Are Large Cities More Pollution Efficient?

It is estimated that the world’s population will reach 10 billion by 2100, with almost 90 percent residing in urban areas. That increase in population will demand energy inputs that will have an effect on the total CO₂ emissions produced. One might assume that cities benefit from economies of scale and operate more efficiently as they increase in size, particularly in the sharing of infrastructure. Past research shows that urban areas with larger populations have energy efficiency advantages over lesser-populated cities, but the next step is to determine if large populations manifest those efficiency advantages in CO₂ emissions.

In “Does Size Matter? Scaling of CO₂ Emissions and US Urban Areas,” Michail Fragkias, Jose Lobo, Deborah Strumsky, and Karen C. Seto study the effects of population size, density, and wealth on CO₂ emissions to determine if larger cities generate emissions more efficiently than smaller cities. They find that there are no economies of scale for larger cities and a one percent increase in population size yields a nearly equivalent increase in CO₂ emissions.

There is quite a bit of historical context for this study, as researchers have been studying the effects of population size on overall urban production for decades. Several studies reference the concept of “urban metabolism,” acknowledging that cities require inputs, including energy. The authors of these studies argue that if CO₂ emissions could be an indirect measure of these needed energy inputs, then a connection could be analyzed between cities and biological features of energy. The biological feature in question is that larger biological organisms are more efficient in their energy consumption than smaller organisms, exhibiting larger economies of scale. Fragkias et al. use this concept to structure their overarching research focus that if cities are often compared to biological structures, larger cities might experience the same economies of scale in emissions efficiency.

The authors use a scaling model to directly calculate cities’ emissions in relation to the population size. They gathered data from Project Vulcan for emissions statistics over a ten-year period from 366 Metropolitan Statistical Areas (MSAs) and 576 Micropolitan Areas, all in the United States. Once scaled, the urban areas were ranked based on their residuals, or the difference between their actual and expected production based on the estimated relationship between emissions and population.

Micropolitan areas have the highest deviations in either direction from their expected emission production meaning that they are the most as well as the least efficient urban areas. Fragkias et al. conclude that large urban areas are slightly more efficient than small areas, yet neither experience a form of emissions efficiency that could be described as economies of scale. A one percent increase in population size is associated with an almost one percent increase in CO₂ emissions. In addition, the authors test for each individual year within the study period and show the same results, indicating invariable results over time. They rank the top 20 MSAs in the United States in 2008 in regard to their residuals, concluding that St. Louis and Minneapolis-St. Paul are the least CO₂ efficient whereas Los Angeles and Seattle are the most efficient.

Considering multiple variables, the authors determine that population size is the most influential factor for CO₂ emissions, with population density and wealth having smaller effects. A one percent increase in population density corresponds with a 0.17 percent reduction in CO2 emissions, which seems to contradict their findings. The authors determine that although emissions drop with an increase in density, the effects of the overall growth of the urban area dominate the potential benefits from the increased density. They also find that a one percent increase in personal income yields a 0.36 percent increase in emissions, yet this relationship was not as conclusive. Overall, population is the strongest component affecting CO₂ emissions across the MSAs and Micropolitan areas evaluated in this study.

These findings contradict the hypothesis that cities maintain the same energy efficiencies as biological organisms. While more populated cities experience economies of scale in many areas, these economies cannot be assumed in CO₂ emissions. As a result, urban size may not be an adequate mitigation strategy for climate change and environmental policy should reflect that. City officials must find other mitigation options to tackle increasing carbon emissions as a result of their cities’ increasing populations. It is also important to note, however, that while larger cities do not experience economies of scale for CO₂ emissions, a policy favoring larger cities may be more effective in causing carbon reductions due to technological advantages and ecological innovations. Overall, any policy decision for urban area growth and sustainability must weigh the implications of increasing population on several urban metrics, CO₂ emissions included.

Feature Photo: cc/(leniners)