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A moisture problem muddles climate work.

The world's 19 best computer climate models differ substantially in the way they stimulate "moist" processes in the atmosphere--a problem scientists must solve to improve global warming predictions, a new study warns.

"This study and others suggest to me that existing models are capable of giving qualitative evaluations but are not capable of making quantitative predictions," says study leader David A. Randall of Colorado State University in Fort Collins.

The study by 31 researchers in eight countries expands on a previous comparison which showed that models disagree in their assessment of how clouds affect radiation leaving or entering the top of Earth's atmosphere. The new study looked at another facet of the climate: energy absorbed and released from Earth's surface.

The scientists compared the models by running identical simulations of simple climate changes. At first, they lowered sea-surface temperatures by 2[degrees]C below present conditions and then let the atmosphere react. Later, they raised sea temperatures by 2[degrees]C above present conditions.

The comparison revealed that models differed greatly in their portrayal of energy entering and leaving Earth's surface, the researchers report in the March 20 JOURNAL OF GEOPHYSICAL RESEARCH. But unlike the previous disagreement, this inconsistency does not trace directly back to the effect of clouds on radiation. Instead, it results from the way each model treats processes involving moisture. Most important among these are surface evaporation, the development of cumulus clouds and the absorption and emission of radiation by water vapor.

The results have important implications for those trying to improve predictions of climate change. Some scientists and administrators have emphasized the need to boost computer power, thereby permitting the use of models with greater spatial resolution. But Randall and his colleagues say this will not solve existing problems. From their study and others, they conclude that "dramatically increased computer power would not, by itself, be sufficient to greatly improve either our ability to simulate the present climate or our confidence in climate-change simulations produced by existing models."

Instead, Randall emphasizes the need for meteorological observations and theoretical investigations aimed at improving scientific understanding of how the climate works.
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Author:Monastersky, Richard
Publication:Science News
Date:Apr 4, 1992
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