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.