Sunlight Sponge? Energy trackers gauge water vapor’s wild dance

Despite decades of study, climate-change researchers still can’t tell what in Earth’s atmosphere is responsible for up to 30 percent of the solar radiation soaked up there. Some scientists argue that water vapor–the atmosphere’s major sunlight absorber–takes in much more solar radiation than has been indicated by measurements and models.

Vapor doesn’t absorb enough radiation to explain the discrepancy fully, suggests a newly reported experiment from the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland.

Andrea Callegari and his colleagues made unprecedented measurements of a property of vigorously vibrating water molecules. That property, the extent of the molecules’ charge separation, is closely related to the amount of light energy those agitated molecules can absorb. Charge becomes separated in water molecules because oxygen and hydrogen atoms don’t share electrons equally.

The team’s findings indicate that the actual values of water vapor’s capacity to absorb sunlight should be within about 10 percent of the theoretical calculations to date. That’s pretty close, says Callegari, so scientists should also look to radiation absorbers other than water vapor to explain the fate of most of the missing solar energy.

He and his coworkers at EPFL and U.S., English, and Russian institutions describe their experiment in the Aug. 9 Science. Since the experiment, Callegari has moved to the University of Lausanne.

Investigators of atmospheric absorption have measured thousands of solar wavelengths at which water vapor strongly sops up the incoming radiation. However, at millions more wavelengths, water vapor absorbs light too weakly to be gauged. To determine the energy going into those weak absorptions–which is sizable because of the huge number of wavelengths–scientists use calculations based on theory: the laws of physics and what’s known about basic properties of atoms.

“This theory needs to be validated. That’s what we are providing,” Callegari says.

The tough-to-detect absorptions tend to make water molecules vibrate with high energy. To make an artificial sample of such excited molecules, Callegari and his colleagues put a low concentration of water vapor–about a thousandth of what’s normally in air–into a half-meter-long metal chamber in which they generated an electric field. By firing a laser pulse into the tube, the scientists excited some vapor molecules to a selected vibrational state. Then, by monitoring the molecules’ interactions with the electric field and laser pulses, the team determined, within about a half-percent accuracy, how those molecules’ charge separations changed with excitation.

That experiment is “a tour de force in modern chemical physics,” says Peter F. Bernath of the University of Waterloo in Ontario, in the same issue of Science.

Although the new work shows that theoretical and experimental results are close, they’re not close enough, Bernath adds.

Theoretician David W. Schwenke of NASA’s Ames Research Center in Mountain View, Calif., agrees. When he heard about the new findings, he says, “I started new calculations to try to understand what went wrong with my previous results.”

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