Researchers study the other greenhouse gas: water vapor

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.

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