Driving down busy Interstate 5 in Los Angeles in a nondescript blue Toyota Prius, Riley Duren of NASA's Jet Propulsion Laboratory, Pasadena, California, is a man on a mission as he surveys the vast urban jungle sprawled around him.

In his trunk, a luggage-sized air-sampling instrument sniffs the outside air through a small tube to measure the greenhouse gases carbon dioxide and methane. While not a very efficient way to study urban emissions, the ground data being collected are helping Duren and his team build confidence in greenhouse gas measurements taken from aircraft and satellites, which can cover large areas more effectively.

At the next exit, Duren pulls over to admire a scene most Angelenos would try to ignore: a large landfill stretched alongside the freeway. The instrument in the trunk quickly detects a large plume of methane emanating from the landfill. A NASA aircraft soon appears overhead, carrying a prototype satellite instrument that records high-resolution images of methane that scientists can use to identify gas plumes. The pilot buzzes the landfill several times to capture images of the invisible gas, then the plane departs and Duren heads off to his next study area.

The instruments in the Prius and airplane are just two of many elements of the Megacities Carbon Project, an international, multi-agency pilot initiative to develop and test ways to monitor greenhouse gas emissions in megacities: metropolitan areas of at least 10 million people. Cities and their power plants are the largest sources of human-produced greenhouse gas emissions and are the largest human contributors to climate change.

Duren is principal investigator for the LA component of the Megacities Carbon Project. He hopes to work with international partners to deploy a global urban carbon monitoring system that will eventually allow local policymakers to fully account for the many sources and sinks, or storage sites, of carbon and how they change over time. Los Angeles and Paris are pilot cities in the initiative. Efforts are underway to add other cities around the world.

When fully established in late 2014, the LA network will consist of 15 monitoring stations around the LA basin. Most will use commercially available high-precision greenhouse gas analyzers to continuously sample local air. The LA network encompasses the portions of the South Coast Air Basin that produce the most intense greenhouse gas emissions in California. Megacities scientists will also periodically take to the road and to the skies to collect mobile measurements of the local atmosphere to better define individual emissions sources and environmental conditions.

"LA is a giant laboratory for climate studies and measurement tests," said Duren. "The LA megacity sprawls across five counties, 150 municipalities, many freeways, landfills, oil wells, gas pipelines, America's largest seaport, mountains, and even dairies, all within an area measuring about 80 miles [130 kilometers] on a side. In theory, you could drive across the whole thing in an hour and a half, or three if it's rush hour."

Urbanization has concentrated more than half of Earth's population, at least 70 percent of fossil fuel carbon dioxide emissions and a significant amount of methane emissions into a tiny fraction of Earth's land surface. The world's 40 largest cities combined rank as the world's third largest emitter of fossil-fuel carbon dioxide -- larger than the total emissions of Japan. That trend is expected to grow.

There's an urgent need to get a handle on explosive growth in carbon emissions from fossil-fuel use by cities and to establish baseline measurements that currently don't exist. The lack of measurements makes it hard to assess emission trends.

Most countries and some states produce annual inventories of their greenhouse gas emissions based on energy statistics and other data, but the same information is typically not available for individual cities.

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NASA, partners target megacities carbon emissions