One way to measure greenhouse gases is simply to capture them at the source: You put an instrument on a smokestack, for example. Cities, however, are full of cars, buses, factories and homes that all use fuel or electricity. No one really knows how much carbon dioxide, the main greenhouse gas, comes from each.
Ecologist Kevin Gurney says he can find out.
"If a molecule of CO2 is coming from the surface due to the combustion of fossil fuel, I want to know about it," he says.
Many mayors want to know, too. Hundreds have pledged to reduce their cities' greenhouse gas emissions, but measurements from satellites and monitoring stations mostly lump together emissions sources for a whole city or county.
At Arizona State University, Gurney has come up with a software package that analyzes what he calls a city's metabolism.
"We look at the landscape as if you were in an airplane, and look down at everything, everything that's burning fossil fuels, that's what I want to see," he says. "I want to see through the walls, through the engine."
Gurney collects piles of information about a city's energy diet — from utilities, transportation departments and air-pollution monitors. When he analyzed Indianapolis, Gurney and colleagues from Purdue University and other institutions could pinpoint emissions down to the level of a building or a street.
"We can actually see, through CO2, people waking up, leaving their homes, getting in their cars, going to work," he says. "They move from sector to sector."
The emissions level moves with them.
Gurney calls his toolkit Hestia after the Greek goddess of the hearth and home. In Indianapolis, Hestia showed which neighborhoods had older, draftier homes that use more heating, and which roads get clogged at rush hour and thus produce more CO2.
The idea is that mayors can find out where to surgically nip and tuck to reduce their carbon footprint.
Scott Bernstein, head of the Center for Neighborhood Technology, advises city managers on how to do that. He says existing tools to measure CO2 are often user-unfriendly.
"People look at these big giant spreadsheets in emissions accounting, and their eyes glaze over badly, but if they can see a color-coded map, if they can see a flyover view, people get engaged," he says.
Bernstein says it may be easier to reduce CO2 emissions up from city streets and buildings than to change the entire country's network of power plants, coal mines and refineries.
Colin Tetreault, the official who's figuring out how to cut carbon emissions in Phoenix, says Hestia has already given him ideas on where to spend city money to lower emissions. It also makes it easier for Phoenix residents to make their own decisions, he says.
"This provides the opportunity for any normal citizen to pull up and see what their CO2 footprint could be over the city," he says. "So that could inform them, 'Do I want to spend money on energy efficiency here in my house, reduce my electricity bill, do I want to change my transportation pattern, to something like our light rail or ride a bike?' "
The Hestia program is described in the journal Environmental Science and Technology. Gurney says he's doing an emissions diagnosis of Los Angeles next.