Researchers study the other greenhouse gas: water vapor

On average, half a year’s worth of precipitation in New Mexico comes during the summer monsoons, Mr. Strong says, with three possible sources for moisture: the Gulf of Mexico, the Gulf of California, and the Pacific Ocean. But the monsoons vary in strength each year. They sometimes display false starts. And the region also has undergone decade-scale droughts.

It’s hard to figure out what’s going on “if we don’t know the relative contributions from each” potential moisture source, Strong says.

In the tropics, researchers have been asking a similar “Where does moisture come from?” question, but with a twist. In New Mexico, the contribution of moisture from plants is relatively small. In the humid, fecund tropics, it’s much larger.

John Worden, a researcher at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues have looked at isotope data from the Aura satellite, a sister orbiter to the Aqua satellite. He found that water vapor over the Amazon and across tropical Africa has a heavier isotopic content than vapor over the ocean. Some of it is even heavier than ocean water itself.

Rain that evaporates before it lands
Essentially, much of the atmospheric moisture over these continental areas appears to come from rain that evaporates before it reaches the ground (it’s called “virga”), as well as moisture given off by the lush tropical plant life through evapotranspiration.

One approach to getting a better handle on these and other atmospheric moisture issues involves a concerted water-vapor monitoring program, says David Noone, a water-cycle specialist at the University of Colorado, Boulder, and a member of Worden’s team.

Last fall, Dr. Noone and colleagues took off-the-shelf laser-based sensors for an isotope-measuring test drive on the slopes of Mauna Loa, on the big island of Hawaii. The low-budget project aims to see if the sensors could help provide a reality check on satellite measurements as well as act as easy-to-use tools for longer-term monitoring from a range of sites around the world – particularly in the subtropics, which tend to be drier.

“There’s a debate about what really controls the dry regions,” Noone says. One camp holds that cloud processes dominate; the other holds that the drier climate is the result of air masses that mix and, in effect, dehydrate the air in the region.

This may sound like debating how many angels can dance on the head of a pin.

But the outcome can be significant, Noone says. “One of the things that’s evident from the climate models is that you can get the right answer for a variety of wrong reasons,” he explains.

By using isotope measurements to get “the right water vapor for the right reasons, we can improve the way the models are representing the water cycle on the climate system.”

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