Still, the stratosphere is even drier than experts suspect it should be. At the same time, scientists have measured a 50 percent rise in water vapor in the stratosphere over the last 50 years. To explain the trend, some researchers have speculated that powerful tropical cyclones might be increasingly punching through the tropopause and spewing water vapor high into the atmosphere. The stratosphere is warm enough to trap the moisture, according to some theories. Circulation patterns would then carry the vapor from the tropics to the poles. "There's all this evidence that a changing climate could change the intensity or frequency of tropical cyclones," Romps said. "We were thinking about what impact tropical cyclones could have on climate." To test the theory, Romps and colleague Zhiming Kuang looked at 23 years worth of satellite data to measure cloud temperature in cyclones and other storms. Temperature readings told them how high the clouds were. The maps they produced, published in Geophysical Research Levels, showed that tropical cyclones account for 15 percent of the water vapor that shoots into the stratosphere and 30 percent of water vapor that shoots the highest -- about 1.5 kilometers (nearly a mile) above the troposphere. "This is not just academic," Hall said. "Water vapor is a greenhouse gas. So, changing water vapor does have global warming potential, and it is significant." Still, Hall said, increases in stratospheric water vapor make a much smaller contribution to warming than do carbon dioxide emissions. In that sense, the new work adds a small piece of the puzzle to our understanding of the forces behind climate change. Related Links: |
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