Cities will save us, experts argue. Higher densities of people mean less energy consumption and lower carbon emissions per capita—a boon for the environment. People supposedly switch from driving cars to using public transportation, for instance. But “density in and of itself isn't changing behavior,” says Conor Gately, a graduate student in Boston University's department of Earth and environment. Gately and his colleagues analyzed 33 years' worth of annual carbon dioxide emissions by on-road vehicles across the U.S. Since 2000 the 50 fastest-growing counties by population decreased their per capita emissions by only 12 percent—a reduction that was not enough to offset the total emissions growth in those same areas. The discrepancy probably comes down to the sprawl of suburbs, the researchers say. If public transportation between city and surrounding neighborhoods, for example, fails to keep pace with growth, more and more people will drive into center cities for work and play and add pollution to areas where they don't live. (That's what's been happening in Salt Lake City.)

Urban areas that are already dense and have the necessary green infrastructure, such as New York City, will see their per capita emissions decline as they grow larger. In 2012 carbon emissions from vehicles on the road accounted for 28 percent of the total fossil-fuel CO2 emissions for the entire country—a sizable chunk to target for reductions. The data suggest that solutions and regulations should be customized for individual cities, not mandated as national catchalls.

Graphic by Tiffany Farrant-Gonzalez