LOS ANGELES (AP) — Every time Los Angeles exhales, odd-looking gadgets in the mountains above the city trace the invisible puffs of carbon dioxide, methane and other greenhouse gases that waft skyward.
Halfway around the globe, similar contraptions atop the Eiffel Tower and elsewhere around Paris keep watch on emissions from smokestacks and automobile tailpipes. And there is talk of outfitting Sao Paulo, Brazil, with sensors that sniff the byproducts of burning fossil fuels.
It's part of a young effort to track the carbon footprints of megacities — urban hubs with more than 10 million people that are increasingly responsible for human-caused global warming.
For years, carbon dioxide and other greenhouse pollutants have been closely monitored around the planet by stations on the ground and in space. Last week, worldwide levels of carbon dioxide reached 400 parts per million at a Hawaii station that sets the global benchmark — a concentration not seen in millions of years.
Now, some scientists are looking at large cities — with LA and Paris as the first —as a first step toward independently verifying whether local climate goals are being met.
For the past year, a high-tech sensor has stared at the Los Angeles basin from a peak in the San Gabriel Mountains that's home to a famous observatory.
Like a satellite gazing down on Earth, it scans more than two dozen points from the inland desert to the coast. Every few minutes, it rumbles to life as it automatically sweeps the horizon, measuring sunlight bouncing off the surface for the unique fingerprint of carbon dioxide and other heat-trapping gases.
On a recent April afternoon, a brown haze hung over the city, the accumulation of dust and smoke particles in the atmosphere.
"There are some days where we have trouble even seeing what's down here in the foreground," said Stanley Sander, a senior research scientist at the NASA Jet Propulsion Laboratory.
Six years ago, elected officials vowed to reduce emissions to 35 percent below 1990 levels by 2030 by shifting to renewable energy and weaning the city's dependence on out-of-state coal-fired plants, greening the twin port complex and airports and retrofitting city buildings.
It's impractical to blanket the city with instruments, so scientists rely on a handful of sensors and use computer models to work backward to determine the sources of the emissions and whether they're increasing. They won't be able to zero in on an offending street or a landfill, but they hope to be able to tell whether switching buses from diesel to alternative fuel has made a difference.
Project manager Riley Duren of JPL said it'll take several years of monitoring to know whether LA is on track to reach its goal.
So far, $3 million have been spent on the U.S. effort with funding from federal, state and private groups. The French, backed by different sponsors, have spent roughly the same.
Scientists hope to strengthen their ground measurements with upcoming launches of Earth satellites designed to track carbon dioxide from orbit. The field experiment does not yet extend to China, by far the world's biggest carbon dioxide polluter. But it's a start, experts say.
"We are at a very early stage of knowing the best strategy, and need to learn the pros and cons of different approaches," said Inez Fung, a professor of atmospheric science at Berkeley who has no role in the various projects.
Associated Press writer Alicia Chang contributed.