Our cities have the potential to be a key climate change solution. Already they are hot-beds of innovation in local and global approaches to the nexus of sustainability and quality of life.  People who live in cities drive less, use less energy to heat, cool, and light their homes, and even their water and sewer lines are shorter and require fewer resources. (See related, “The City Solution.”)

But all of those advantages – and the ability to save more land for nature and agriculture – will be cancelled out if our cities are ringed with suburbs that are profligate in their use of energy and resources. In my laboratory – the Renewable and Appropriate Energy Laboratory (RAEL) — at the University of California, Berkeley, Energy and Resources Group, we decided to take a closer look at whether cities in the United States really are helping to shrink the nation’s carbon footprint. (See related Quiz: What You Don’t Know About Cities and Energy.)

Our results, published this week in Environmental Safety & Technology, should give urbanization advocates kudos for what well-designed urban cores can do. But it also should give them pause, to not forget the suburbs. Population-dense cities indeed are contributing fewer greenhouse gas emissions per person than other areas of the country, but these cities’ extensive suburbs essentially “take back” the climate benefits of their cores.  We are not talking just about commuting, either.

U.S. suburbs, in fact, account for half of all U.S. household greenhouse gas emissions, even though they house less than half the population. Taking into account the impact of all urban and suburban residents, large metropolitan areas have a slightly higher average carbon footprint than smaller metro areas.

The bottom line: Cities are not islands. They exist in a complex landscape that we need to understand better, both theoretically and empirically.

We wanted to present in a visually striking way the impacts and interactions of our energy, transportation, land use, shopping, and other choices in both cities and suburbs. It is plain to see the trends at our interactive carbon footprint maps for more than 31,000 U.S. zip codes in all 50 states at: http://coolclimate.berkeley.edu/maps.

Los Angeles map

“Metropolitan areas look like carbon footprint hurricanes, with dark green, low-carbon urban cores surrounded by red, high-carbon suburbs,” says Christopher Jones, the doctoral student who worked with me on the project. “Unfortunately, while the most populous metropolitan areas tend to have the lowest carbon footprint centers, they also tend to have the most extensive high carbon footprint suburbs.”

In fact, the average carbon footprint of households living in the center of large, population-dense urban cities is about 50 percent below average, while households in distant suburbs are up to twice the average: a four-fold difference between lowest- and highest-emissions locations.

The primary drivers of carbon footprints are household income, vehicle ownership and home size, all of which are considerably higher in suburbs. Other important factors include population density, the carbon-intensity of electricity production, energy prices and weather. In all, we looked at 37 different variables in our analysis of household carbon footprints.

When it comes to crafting a city solution for climate change, we found that density is not enough. A 10-fold increase in population density in central cities yields only a 25 percent reduction in greenhouse gas emissions.

“That would require a really extraordinary transformation for very little benefit, and high carbon suburbanization would result as a side effect,” Jones says. Even more disturbing: These population-dense suburbs tend to create their own suburbs—bad news for the climate.

Does this mean that we should abandon the idea that cities and urbanization can aid in addressing climate change?

Quite the contrary.

A number of cities nationwide have developed exceptionally interesting and thoughtful sustainability plans, many of them very innovative. (See related “Pictures: Twelve Car-Free City Zones” and “Boston Tops Ranking of Energy-Efficient Cities.”) But we should keep in mind that there is no one-size-fits-all solution. In some locations, motor vehicles are the largest source of emissions, while in other locations it might be electricity, food, or goods and services. California, for example, has relatively low emissions associated with household electricity, but large emissions from transportation. The opposite is true in parts of the Midwest, where electricity is produced largely from coal. The important thing is that cities need information on which actions have the highest potential to reduce greenhouse gas emissions in their communities. (Related: “With a Deep Dig Into Its Past, Perugia Built an Energy-Saving Future“)

The real opportunity is tailoring climate solutions to demographically similar populations within locations. Suburbs actually are excellent candidates for a combination of solar photovoltaic systems, electric vehicles and energy-efficient technologies. When you package low-carbon technologies together, you find real financial savings and big social and environmental benefits.  (See related story: “IEA Report Offers Prescription to Ease Urban Transit Congestion.”)

Cities need to step out of traditional roles in planning urban infrastructure and learn how to better understand the needs of residents in order to craft policies and programs that enable the adoption of these energy- and carbon-efficient technologies and practices. One example of this is the CoolCalifornia Challenge (http://www.coolcalifornia.org and http://coolclimate.berkeley.edu), a statewide carbon footprint reduction competition to name the “Coolest California City.” Jones and I, who will be running the program sponsored by the California Air Resources Board and Energy Upgrade California, will be accepting applications for new cities in February. The idea is for each city to create its own, targeted strategy to reduce barriers and increase motivation to engage residents in climate action.

The strategies that work best will make the suburbs part of the city solution.