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 with sensors that sniff the byproducts of burning fossil fuels.
It is part of a budding effort to track the carbon footprints of megacities, urban hubs with over 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 — and aiming to observe emissions in the atmosphere as a first step toward independently verifying whether local — and often lofty — climate goals are being met.
Although the focus on megacities, other researchers have worked to decipher the carbon footprint of smaller cities such as Boston, Indianapolis, and Oakland.
In the Los Angeles study, a high-tech sensor has been operating for the past year on Mount Wilson, a peak in the San Gabriel Mountains that’s home to a famous observatory and communication towers.
There are plans to expand the network. This summer, technicians will install commercial gas analyzers at a dozen more rooftops around the greater LA region.