A unified theory of urban living

core must be developed. Such an ambitious programme requires major international commitment and dedicated transdisciplinary collaboration across science, economics and technology, including business leaders and practitioners, such as planners and designers. Developing a predictive framework applicable to cities around the world is a daunting task, given their extraordinary complexity and diversity. However, we are strongly encouraged that this might be possible. UNIVERSAL FEATURES A unified theory of urban living It is time for a science of how city growth affects society and environment, say Luis Bettencourt and Geoffrey West. A t the start of the twenty-first century, cities emerged as the source of the greatest challenges that the planet has faced since humans became social. Although they have proven to be humanity’s engines of creativity, wealth creation and economic growth, cities have also been the source of much pollution and disease. Rapid urbanization and accelerating socioeconomic development have generated global problems from climate change and its environmental impacts to incipient crises in food, energy and water availability, public health, financial markets and the global economy1,2. Urbanization is a relatively new global issue. As recently as 1950, only 30% of the world’s population was urbanized. Today, more than half live in urban centres. The developed world is now about 80% urban and this is expected to be true for the entire planet by around 2050, with some 2 billion people moving to cities, especially in China, India, southeast Asia and Africa2. Cities are complex systems whose infrastructural, economic and social components are strongly interrelated and therefore difficult to understand in isolation3. The many problems associated with urban growth and global sustainability, however, are typically treated as independent issues. This frequently results in ineffective policy and often leads to unfortunate and sometimes disastrous unintended consequences. Policies meant to control population movements and the spread of slums in megacities, or to reverse urban decay, have largely proven ineffective or counterproductive, despite huge expenditure. In New York City in the 1970s, for example, a strategy of ‘planned shrinkage’ intentionally removed essential services from some urban areas — notably the Bronx — to prompt people to move away and allow for redevelopment. Instead, this strategy led to increases in crime and general socio-economic degradation. In North America in the 1950s to 1970s (and earlier in Europe), policies of urban renewal intended to reduce high urban densities, by razing poorer old neighbourhoods and creating infrastructure, actually ended up encouraging urban sprawl3. Similar debates continue to play out in rapidly developing cities around the world today, from Beijing to Rio de Janeiro in Brazil, often leading to similar mistakes. But cities supply solutions as well as problems, as they are the world’s centres of creativity, power and wealth. So the need is urgent for an integrated, quantitative, predictive, science-based understanding of the dynamics, growth and organization of cities. To combat the multiple threats facing humanity, a ‘grand unified theory of sustainability’ with cities and urbanization at its 9 1 2 | NAT U R E | VO L 4 6 7 | 2 1 O C T O B E R 2 0 1 0 © 2010 Macmillan Publishers Limited. All rights reserved Cities manifest remarkably universal, quantifiable features. This is shown by new analyses of large urban data sets, spanning several decades and hundreds of urban centres in regions and countries around the world from the United States and Europe to China and Brazil4,5. Surprisingly, size is the major determinant of most characteristics of a city; history, geography and design have secondary roles4,6. Three main characteristics vary systematically with population. One, the space required per capita shrinks, thanks to denser settlement and a more intense use of infrastructure. Two, the pace of all socioeconomic activity accelerates, leading to higher productivity. And three, economic and social activities diversify and become more interdependent, resulting in new forms of economic specialization and cultural expression. We have recently shown that these general trends can be expressed as simple mathematical ‘laws’. For example, doubling the population of any city requires only about an 85% increase in infrastructure, whether that be total road surface, length of electrical cables, water pipes or number of petrol stations4. This systematic 15% savings happens because, in general, creating and operating the same infrastructure at higher densities is more efficient, more economically viable, and often leads to higher-quality services and solutions that are impossible in smaller places. Interestingly, there are similar savings in carbon footprints7,8 — most large, developed cities are ‘greener’ than their national average in terms of per capita carbon emissions. It is as yet unclear whether this is also true for cities undergoing extremely rapid development, as in China or India, where data are poor or lacking. Similar economies of scale are found in organisms and communities like anthills and beehives, where the savings are closer to 20%9. Such regularities originate in the mathematical properties of the multiple SCIENCE AND THE CITY Full content and enhanced graphics at: nature.com/cities OLIVER MUNDAY COMMENT log (metric/metric average) networks that sustain life, from the cardioof population size: diseases spread faster, vascular to the intracellular9. This suggests businesses are born and die more often and that similar network dynamics underlie people even walk faster in larger cities, all by economies of scale in cities. approximately the same 15% rule4. MoreCities, however, are much more than giant over, this social network dynamic allows the organisms or anthills: they rely on longgrowth of cities to be unbounded: continuous range, complex exchanges of people, goods adaptation, not equilibrium, is the rule. and knowledge. They are invariably magnets Open-ended growth is the primary for creative and innovative individuals, and assumption upon which modern cities and stimulants for economic growth, wealth economies are based. Sustaining that growth production and new ideas — none of which with limited resources requires that major have analogues in biology. innovations — such as those historically assoThe bigger the city, the more the averciated with iron, coal and digital technology age citizen owns, produces and consumes, — be made at a continuously accelerating rate. whether goods, resources or ideas4. On averThe time between the ‘Computer Age’ and age, as city size increases, per capita socio-economic quantities such as PREDICTABLE CITIES wages, GDP, number of patents proData from 360 US metropolitan areas show that metrics such as wages and crime scale in the same way with population size. duced and number of educational 2.5 and research institutions all increase METRIC: 2.0 by approximately 15% more than the Crime 4 expected linear growth . There is, 1.5 GDP Income however, a dark side: negative met1.0 Patents rics including crime, traffic conges0.5 tion and incidence of certain diseases 0 all increase following the same 15% rule4. The good, the bad and the ugly −0.5 come as an integrated, predictable, −1.0 package. −1.5 Our work shows that, despite −2.0 appearances, cities are approximately −2.0 −1.5 −1.0 −0.5 0 0.5 1.0 1.5 scaled versions of one another (see log (city population/city population average) graph): New York and Tokyo are, to a surprising and predictable degree, nonlinearly scaled-up versions of San Franthe ‘Information and Digital Age’ was some cisco in California or Nagoya in Japan. These 20 years, compared to thousands of years extraordinary regularities open a window on between the Stone, Bronze and Iron Ages. underlying mechanism, dynamics and strucMaking major technological paradigm shifts ture common to all cities. systematically faster is clearly not sustainable, Deviations from these scaling laws, illuspotentially leading to collapse of the entire trated by the spread of data in the figure, urbanized socio-economic fabric. Avoiding measure how each city over- or under-perthis requires understanding whether we can forms relative to expectations for its size6. continue to innovate and create wealth withRelatively large deviations (as much as 30%) out continuous growth and its compounded are seen for quantities with small numbers, negative social and environmental impacts. such as patents and murders, whereas much smaller deviations (with variances less than ACTING ON EVIDENCE 10%) are seen for economic properties. We The job of policy-makers is to enhance the also find that quantities such as GDP are performance of their city relative to basemore variable for urban centres in developing lines for their size defined by scaling laws. countries, such as China and Brazil, than for Although a scientific understanding of how older cities in developed areas such as North cities work may not be prescriptive for polAmerica or Japan. It is unclear whether this icy-makers, recent work should help them to is a fundamental property of developing encourage positive urban development. nations or an artefact of data collection. Our research shows that cities are remarkIn biology, the network principles underably robust: success, once achieved, is suslying economies of scale have two profound tained for several decades or longer6, thereby consequences. They constrain both the pace setting a city on a long run of creativity and of life (big mammals live longer, evolve slower, prosperity. A great example of success is and have slower heart rates, all to the same metropolitan San Jose, home to the Silicon degree9), and the limits of growth (animals Valley, which has been consistently overgenerally reach a stable size at maturity10). In performing relative to expectations for its size contrast, cities are driven by social interacfor at least 50 years, well before the advent of tions whose feedback mechanisms lead to modern hi-tech industry. Unfortunately, the the opposite behaviour. The pace of urban life reverse is also true: it is very hard to turn systematically increases with each expansion around urban decay swiftly. Ineffective policy and unrealistic short-term expectations can condemn a city to decades of under-performance: witness former industrial cities such as Buffalo, New York. Today’s rapid development and urbanization provides an opportunity to collect detailed data that will illuminate the links between economic development and its undesirable consequences. Policy initiatives in developed and developing cities should be viewed as experiments that, if carefully designed and measured, can help support the creation of an integrated, predictive theory and a new science of performance-based planning. Examples of this approach are increasingly common, both among poster children such as Barcelona in Spain or Curitiba in Brazil, and as part of new initiatives in New York or London. Ideally, by coupling general goals (such as lower carbon emissions) to actionable policies and measurable indicators of social satisfaction, successes and failures can be assessed and corrected for, guiding development of theory and creating better solutions. Cities are the crucible of human civilization, the drivers towards potential disaster, and the source of the solution to humanity’s problems. It is therefore crucial that we 2.0 understand their dynamics, growth and evolution in a scientifically predictable, quantitative way. The difference between ‘policy as usual’ and policy led by a new quantitative understanding of cities may well be the choice between creating a “planet of slums” or finally achieving a sustainable, creative, prosperous, urbanized world expressing the best of the human spirit. ■ Luis Bettencourt is a scientist at Los Alamos National Laboratory and external professor at the Santa Fe Institute. Geoffrey West is distinguished professor at the Santa Fe Institute and senior fellow at Los Alamos National Laboratory. e-mail: gbw@santafe-edu 1. Schellnhuber, H. J., Molina, M., Stern, N., Huber, V. & Kadner, S. (eds) Global Sustainability: A Nobel Cause (Cambridge Univ. Press, 2010). 2. UN-Habitat. State of the World’s Cities 2010/2011 — Cities for All: Bridging the Urban Divide (2010); available at http://www.unhabitat.org 3. Jacobs, J. The Death and Life of Great American Cities (Random House, 1961). 4. Bettencourt, L. M. A., Lobo, J., Helbing, D., Kühnert, C. & West, G. B. Proc. Natl Acad. Sci. USA 104, 7301–7306 (2007). 5. Batty, M. Science 319, 769–771 (2008). 6. Bettencourt, L. M. A., Lobo, J., Strumsky, D. & West, G. B. PLoS ONE (in the press). 7. Brown, M. A., Southworth, F. & Sarzynski, A. Policy Soc. 27, 285–304 (2009). 8. Dodman, D. Environ. Urban. 21, 185–201 (2009). 9. West, G. B., Enquist, B. J. & Brown, J. H. Science 276, 122–126 (1997). 10. West, G. B., Brown, J. H. & Enquist, B. J. Nature 413, 628–631 (2001). 2 1 O C T O B E R 2 0 1 0 | VO L 4 6 7 | NAT U R E | 9 1 3 © 2010 Macmillan Publishers Limited. All rights reserved L. BETTENCOURT; N. RODRIGUEZ; G. WEST COMMENT