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Researchers study the other greenhouse gas: water vapor

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

By Peter N. SpottsStaff writer of The Christian Science Monitor / January 29, 2009

The sky over Phoenix at dawn, seen from Camelback Mountain. Where did the moisture come from to make these?

Melanie Stetson Freeman/Staff/File

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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.

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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.

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