve trade-offs between efficiency and resilience, says Barnett, a principle under-acknowledged by many urban planners and policymakers.

Take water, an essential resource for every city—and therefore a chronic source of concern for city managers without an ample supply. An efficient way of getting more would be to tap into groundwater; wealthy cities might even import water from afar. The more resilient approach, according to Elmqvist, would be for city managers to consider the dynamics of the larger watershed. They could negotiate agreements with rural landowners, paying them to manage their property in a way that provides the city with a certain amount and certain quality of water.

“We actually see this happening in a lot of places in Latin America,” says Elmqvist, “Cities are using revenues from water and energy taxes to compensate land owners for landscape management—often by conserving forests to secure a flow of clean water for the city.” A similar scheme is in place in Capetown, South Africa, where the city is paying landowners to remove exotic plants from the watershed. Alien species consume copious amounts of water, so their destruction means more water flows into the streams that eventually reach the city.

Watersheds highlight a simple but crucial point about cities: They are highly dependent, open systems, meaning they consume far greater resources, over a much larger scale, than are available within their own borders. Hong Kong, for instance, depends on a land area 2,200 times its actual size to support its inhabitants—and only 30 percent of this land is Chinese. More than 95 percent of its seafood supply comes from the marine waters of other nations. The resilience of a city, in other words, is highly contingent on the resilience of other places.

The Human Dimension

On the coral reefs around Jamaica, a variety of fish once helped keep algae firmly in check. When intensive harvesting eliminated many of these algae-grazers, long-spined sea urchins took over that niche—and the urchins’ numbers exploded with less competition for food. But then a one-two punch of a bacterial pathogen and a hurricane devastated the urchin population, and algae growth surged, strangling the coral. A coral-dominated system abruptly shifted into a state of algal-dominance.

Abundant biodiversity is critical, as most people know, because it means being able to fill the numerous niches of a healthy ecosystem. But it also increases the odds that some of those species, like fish and urchins, will share niches and have overlapping roles in the ecosystem. This redundancy can help a system absorb disturbance—or when it’s lost, make it vulnerable to attack. In the case of the Jamaican reefs, an infection or storm that might once have been easily surmountable suddenly become lethal.

When it comes to human populations, ecologists are hesitant to stretch metaphors too far—a biodiverse ecosystem is not the same as a diverse population. (After all, says Elmqvist, a very heterogeneous society can also mean a lot of conflict). On the other hand, he says, a good analogy can be drawn to ecosystem redundancy.

“It’s important that you have institutions and functions in society that also overlap,” says Elmqvist. “If one member of the group is lost, there will be another that can maintain the function, so the function of the system as a whole is maintained.”

The tension between efficiency and redundancy is “a big, complex issue for society to struggle with,” he says. “On one hand you have demands to build a very efficient society, particularly when it comes to using taxpayers’ money in public administration. On the other hand, we must learn not to drive efficiency to the extent that we lose the backup capacity.”

Social equity and access to resources, Elmqvist believes, will also emerge as hugely important components of resilience. Though human behavior is new territory for resilience experts, numerous social scientists have documented the erosion of civic engagement, and even violence, in areas marked by high levels of social stratification. Riots, for instance, broke out in Paris in 2005 after a couple of North African youth, running from the police, hopped a fence and were electrocuted by a transformer. Immigration has long been a powder keg across Europe, and with rising fears over international terrorism, those tensions have arguably grown even stronger. Such incidences, of course, aren’t isolated to Europe—Los Angeles has seen its own share of racially motivated riots, and the megacity of Mumbai perennially convulses with conflicts sparked by religion and class discrimination. Social dissonance becomes almost the norm when certain groups are denied education, voting rights, jobs, and other basic civil liberties.

These ideas may seem intuitive, but many cities are in fact moving in the opposite direction: From Phoenix to Dakar, metropolitan regions are increasingly being built around housing developments that cater to specific price ranges, creating pockets of extreme homogeneity and income inequality across the metropolitan area.

“If you have huge inequity, you will not have information being shared,” says Elmqvist. In a more equitable society, conflicts will still be there, but people will be more prepared to share information, he says. “It’s a matter of social trust.”

Ecology in the City

When a mysterious ailment known as Colony Collapse Disorder decimated honeybee populations across the US in 2007, threatening a $14 billion fruit and nut industry, it became painfully apparent just how valuable the pollination services of this single winged species are. Similarly, the large-scale destruction of tropical rain forests has called attention to their invaluable role as living carbon sinks.

Urban resilience calls attention to the ecosystem services within cities themselves, to the medley of blue and green spaces, both natural and man-made, that can buffer a city against change. Things like urban parks, green roofs, community gardens, and coastal wetlands perform numerous functions, from cooling the city’s microclimate to purifying its rainwater to serving as built-in flood control.

In New Orleans, for example, more than 60 percent of wetlands have been lost in the last 60 years, due partly to oil and natural gas exploration and partly to the levies that were built to keep the Mississippi from flooding the city. Ironically, the loss of these wetlands contributed very directly to the disastrous effects of Hurricane Katrina. Researchers have since calculated that restoring 1 kilometer of wetland would reduce the wave height by one meter, and now efforts are underway to begin rebuilding the southern Louisiana coastline.

Douglas Meffert, an environmental engineer at Tulane University and leader of the Urban Network’s New Orleans research team, says that coastal restoration has emerged as a top priority both at the city and national levels. “The fact that our wetlands are still eroding, and climate change is more threatening is a critical concern,” says Meffert, “We need to acknowledge that we will have to live with flooding, and an abundance of water. That is going to be key—how we live with water on a day to day basis.”

As important as these environmental boons are the social benefits—the various aesthetic, educational, recreational, psychological, and health advantages—that urban nature confers.

In New York City, for example, MillionTreesNYC is an initiative to plant one million new trees across the city’s five boroughs over the next decade. This urban forest will have a cooling, shading effect, will reduce air pollution, and will sequester megatons of carbon—issues, according to Elmqvist, that have recently become high on the urban agenda and will likely become higher in the future. The added greenery could also have surprisingly large health effects: A recent study found that tripling the number of street trees could reduce asthma among children by 25 percent. “If you add all the social and ecological values together, you come up to astronomical figures,” says Elmqvist.

The frontier of resilience research involves defining more precisely what those astronomical values are, and giving decision-makers access to this data. Towards this end, the Urban Network is in the process of building a Social-Ecological Atlas, which will give urban planners tools to measure and map ecosystem services in the urban landscape.

According to Elmqvist, the Atlas incorporates Global Information System (GIS) functions, so users can use Google maps to upload and analyze data on various social and ecological variables. The Stockholm research team, for instance, is currently using the Atlas to map biodiversity distribution across the city, as well as creating a “sociotrope”—a map of the social value of green spaces in the Swedish capital. High social values means that various groups frequently derive benefits from using a site, whether by exercising, bird-watching, or simply relaxing away from the bustle of the city streets.

Using this tool, the researchers can see how the biological and social values compare. Do places with high biodiversity and high social values overlap? If not, then the sites could be vulnerable, says Elmqvist. If the areas are unknown, and used by very few, people might not object to their destruction. The perfect counterexample is New York City’s prized—and hugely popular—Central Park. “No one in their right mind would propose a plan to construct buildings in Central Park,” says Elmqvist. “ That idea would be killed immediately.”

Who Governs the World?

No city today could survive on its own resources. This goes for energy, water, food, information, and various other inputs that fuel urban activity. But it also holds true for governance, as evidenced by the network of cities that is becoming increasingly prominent in the global policy arena.
As urban areas grow in size and complexity, they are catalyzing a shift in power: Increasingly, it is cities—financial centers, hubs of innovation and human capital—that are driving the agenda. This has happened in the US, for example, as cities that took the lead in instituting climate policies after the Bush Administration showed no initiative in doing so. At last year’s annual conference of Convention on Biological Diversity (CBD)—the world’s principle legally binding international treaty to protect biodiversity—more than 50 cities said that they wanted to be a part of implementing its agenda.

From a systems standpoint, what cities are doing is creating a network—which in itself could strengthen resilience. Knowledge generated in one place could be used in another, and experiences and best practices could be shared. But this power shift raises an interesting governance question, as every organization in place today when it comes to global governance—the CBD, the United Nations, the Law of the Sea—is based on the nation state. Now on the sidelines, a very powerful network of cities is growing, sharing information technology, and driving an ambitious sustainability agenda.

Cities could be more nimble and more effective in areas where others have failed. But the key will be creating a global governance system that can incorporate them, creating the critical links to regional and worldwide scales. “It’s very complex,” says Elmqvist, “But you need to have these ‘cross-scale’ interactions.” For example, a community in rural Holland might develop in textbook resilience fashion—building redundancy, monitoring diversity, evaluating ecosystem services—but unless it is linked both nationally and globally, it will be very vulnerable. This idyllic community could be completely wiped out, for instance, by an EU decision to cut support for organic agriculture.

The complex systems view, says Elmqvist, is that you will need a diversity of citizens and groups collaborating to design solutions that fit on a local level. But you will also need to forge connections higher up in the hierarchy. With this vision in mind, the Stockholm Resilience Center has recently partnered with UNESCO to launch URBIS, a international network that focuses on linking innovative urban science to policy-making at local, regional, and global levels.

“We are going into a very interesting new era when it comes to global governance,” says Elmqvist. “We will have nation states, but we will also have very powerful cities raising their voices about the future and the nature of sustainable development.”

The Road Ahead

This May, some 70 million people will descend on Shanghai for the 2010 World Expo, whose theme is “Better city, Better life.” The 12-city Urban Network and URBIS will both be participating, says Elmqvist, organizing several workshops and symposia on the importance of the resilience perspective on urban development. “It’s a fantastic place to try to communicate other perspectives on urban development,” he says. “We look forward to sharing these ideas and the results of what we are doing at the 12 sites.”

The Urban Network will also push to complete its Social-Ecological Atlas—for publication as a book in 2010, and more importantly as a web portal for urban scientists and policymakers. Ultimately, the Atlas will incorporate data from all 12 cities, focusing on freshwater services, carbon sequestration, and cultural values across the dozen. This will enable users for the first time to compare cities of different size and wealth—compare Bangalore and New York and Istanbul, for example—to see how varying social contexts, varying customs and norms, affect urban ecosystems and vice-versa.

Cross-city analysis might reveal standouts among urban systems: Shanghai, for example, had just 900 hectares of green space in 1975. By 2005 it had 27,000. So despite the city’s tremendous growth, its proportion of urban nature is actually increasing.

Questions of green space and optimal urban density are just the beginning. Can the world’s mega-cities keep growing? Are other patterns of urban growth preferable? Urbanization is inevitable, but can it be directed so that cities can be harnessed as generators of innovation, and core contributors to future sustainability? As scientists make headway on these macro-issues, can they develop tools to help decision-makers build for social, economic, and ecological resilience?

These are tall orders and extremely complex questions. So complex, in fact, that many doubt the ability of resilience science to get beyond the theory stage. It’s a common criticism levied against systems scientists in general: they are so wrapped up in their “emergent behaviors,” “thresholds,” and “response diversities”—so smitten with the notion of unifying principles—that they gloss over inherent variation. And what look like great underlying patterns—a coral reef looks like a city, and a financial network looks like an ecosystem—are actually riddled with exceptions, irreducible to any sort of blueprint.

Elmqvist is familiar with these charges. “Those critics are right,” he says. “But what I think the resilience area is contributing is not that blueprint. It is contributing guidelines and rules of thumb.”

At the outset of 2010, volatility is the watchword of the day. Some things are certain: economies will grow, greenhouse gases will accumulate, more people will be born than will die across the planet. But how exactly consumption, climate, population, and other factors will interact is anyone’s guess. In that context, when risk and uncertainty are inevitable, providing the capacity to absorb change—building for resilience—is the only rational response.