WASHINGTON — For years, scientists have been trying to solve what they call the "Mystery of the Missing Sinks.''
No, they're not talking about misplaced kitchenware. These "sinks'' are the world's forests, pastures, crops and soil, which soak up the excess carbon — in the form of carbon dioxide — that's a major driver of global warming. Even golf courses and suburban lawns serve as carbon sinks.
"Humans dump about 9 million tons of carbon daily into the atmosphere, but only half stays there,'' said David Crisp, principal investigator for NASA's Orbiting Carbon Observatory.
The rest is returned to Earth, but where much of it ends up is uncertain. About a quarter of the recycled CO2 is drawn into the ocean, and land vegetation absorbs another quarter.
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"We don't know where the other half is going,'' said Crisp, who's based at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
To solve the mystery, NASA and the Japan Aerospace Exploration Agency are sending up complementary scientific satellites. The Japanese Greenhouse Gases Observing Satellite was boosted into orbit last Friday. NASA's Orbiting Carbon Observatory is scheduled for launch on Feb. 23.
The two spaceships will circle the Earth on overlapping paths, more than 400 miles high, analyzing plumes of CO2 rising and falling through the air. (CO2 contains two atoms of oxygen for every atom of carbon.)
"Carbon dioxide 'sources' increase the local concentrations of this gas. Carbon dioxide `sinks' reduce the concentrations,'' Crisp explained at a NASA briefing Thursday.
For example, cities such as New York and Los Angeles are carbon sources year-round. On the other hand, Iowa is a major carbon sink in summer because its cornfields take up tons of CO2.
"We want to understand why, how and where these sinks are and what's going to happen in the future,'' said Anna Michalak, an Orbiting Carbon Observatory team member from the University of Michigan, in Ann Arbor. "There's no consensus whether these sinks will increase or decrease.''
For example, as the world warms, Arctic tundra is thawing and releasing more CO2. At the same time, forests are expanding farther north, storing more CO2.
"We can only account for about half of the carbon that doesn't remain in the atmosphere,'' said Eric Ianson, Orbiting Carbon Observatory project manager. Identifying where it goes will "help policymakers make informed decisions about carbon management,'' he said.
Earth has only about 100 land stations that are able to measure CO2 with sufficient accuracy. "Large parts of the Earth have no stations at all,'' Crisp said.
Scott Denning, an atmospheric scientist at Colorado State University in Fort Collins, pointed out that "the "missing sinks' aren't really missing. They are just poorly understood,'' he said.
"It would be great if we could measure how much carbon every tree, shrub, peat bog and blade of grass takes in. But the world is too big and too diverse and is constantly changing,'' Denning said. "The solution isn't measuring carbon in trees. The solution is measuring carbon in the air.''
The new satellites will let scientists depend less on sometimes inaccurate computer models than they do now and more on data to understand the causes of global warming.
The U.S. satellite will continuously measure CO2 in patches a little bigger than a square mile, accumulating 8 million observations that will be repeated every 16 days. The Japanese version will monitor wider areas, about 20 square miles, and provide a series of 56,000 snapshots about 60 miles apart that will be repeated every three days.
The images will be taken by spectrographs, instruments that can measure the amount of carbon dioxide in the air with an accuracy of one part in a million.
"This approach will provide the first regional-scale, global maps of carbon dioxide over the entire globe,'' Crisp said
The $500 million Japanese satellite is supposed to last five years. The $278 million U.S. system is designed for two years.
The two countries intend to collaborate closely. "Fortunately, we have two different systems using very different methods to make these measurements at the same time,'' Crisp said. "Their orbits will cross several times a day.''
Martin Heimann, a geochemist at the Max Planck Institute in Jena, Germany, who isn't a member of the American or Japanese teams, praised the satellites but said that they wouldn't last long enough to do an adequate job.
"Some of the most fundamental questions on the mechanisms of the Earth's carbon cycle in a changing climate just cannot be tackled with less than half a decade of data,'' he wrote in the January issue of Nature Geoscience.
Nevertheless, Denning said that the satellite data would "buy us more time to develop alternative energy and other mitigation measures'' against global warming.
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