Researchers study the other greenhouse gas: water vapor

By tracking specific origins of moisture, scientists can better predict regional rain and snowfall.

Melanie Stetson Freeman/Staff/File
The sky over Phoenix at dawn, seen from Camelback Mountain. Where did the moisture come from to make these?

For years, “follow the water” has been a mantra for exploring one planet in our solar system, Mars. With a slight change, the phrase is also becoming a mantra for exploring Earth’s climate system: Follow the water vapor.

The details of how water behaves after it evaporates, the processes that parcels of moist air undergo as they travel across the planet, and the sources of moisture for several regions around the globe are poorly understood.

Yet that information is key to better forecasts of seasonal changes, such as monsoons, as well as to more reliable projections of global warming’s effects on regional rain and snowfall patterns, researchers say.

“If you look at model projections of rainfall in arid regions – the American Southwest, the Sahel [in Africa], India, China – for 2050 or 2100, half the models say one thing, half the models say another thing,” says Gavin Schmidt, a climate modeler at the National Aeronautics and Space Administration’s Goddard Institute for Space Studies in New York.

Hundreds of millions of people live in these regions, he continues, and they are deeply concerned about the future of their water supplies.

Now scientists are taking advantage of techniques that allow them to more easily read the story of water vapor’s travels and travails. The broad approach involves teasing out the relative abundance of heavier and lighter forms (isotopes) of oxygen and hydrogen atoms that water-vapor samples contains.

This real-world isotope information, which is incorporated into climate simulations, is expected to provide a valuable test for the models as researchers try to sort out which ones do the best job of approximating water vapor’s behavior outside the confines of a computer.

In particular, researchers will be looking to see how well a new generation of models reproduce past events, such as megadroughts that have hit the US Southwest, or the so-called “green Sahara” period some 6,000 years ago. These events are told in isotope records from the affected regions.

Indeed, improving models’ treatment of the hydrological cycle of our planet is one of the key goals set by the Inter­governmental Panel on Climate Change as it looks ahead to its next set of climate reports, currently set for release beginning in June 2013, Dr. Schmidt says.

While water vapor’s largely invisible hand is most obvious in the clouds and precipitation it forms, it’s also the most abundant greenhouse gas in the atmosphere, followed by carbon dioxide and trace amounts of other gases. As CO2 concentrations have risen and warmed the atmosphere, the warming has allowed the atmosphere to hold more water vapor, which in turn further warms the atmosphere.

This effect was most recently documented last October in the journal Geophysical Research Letters, when researchers at Texas A&M University in College Station published the results of a study of the link between global average temperatures and water vapor between 2003 and 2008.

During that period, surface temperatures fell by about 1 degree F., in large part because of a shift from El Niño to La Niña in 2007 and into 2008. (La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific. El Niño describes the condition when these temperatures are unusually warm.)

Using satellite measurements of water-vapor trends during the warmer and cooler portions of those years, researchers found a strong positive feedback from water vapor. It was similar in strength to what the feedback models estimate. If CO2 emissions continue to grow at a business-as-usual pace during the rest of this century, the positive feedback “is virtually guaranteed to produce warming of several degrees Celsius,” the researchers conclude.

It’s still hard to validate models regarding how this feedback plays out on century-long time scales, notes Andrew Dessler, an atmospheric scientist who led the team. To do so would require a century’s worth of data. Still, he adds, “the models seem to be getting the feedback in response to short-term fluctuations right. So it’s hard to believe they’re not getting the long-term feedback right.”

With or without an increase in water vapor, researchers are increasingly interested in where it comes from and where it goes. The tropical oceans, where the sun’s heat is strongest, is the most obvious source. But for regions interested in their water supplies, the devil is in the details.

At 18,000 feet in an ultralight aircraft
Which is why Mel Strong rousted himself up before dawn on cloudless days for six weeks in the spring of 2005. By sun-up, he was headed toward 18,000 feet in a cross between a propeller-driven go-cart and a parachute. His ultralight aircraft had a tiny weather station that gathered fresh information every 60 seconds. His payload consisted of 10 glass sampling bottles.

At 18,000 feet, he would kill the engine and glide back to earth, capturing air samples in the bottles every 1,000 feet. Back at the University of New Mexico, Albuquerque, where he was working on his PhD, another set-up was gathering roof-top air samples.

Mr. Strong analyzed oxygen- and hydrogen-isotope ratios to try to determine where the moisture in the samples came from. The ocean – even different parts of the ocean – has a distinct set of ratios. As water evaporates, the vapor tends to host more of the lighter isotopes than the heavier ones. When it rains, the heaviest isotopes tend to rain out first, leaving the remaining water vapor and any subsequent precipitation even poorer in heavy isotopes.

By looking at this depletion and working with models that backtrack moving parcels of air, his results point to springtime moisture coming into the state from the Gulf of California and the Gulf of Mexico. The results appeared in the Feb. 17, 2007, issue of Geophysical Research Letters.

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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