With One Space Observatory Down, NASA Uses Another to Map CO2

Newly released data show carbon dioxide “more lumpy” than expected

By Davide Castelvecchi. From ScientificAmerican.com, December 17, 2009.

SAN FRANCISCO—The Orbiting Carbon Observatory, or OCO, was designed to monitor the movement of carbon dioxide in Earth’s atmosphere starting this year, but instead it plunged into the ocean in February due to a launch malfunction. Now an instrument on another NASA probe is enabling researchers to map atmospheric CO2, and revealing that its global distribution is surprisingly uneven, with regional variations of up to 5 percent.

A team of scientists presented the new CO2 data—courtesy of the Atmospheric Infrared Sounder (AIRS), an instrument on board NASA’s Aqua probe—Tuesday at a meeting here of the American Geophysical Union. The team’s map shows the day-by-day evolution of global CO2 concentrations in the mid-troposphere, or between five and 12 kilometers above Earth’s surface. The data cover the seven years from 2002—when Aqua was launched—to the present.

“Contrary to prevailing wisdom, CO2 is not well mixed in the mid-troposphere,” said Moustafa Chahine of the NASA Jet Propulsion Laboratory (JPL), AIRS’s principal scientist, during a press conference. While showing animations of CO2 fluctuations over time, Chahine also pointed out that global concentrations are higher in the Northern Hemisphere, where much of the gas is produced. “The Southern Hemisphere is a net sink—some people call it the ‘garbage dump’ of the Northern Hemisphere’s CO2,” Chahine said.

The data are especially interesting because the mid-troposphere is precisely the layer where CO2 blocks Earth’s thermal radiation from escaping into space (the planet’s main self-cooling mechanism), says JPL’s David Crisp, who was OCO’s lead scientist. “In the spectrum AIRS measures, you see a direct measurement of the greenhouse effect,” he adds.

On average, the global atmospheric CO2 concentration today is about 380 parts per million (ppm), but AIRS showed that it can vary from one region to another by as much as 20 ppm. For comparison, the average CO2 concentration in Earth’s atmosphere is rising by about two ppm per year. The new data will help researchers understand and predict regional warming’s impact on the overall greenhouse effect, although Crisp points out that clouds and water vapor vary even more markedly than CO2 concentrations and thus can have even stronger local effects on climate.

Chahine said that one of the most important problems for climate researchers is understanding CO2 sinks. “We put out eight gigatons of CO2 per year,” Chahine said. “Half stays in the atmosphere. Where does the other half go?”

Crisp agrees: “If there are natural processes that are absorbing CO2,” he says, “it would be a good idea to find out and not to destroy those processes.” Tracking carbon sources and sinks will also be crucial to any future global cap-and-trade scheme, he points out.

It was only two years ago that the team became confident that they could process their data and tease enough spectroscopic information from it to yield the CO2 maps, according to AIRS team member Joel Susskind of the NASA Goddard Space Flight Center in Greenbelt, Md. AIRS was mainly intended to study weather by mapping moisture and temperature on a global scale and to predict extreme events such as hurricanes: JPL hydrologist Eric Fetzer pointed out at the press conference that the AIRS data on water vapor and temperature could now be used to anticipate where tropical cyclones will make landfalls.

In addition to its effects on weather, the atmospheric water vapor load is also thought to magnifiy CO2’s global warming effect, in part because warming causes more evaporation and water vapor is a strong contributor to the greenhouse effect.

Andrew Dessler of Texas A&M University’s Department of Atmospheric Sciences said at the press conference that thanks in part to AIRS, water vapor’s warming feedback “is not much of an uncertainty anymore.” Dessler and his collaborators put theoretical models of this feedback to the test by looking at the temperature changes between the 2007 El Niño event and the 2008 La Niña—when global average temperature changed by nearly as much as global warming’s total temperature change over the 20th century. Thus, they used El Niño as a “proxy for climate change,” he said. “The models are doing an outstanding job at predicting how water vapor reacts to warming.”

In addition to water vapor and CO2, AIRS also maps other greenhouse gases, such as methane, ozone and carbon monoxide. For example, the instrument detected a plume of carbon monoxide emerging from southern California during this summer’s Station Fire. The fire burned thousands of acres of land and threatened JPL headquarters in Pasadena.

OCO was designed to measure CO2 concentrations based on the amount of solar radiation the gas reflects back into space, whereas AIRS detects CO2 via the gas’s emission of particular wavelengths of infrared radiation. Because of their technical differences, OCO would have been able to map CO2 at lower altitudes, and thus more precisely pinpoint the location of sources and sinks. Crisp says he and his team are working feverishly to get an OCO replacement probe into orbit within three years. Last week, Congress set aside $50 million in NASA’s 2010 budget to restart the mission.

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