A new carbon detective to study how half of gas disappears

By Kenneth Chang
Published: February 22, 2009

Thirty billion tons of carbon dioxide go into the air from the burning of fossil fuels each year. About half of it stays in the air. The other half disappears. Where it goes, nobody quite knows.
With the Orbiting Carbon Observatory, a NASA satellite scheduled to be launched Tuesday morning from Vandenberg Air Force Base in California, scientists hope to better understand the comings and goings of carbon dioxide, the main greenhouse gas driving the warming of the planet.
The new data could help improve climate models and improve the understanding of the "carbon sinks" like oceans and forests that absorb much of the carbon dioxide.
In some years, all of the excess carbon dioxide disappears. In other years, all of it stays in the air. The variations indicate that some of the sinks might fill up and spill carbon dioxide back into the air.
"Something out there is changing dramatically," said David Crisp, a scientist at the NASA Jet Propulsion Laboratory in Pasadena, California, and the principal investigator of the mission.

Natural sources like the decay of dead plants account for 98 percent of the carbon dioxide emissions. Humans account for only 2 percent, but that is enough to tip the balance. Before the beginning of the Industrial Revolution two centuries ago, carbon dioxide levels were at about 280 parts per million. Today, the level is 387 parts per million and projected to rise sharply in the coming decades.
Scientists have good estimates of how much carbon dioxide is released by the burning of fossil fuels, but other human influences like the clearing of forests and the harvesting of crops affect carbon dioxide "in ways we don't understand," Crisp said.
The Orbiting Carbon Observatory will measure carbon dioxide by using an instrument with three spectrometers to analyze light reflected off Earth. Carbon dioxide absorbs certain wavelengths of light, particularly in the near infrared, and by measuring how dim those parts of the spectrum are, the observatory can determine how many carbon dioxide molecules the light has passed through.
At the same time, the instrument will make a similar measurement for oxygen. Combining the two measurements gives the concentration of carbon dioxide in the air. Because carbon dioxide mixes quickly with the other gases in air, the measurements will have to pick out small variations, expected to be less than 5 percent.
Crisp said the spacecraft would be able to pick out emissions from a power plant or from along highways. More difficult will be picking out the carbon sinks, which tend to be spread out over large areas. Scientists know that the oceans are by far the largest sinks, but the absorbing power of forests, for example, is still uncertain.