You might think that cities would spew less carbon dioxide and other greenhouse gases than rural areas. After all, urban areas often provide opportunities to walk or take public transportation nearly everywhere you'd want to go. But a new study reports that urban areas likely account for nearly two thirds of driving-related greenhouse gases, and carbon emissions grow disproportionately with population density, conclusions that call into question national and local smart-growth policies.
Conclusions like that are somewhat at odds with the view that cities are leaders in the fight against global climate change. Indeed, some have advocated looking toward major American cities for inspiration, and from Santa Fe, New Mexico's residential building codes to Chicago's efforts to make itself more bicycle friendly, there's plenty to get you inspired.
At the same time, Boston University graduate student Conor Gately and professors Lucy Hutrya and Ian Sue Wing argue, not enough is known about how much carbon cities actually send into the atmosphere. In particular, studies often assume that vehicular carbon emissions grow in proportion to a city's population density, a presumption that's controversial at best. There's also a dearth of data on how driving habits change as a city grows and its population profile changes over time.
Urban centers' supposed advantage—per capita emissions that decline as density increases—don't really kick in until you get above 1,250 people per square kilometer.
What's needed, they write, is data speaking directly to traffic patterns—something the researchers had to put together themselves. Using Federal Highway Administration surveys of traffic on individual segments of roads from 1980 to 2012, along with emissions data tailored to the year and state in question, they constructed a one-kilometer resolution database of vehicle emissions.
A quick analysis of that data revealed carbon emissions in America are up from about a gigatonne—one trillion kilograms—in 1980 to 1.55 gigatonnes in 2012, with higher-density urban areas accounting for roughly 80 percent of the increase.
But digging deeper, Gately, Hutrya, and Wing discovered a complicated relationship between density and emissions. Total emissions climb dramatically as a function of population density, but only up to about 1,650 people per square kilometer—roughly the density of fairly low-density Portland, Oregon. That's in part because urban centers' supposed advantage—per capita emissions that decline as density increases—don't really kick in until you get above 1,250 people per square kilometer.
That pattern is a significant obstacle to current thinking about city growth and climate change, the authors argue. Eighty-seven percent of the United States population lives in places with densities under 1,000 people per square kilometer, and the 50 fastest growing counties in the country since 2000 averaged about 350 people per square kilometer—right where emissions grow the most with population. That will likely make for "tension between climate change policy at national and regional scales, because the reductions in national emissions provided by urbanization will not necessarily occur in the urban areas that are actually growing denser," the researchers write. In other words, New York, Santa Fe, and Chicago may be the places to watch, but they may not be the places that matter.
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