With cities in the midst of a revival, there's reason to believe they can help Americans be more frequent users of sustainable transportation. Dense urban centers with robust public transit—à la New York City—make cars seem almost unnecessary. Commuting to work with a greener alternative—the subway for instance, or the greenest alternative, walking—is often more convenient for those who live and work in a tight downtown. But new evidence shows that CO2 emissions from cars in urban hubs are still on the rise, and curbing them is more complicated than simply planning dense, livable downtowns.
A trio of researchers from Boston University has developed a database that is useful for analyzing this very issue. The data set, called DARTE, shows that 80 percent of CO2 emissions growth came from on-road vehicles in urban metros between 1980 and 2012. In 2012 alone, urban metros accounted for 63 percent of total vehicular carbon emissions in the United States. A full report on the new database and its findings were published Monday in the science journal PNAS.
This information may not come as a complete surprise, as the number of Americans living in cities is on the upswing. Alternative transit options may exist in these urban hubs, but cities will inevitably contribute a larger share of the vehicular carbon footprint as more people move in.
"It’s pretty hard to add people anywhere without getting some vehicle activity," explains Conor Gately, a Ph.D candidate at Boston University, and co-developer of the DARTE database.
Still, DARTE has tremendous value because it reveals how spatiality is correlating with per capita auto emissions at the metro level. In other words, it shows us many suburban commuters are pumping air pollution into cities.
Shown above, per capita on-road emissions levels nearly doubled in metro Salt Lake between 1980 and 2010. The city hardly densified during this period, raising the concern of whether air pollution increases could have been mitigated by smarter downtown development. However, the BU team concludes that carbon emissions levels increased mainly because of the behavior of suburban commuters.
"We’re seeing emissions occurring—we’re seeing rising emissions in the city itself—most likely due to the fact that even though people have decided to live outside of Salt Lake City, they’re going to drive in for work," Gately says.
This hypothesis—that emissions from suburban car commuters negates the benefits of green transit in urban cores—is supported by a 2014 study by the University of California. That research concluded that populous cities with small carbon footprints are generally surrounded by gas-guzzling suburbs.
It's still true that people living in dense downtowns are generally traveling in greener ways. As the image above demonstrates, residents of denser metros associated with strong public transit networks—San Francisco, Boston, New York Chicago—accounted for lower levels of vehicular CO2 in 2010. Residents of sprawling metros like Salt Lake City, Houston, and Atlanta, meanwhile, generally emitted greater carbon volumes through driving.
But to expect that every city can become a dense haven of public transit is unrealistic. American cities of all shapes and sizes are contributing to carbon emissions from cars. Tackling pollution emitted by suburban commuters, rather than focusing only on downtown density, could prove crucial to keeping our skies clean.