To paraphrase Leo Tolstoy: Every polluting city pollutes in its own way. Yet until recently, just how and whence Los Angeles, Bangkok, and eight other global cities exhaled their climatec-hanging vapors was a topic shrouded in mystery. Now, a 10- city comparison of greenhouse gas emissions per capita is showing metropolises “exactly where their emissions are coming from,” says Christopher Kennedy, an associate professor of civil engineering at the University of Toronto and the study’s lead author. The research “could also help cities learn from each other,” he adds.
Aside from the usual finding that North American cities are the heaviest breathers, Kennedy and his team reveal that each urban area has a distinct emissions profi e. (See these profiles on the graph below.) Mile-high Denver and temperate Toronto burn lots of fossil fuels to generate electricity for their businesses and industries, as well as to stay warm during their frostier months. At the same time, hydropower keeps Geneva’s electricity- related emissions low. Yet cold winters drive up Geneva’s heating oil-induced effluvia, as they do for New York and Prague. But New York spares the air many of its transportation-related fumes with high population density and good public transit, as do London and Barcelona.
The study’s 10 authors not only calculated how many tons of carbon dioxide equivalents (a measure of greenhouse gas emissions) each city generates through electricity, heating and industrial fuels, and ground transportation; they also assessed how much each city’s industrial processes, aviation, marine travel, and waste contribute to the climate-changing mix. Theirs is the first study comparing the same indicators of emissions, calculated in the same ways, across cities. An ongoing project is expanding this study to include 44 cities.
One easy way for cities to reduce their greenhouse gas emissions, says Kennedy, is to heat homes without using fossil fuels. “You just need heat, not electricity or some other form of energy,” he notes. “You can get heat from the ground or the sun.”
But the lowest-hanging fruit for cities to pluck is the gases exuded by their landfills. For some cities, this miasma makes up about 10 percent of emissions. Many cities in the western United States are already capturing waste-generated methane, notes Kennedy.
Meanwhile, the greatest challenge to clean air and a stable climate is urban transportation. “It’s good to build public transit, but it’s a long, hard struggle that takes decades,” says Kennedy. He recommends that until cities improve their public transit systems, citizens switch to electric cars. “Although the coal needed to generate the electricity is bad, the efficiency of the electric engine makes it a better option than a gasoline engine,” he says. He also suggests that cities curb sprawl and encourage higher-density living. “When people live close together, you have enough users to pay for public transportation, and you don’t have to build as big a system.”
Christopher Kennedy, Julia Steinberger, Barrie Gasson, et al., “Greenhouse Gas Emissions from Global Cities,” Environmental Science and Technology, 43, 2009.
Read more stories by Alana Conner.