Allen Robinson, an air quality expert at Carnegie Mellon University, said he prefers more attention paid to measuring a city's methane emissions since scientists know less about them than carbon dioxide release.
Nearly 58 percent of California's carbon dioxide emissions in 2010 came from gasoline-powered vehicles, according to the U.S. Energy Department's latest figures.
In much of the country, coal —usually as fuel for electric power — is a major source of carbon dioxide pollution. But in California, it's responsible for a tad more than 1 percent of the state's carbon dioxide emissions. Natural gas, considered a cleaner fuel, spews one third of the state's carbon dioxide.
Overall, California in 2010 released about 408 million tons of carbon dioxide into the air. The state's carbon dioxide pollution is greater than all but 20 countries and is just ahead of Spain's emissions. In 2010, California put nearly 11 tons of carbon dioxide into the air for every person, which is lower than the national average of 20 tons per person.
Gregg Marland, an Appalachian State University professor who has tracked worldwide emissions for the Energy Department, said there's value in learning about a city's emissions and testing techniques.
"I don't think we need to try this in many places, but we have to try some to see what works and what we can do," he said.
Launching the monitoring project came with the usual growing pains. In Paris, a carbon sniffer originally tucked away in the Eiffel Tower's observation deck had to be moved to a higher floor that's off-limits to the public after tourists' exhaling interfered with the data.
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.
With the focus on megacities, others have worked to decipher the carbon footprint of smaller places like Indianapolis, Boston and Oakland, where University of California, Berkeley researchers have taken a different tack and blanketed school rooftops with relatively inexpensive sensors.
"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.
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