Cities and metros are not only leading the way in sustainability; they offer an intrinsically greener, less wasteful and more energy efficient way of life. Earlier this week I mapped the geography of carbon emissions across U.S. cities and metros. Today I turn to the connection between greenness and city size.
The size and density of cities confers considerable economic advantages. Denser cities are seed-beds of innovation and productivity improvement, as Jane Jacobs long ago argued. Pioneering studies of "urban metabolism" by Geoffrey West and his colleagues at the Santa Fe Institute find that as metro areas get larger their metabolic rate essentially speeds up, making them more productive and inventive
The environment benefits from density and size as well. Larger, denser cities are cleaner and more energy efficient than smaller cities, suburbs, and even small towns. Ecologists have found that by concentrating their populations in smaller areas, cities and metros decrease human encroachment on natural habitats. Denser settlement patterns yield energy savings; apartment buildings, for example, are more efficient to heat and cool than detached suburban houses. Urban households emit less carbon dioxide than their suburban and rural counterparts. In his book Green Metropolis, David Owen lauds the dense, concentrated built environment of Manhattan – where most people live in apartments and use mass transit — as the greenest place in America. When it comes to greenness, size matters; as urban regions grow their populations, the rate of growth in their emissions actually declines.
That larger and denser cities and metro areas might be energy efficient makes intuitive sense, but the idea that their emissions might actually be lower than smaller cities’ seems like a stretch. It is, however, something that can be tracked empirically, which is what my MPI colleague Kevin Stolarick and Jose Lobo of Arizona State University and I did, using the Project Vulcan data I described previously.
The graph above by Lobo plots the relationship between CO2 emissions and economic output (measured as Gross Metropolitan Product) for U.S. metros. (Because of the size difference between the largest and the smallest metros, the data is expressed in logarithmic form so as to facilitate comparison.) Not surprisingly, the total amount of CO2 emissions increases alongside the economic output of metros. But, it does so at a declining rate, as the graph above shows. According to Lobo, a 1 percent increase in population generates a slightly smaller (between 0.8 and 0.9 percent) increase in emissions. Emissions are reduced as metros become larger. In other words, increasing metropolitan output is associated with decreasing emissions.
When it comes to being green, bigger really is better.
Next time, we’ll take an even closer look at the numbers.
Top image: Peter Foley/Reuters