Sound a cautious climate alarm

WHEN the American Association for the Advancement of Science (AAAS) recently warned us to prepare for global warming, it added an important caveat. It urged climatic scientists not to overstate the threat. Would that those scientists always heeded this advice. In these days of sound bites and facile imagemaking, some of them seem prone to alarmist speculation. The press then tends to take such speculation out of context and highlight it.

During last summer's North American drought, a few experts suggested we were seeing the beginnings of man-made warming. The suggestion reverberated in Congress and through the news media. Yet there's no scientific basis for such a claim. ``We cannot tell whether or not the climate is changing by looking at a single year or even the sequence of a decade,'' the AAAS climate analysis team says.

More recent speculation suggests that a warmer Earth would have stronger storms - hurricanes with winds of 225 miles an hour or more. This stretches the credibility of the computer simulations on which it is based. The expert offering the projection may note its uncertainties. But it's the 225 m.p.h. winds that stand out in news reports and that people remember.

As the AAAS notes: ``The phrase `future shock' expresses the emotions of insecurity and confusion evoked by [climate] change. Uncertainty about what scientists agree upon ... adds to the confusion.'' So the association says: ``Those reporting about climate change bear a special responsibility for accuracy, conveying the real complexities and uncertainties, and not oversimplifying. Scientists must make extra effort to explain [these matters] in conservative and understandable terms.''

What scientists do agree on is that human activity is putting enough excess carbon dioxide and other heat-trapping gases into the air to raise the planet's average temperature by several degrees in another 50 years or so. They agree that this will likely change regional climates, intensifying drought in some areas, and raise sea level anywhere from a few centimeters to a meter or more.

They also agree they cannot foretell how fast such warming will develop or what its specific regional effects will be. And they agree that the knowledge needed to predict these effects will develop gradually over the next decade or two. Among other requirements, they need computers several times more powerful than are now available to simulate regional climatic changes in credible ways.

The AAAS is not suggesting we ignore the challenge of climate change until we understand it better. Its new study reflects general scientific opinion in recommending that, despite such uncertainty, we begin planning to cope with global warming's possible effects. We should be prepared for shifts in water supply and changes in agricultural conditions, especially in traditionally dry regions. And we should begin to curb the burning of fossil fuels, which produces the excess carbon dioxide.

But in urging such action now, some scientists overemphasize possible effects - such as monster hurricanes - when these are only speculations. Eventually, this kind of exaggeration, amplified by the press, will turn people off.

Climatic prophets should heed the AAAS's advice and be wise: ``Explain [your message] clearly in conservative and understandable terms.'' A Tuesday column