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ROBERT SIEGEL, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

MELISSA BLOCK, host:

I'm Melissa Block. And it's time now for Climate Connections - our yearlong series with National Geographic about the Earth's changing climate. And we have two stories today.

In a few moments, we'll talk with former speaker of the house Newt Gingrich about what he's calling his "Contract With the Earth."

First, we'll examine how water warms our planet. We all know greenhouse gases trap heat from the sun. But the most important greenhouse gas isn't carbon dioxide or methane. It's water vapor. This summer, NASA sent a team to San Jose, Costa Rica to learn more about water's role in global warming.

NPR's Jon Hamilton tagged along.

JON HAMILTON: The planes at Juan Santamaria Airport usually carry business travelers or tourists heading for the beach. So you'll notice when something that looks like U-2 spy plane touches down and taxis toward a temporary hangar shaped like a clam shell.

Dr. PAUL NEWMAN (Atmospheric Scientist, NASA Goddard Space Flight Center): Yeah, here comes our plane after about a five-hour flight here, wheels down at, I guess, 12:57.

HAMILTON: Paul Newman is from NASA Goddard Space Flight Center. He says the sleek white jet is not a spy plane. It's a civilian version of the U-2 called the ER-2. And this one set up for science. The plane spent the morning gathering information about the atmosphere more than 10 miles above us. It wasn't a perfect flight. Newman just found out that one piece of equipment didn't work.

Dr. NEWMAN: And it was not long after takeoff that it failed. So…

HAMILTON: It's a minor setback. A day earlier, lightning punched a small hole in another NASA plane. It's all part of working in this area of the tropics, which may be the best place on Earth to learn more about the role of water in global warming. And Newman says scientists need to learn more.

Dr. NEWMAN: Water is a tremendous greenhouse gas. It can condense and form all these clouds. Clouds can reflect radiation. Clouds can actually insulate the surface. If you got a cloud layer overhead, the surface stays a little bit warmer. So water is really crucial to the whole process.

HAMILTON: To appreciate the importance of water in the atmosphere, you have to imagine what the Earth would be like without it.

Brian Toon has come to Costa Rica from the University of Colorado.

Dr. BRIAN TOON (Atmospheric and Oceanic Sciences, University of Colorado; Mission Scientist, TC4 Mission): If we didn't have an atmosphere with water in it, the Earth's average temperature would be about 30 degrees Fahrenheit below freezing.

HAMILTON: So we need water in the atmosphere to survive. Unfortunately, Toon says, water amplifies the effects of other greenhouse gases.

Dr. TOON: You add carbon dioxide to the air, burn a fire, you ride a car, something like that, and that carbon dioxide floats up to the upper atmosphere and blocks some of the light from escaping to space, so the Earth warms up a little bit.

HAMILTON: That causes water in the atmosphere to radiate more energy. So the oceans get warmer.

Mr. TOON: And when you make water warmer, as everybody knows in their kitchen, more water evaporates and so the atmosphere gets more water in it. And that warms things up even more. So the net effect of all this is is that if you just put in the carbon dioxide, you get a temperature change, but all that water gives you three times more temperature change than you get with just the carbon dioxide alone.

HAMILTON: Most water doesn't linger in the atmosphere. It usually falls back to Earth in a week or so.

But in the past couple of decades, it's become clear that if water reaches the stratosphere, it can stay there for years, just like other greenhouse gases. It's also clear that the amount of water in the stratosphere is increasing.

What's not clear is whether that extra water is the result of human activities and whether it's accelerating global warming.

(Soundbite of airplane flying overhead)

HAMILTON: NASA picked Costa Rica to study these questions because it's one of the places where large amounts of water can actually reach the stratosphere. The reason is massive thunderheads during the rainy season. Newman says these storms create powerful updrafts that can carry water miles into the air.

Dr. NEWMAN: In fact, some of these clouds do poke up to very high altitudes, 60,000 feet. And you get cirrus particles, small ice particles, you know, the very feathery kind of clouds you see, they will come off of these huge thunderstorms.

HAMILTON: Transporting water into a part of the atmosphere that is normally very dry.

Today's flight took the ER-2 into high-altitude clouds that were only discovered a few years ago. They're cirrus clouds, so thin, they're invisible from the ground.

Dr. NEWMAN: We actually flew along with the airplane, and we had a laser that shoots out at the bottom of the plane. And you could tell that this sub-visible cirrus was there, so we actually measure it with the laser. And then, we flew both just right above this cirrus and right below it. And you could look at the infrared properties of the cloud.

HAMILTON: Scientists hope the infrared measurements will show whether these clouds are contributing to the greenhouse effect by reflecting heat back to Earth. That won't make climate predictions perfect. But it will make them better.

Jon Hamilton, NPR News.

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