"Modeling the atmosphere is very difficult because there are so many different interactions that have to be considered. It's like trying to put together a gigantic jigsaw puzzle. The problem modelers face is, up to one-third of the pieces of their puzzle are not defined very well. My group tries to work on individual pieces of the puzzle to help make the modelers' job a little easier."

Climate change is one of the most pressing issues of our time. For Associate Professor of Chemistry Karen Castle, understanding this complex topic is looking up. Way up.

Castle studies the physical and chemical processes in Earth's upper atmosphere, roughly 60 to 120 kilometers above Earth. As in the lower atmosphere, levels of greenhouse gases such as carbon dioxide have increased in the upper atmosphere. The effect, however, is cooler temperatures.

"In the last 30 years we've seen about a 10 Kelvin decrease in temperature at about 300 kilometers altitude," Castle says. Ten Kelvin (which is equivalent to a 10 degree Celsius change) is a significant change in a region that is normally very stable. This rapid response to changes in gas concentrations means that monitoring conditions in the upper atmosphere could provide an early indicator of climate-changing processes taking place in the lower atmosphere.

The mechanism behind upper atmosphere cooling is essentially the same as lower atmosphere warming - both are a matter of following the energy as it moves through a game of molecular pinball. The upper atmosphere is rich with fast-moving oxygen atoms. As small molecules such as carbon dioxide collide with the oxygen atoms, they pick up some energy, which excites them. Eventually the excited molecule relaxes by releasing an infrared photon, which is essentially a packet of energy. Because the gas density in the upper atmosphere is so low, chances are good that photons will zoom straight out into space, creating a net loss of energy at a given altitude, which results in cooler temperatures.

In the lower atmosphere, carbon dioxide and other small molecules pick up energy from the infrared radiation that is radiated from Earth's surface. As the molecules relax and release that energy, much of it comes back down toward Earth. The energy remains essentially trapped under a blanket of greenhouse gases, resulting in warmer temperatures.

Castle and her students use a technique called laser spectroscopy to measure the energy transfer in collisions between gas molecules in the laboratory. The information will inform models of climate change. "Modeling the atmosphere is very difficult because there are so many different interactions that have to be considered," Castle says. "It's like trying to put together a gigantic jigsaw puzzle. The problem modelers face is, up to one-third of the pieces of their puzzle are not defined very well. My group tries to work on individual pieces of the puzzle to help make the modelers' job a little easier."

Castle's work has implications not only for climate change, but also for the design of satellites, which travel through the upper atmosphere, and for understanding the atmospheres of Mars and Venus.

She may spend a lot of time worrying about the upper atmosphere, but Castle also knows how to bring herself back down to Earth. She started college as a music major, and still enjoys singing and playing piano. "It gives you a little escape," she says. "I think it does make you a better scientist if you can step away sometimes and do something totally different, not think about it for a while, and then come back to your problem and tackle it again."

Posted Sept. 8, 2009

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