Published: 
Jul 13, 1997

How Dry is the Tropical Free Troposphere?

Implications for Global Warming Theory

July 14, 1997

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Over the past century, global measurements of the temperature at the Earth's surface have indicated a warming trend of between 0.3 and 0.6 degrees C. But many - especially the early - computer-based global climate models (GCM's) predict that the rate should be even higher if it is due to the man-made "Greenhouse Effect". Furthermore, these computer models also predict that the Earth's lower atmosphere should behave in lock-step with the surface, but with temperature increases that are even more pronounced.

However global temperature measurements obtained from satellites of the Earth's lower atmosphere reveal no definitive warming trend over the past two decades. The slight trend that is in the data actually appears to be downward. These satellite data are verified by in-situ measurements of the lower atmosphere made by balloon-borne observations around the world.

Some scientists now believe that this apparent "disagreement" between the predictions by computer models and the measurements may be due to a less-than-accurate modeling of the role of water-vapor in the atmosphere of the GCM's.

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Click here for the most recent data on the globally measured average temperature trends in the troposphere and stratosphere as measured from space.

In the June 1997 edition of the Bulletin of the American Meteorological Society, Dr. Roy Spencer of NASA/Marshall Space Flight Center and the Global Hydrology and Climate Center, and Dr. William Braswell of Nichols Research Corporation. have shown that the low humidity of the tropical free troposphere is playing a previously-overlooked central role in the dynamics of the atmosphere. The very low humidity in this region allows a larger portion of the infrared radiation to escape from the Earth, thereby cooling the atmosphere. Current computer models do not accurately handle the processes which control the humidity in this region, which are related to how much cloud material falls out of rainfall systems.

Dr. Spencer, NASA's Senior Scientist for Climate Change, will be discussing these latest published results, as well as the state of the global warming debate, during the July 15, 1997 forum on The Implications of Climate Change Policy at the National Press Club in Washington, D. C.


Climate Information Resource Center

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Authors: John Horack, Roy Spencer
Curator: Bryan Walls
NASA Official: John M. Horack