Early Tuesday morning, a Taurus XL rocket will take off from Vandenberg Air Force Base, in California, bearing NASA’s Orbiting Carbon Observatory (OCO) satellite. It will be the space agency’s first satellite dedicated to measuring CO₂ in the atmosphere, and it could fill huge gaps in researchers’ understanding of climate change. [Note: The launch failed.]

On average, about half of the 30 billion tons of CO₂ emitted by the burning of fossil fuels each year doesn’t stay in the atmosphere, and scientists aren’t entirely sure where this CO₂ ends up. They have a general idea that it’s taken up by the ocean and by plants. But they don’t know exactly where this happens on the planet or what mechanisms are involved. As a result, they can’t predict how much CO₂ will be absorbed in the future. According to some theories, CO₂ absorption will increase, slowing global warming. According to others, carbon “sinks” such as the ocean may soon stop absorbing CO₂, and could actually start releasing it, potentially accelerating global warming.

“People are asking us to predict how much the climate will change over the next 50 years,” says David Crisp, who is the principal investigator for the OCO project at NASA’s Jet Propulsion Laboratory (JPL), in Pasadena, CA. “How can I tell you how much CO₂-induced climate change there’s going to be if I don’t know how much CO₂ there’s going to be in the atmosphere?” he says. Even if it were possible to predict how much CO₂ humans will put into the atmosphere, “that’s still only half the puzzle,” he says. “I still need to know how much is going to be absorbed by the earth.”

Scientists haven’t been able to answer this question because they have too few CO₂ monitoring stations–only about 100 scattered over the globe–and because most of these only sample CO₂ at ground level. “There’s huge geographic vacuums in those observations,” says Ronald Prinn, professor of atmospheric science at MIT.

“We’re still in a situation where we don’t understand the processes that the earth is using to absorb this carbon dioxide,” Crisp says. “We don’t know where they’re occurring. We don’t know why they’re occurring. And we don’t know whether they’re going to continue.”

The new satellite will provide global coverage, taking hundreds of thousands of measurements each day, with resolution an order of magnitude better than that of other atmosphere-monitoring satellites. Scientists will be able to track where CO₂ is being emitted and where it is being absorbed, including over vast stretches of ocean or rain forest, where it’s previously been difficult or impossible to measure CO₂ levels.

This will allow scientists to test different theories about how CO₂ is being absorbed. According to some theories, it is being absorbed largely by the ocean and by relatively new forest growth–trees planted after the days of the dust-bowl conditions in the 1930s, which are entering their prime in terms of their ability to absorb CO₂. Both of these carbon sinks could soon reach the limit of their capacity to absorb more CO₂. Indeed, as the ocean warms, it may start releasing more carbon than it absorbs, Crisp says. Yet another theory holds that much of the CO₂ is being absorbed by rain forests that can continue to absorb even more. As CO₂ levels in the atmosphere rise, this gas could act as a fertilizer, prompting more growth and hence more absorption of CO₂.

Data from the satellite will help scientists determine where the CO₂ is being absorbed and how much different areas contribute to overall absorption. It will also suggest where scientists should go to study the absorption mechanisms that are involved in more detail, Crisp says.

The satellite will use three fast, high-resolution spectrometers to measure the amount of CO₂ between the satellite and the ground. The satellite will travel at 7 kilometers per second, and will take snapshots of CO₂ levels three times a second, so each snapshot will cover a stretch about 2.5 kilometers long. The spectroscopy systems used on other satellites are much slower: their exposures take longer, so they sample a longer stretch–about 50 kilometers, Crisp says. The faster timing will allow the satellite to better see small gaps between clouds that would otherwise block the measurements. The satellite has a near-polar orbit, circling the earth in a north-south orientation. As the OCO orbits, the earth spins beneath it, so that on each pass, the satellite sees a different swath of earth.

“It’s going to be a vast increase in the amount of data that carbon-cycle scientists have had to work with,” says Britton Stephens, who studies the emissions and absorption cycles of CO₂ at the National Center for Atmospheric Science, in Boulder, CO. “It’s really going to revolutionize the way they do their work.”

Once scientists have analyzed the data, it could have direct policy implications, Stephens says. The results could suggest, for example, how much emphasis should be placed on giving countries incentives to leave forests intact. Or the results might suggest the need to act more quickly to reduce CO₂ emissions.

The satellite will also give researchers a better sense of carbon emissions from sources other than burning fossil fuels. While scientists have an accurate understanding of how much CO₂ is emitted by sources such as power plants and automobiles, they don’t know much about emissions from burning firewood or other biomass, or from clearing land for agriculture with “slash and burn” techniques. The satellite could therefore provide a clearer picture of how much a given country is emitting altogether, which could be important for enforcing emissions treaties. “Countries will claim reductions in their emissions in order to get credit for them. Who’s checking on those claims? How do you check them? The only way is to measure this greenhouse gas around the world. As far as society is concerned, that’s the greatest potential value of this satellite,” Prinn says.

But long-term monitoring will likely have to come from future projects, he says. The satellite is only designed to last two years, and it lacks the redundant systems needed to keep it going if some components fail, says Mike Miller, head of the earth-sciences satellite group at Orbital Sciences, based in Dulles, VA. Orbital Sciences is the company that built the satellite and will launch it. (NASA’s JPL developed the instruments onboard the satellite.)

However, if the OCO validates the spectroscopy approach to measuring CO₂ levels, similar satellites could follow the OCO’s lead. The recent economic stimulus bill could help too, since it includes nearly a billion dollars for NASA and the National Oceanic and Atmospheric Administration (NOAA).