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Mayors of cities across the country have pledged to reduce greenhouse gases from factories, homes and vehicles. The idea is to curb global warming by acting locally. But it's a tough job if you don't know exactly where those gases are coming from. So scientists have invented a new way to pinpoint sources. NPR's Christopher Joyce has more on how it works.

CHRISTOPHER JOYCE, BYLINE: One way to measure greenhouse gases is simply to capture them at the source. You put an instrument on a smokestack, for example. But cities are full of cars, buses, factories and homes. They all use fuel or electricity but no one really knows how much carbon dioxide, the main greenhouse gas, comes from each. Ecologist Kevin Gurney says he can find out.

KEVIN GURNEY: If a molecule of CO2 is coming from the surface due to the combustion of fossil fuel, I want to know about it.

JOYCE: A lot of mayors want to know too. Hundreds have pledged to reduce their city's greenhouse gas emissions, but current 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.

GURNEY: We look at the landscape as if you were in an airplane, and look down at everything that's burning fossil fuel.

JOYCE: What he does is collect piles of information about a city's energy diet - from utilities, transportation departments, 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.

GURNEY: We can actually see, through CO2, people waking up, leaving their homes, getting in their cars, going to work. They move from sector to sector.

JOYCE: And 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 is someone who advises city managers how to do that. He's head of the Center for Neighborhood Technology. He says existing tools to measure CO2 are often user-unfriendly.

SCOTT BERNSTEIN: 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 fly-over view, people get engaged.

JOYCE: Bernstein says it may be easier to reduce CO2 emissions up from city streets and buildings than trying to change the entire country's network of power plants, coal mines and refineries. That's what Colin Tetreault in Phoenix, Arizona is thinking. He's the city official who has to figure out how to cut carbon emissions there. He says Hestia has already given him ideas on where to spend city money to lower emissions. But it also makes it easier for Phoenix residents to make their own decisions.

COLIN TETREAULT: This provides the opportunity for any normal citizen to pull up and see what their CO2 footprint could be over the city. So that can inform them - do I want to spend money on energy efficiency here at my house, reduce my electricity bill? Do I want to change my transportation pattern to something like our light rail or ride a bike?

JOYCE: The Hestia program is described in the journal Environmental Science and Technology. Gurney says besides Indianapolis and Phoenix, he's doing an emissions diagnosis of Los Angeles next. Christopher Joyce, NPR News.

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