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.

Page: 1 | 2