LOS ANGELES - Every time Los Angeles exhales, odd-looking gadgets anchored 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 a pulse 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 budding 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 eyeing large cities - with Los Angeles and Paris as guinea pigs.
For the past year, a high-tech sensor poking out from a converted shipping container has stared at the Los Angeles basin from its mile-high perch on Mount Wilson, a peak in the San Gabriel Mountains that's home to a famous observatory and communication towers.
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.
In a storage room next door, commercially available instruments that typically monitor air quality double as climate sniffers. And in nearby Pasadena, a refurbished vintage solar telescope on the roof of a laboratory on the California Institute of Technology campus captures sunlight and sends it down a shaft 60 feet below where a prism-like instrument separates out carbon dioxide molecules.
On a recent April afternoon atop Mount Wilson, a brown haze hung over the city, the accumulation of dust and smoke particles in the atmosphere.
"There are some days where we can see 150 miles way out to the Channel Islands, and 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.
What Sander and others are after are the mostly invisible greenhouse gases spewing from factories and freeways below.
Scientists also plan to drive around the city in a Prius outfitted with a portable emission-measuring device and fly a research aircraft to pinpoint methane hotspots from the sky. (A well-known natural source is the La Brea Tar Pits in the heart of L.A., where underground bacteria burp bubbles of methane gas to the surface.)
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 port areas and airports and retrofitting city buildings.
Project manager Riley Duren of JPL said it'll take several years of monitoring to know whether L.A. is on track to reach its goal.