Edition: U.S. / Global

When Scientific Predictions Are So Good They're Bad

Published: September 29, 1998
(Page 2 of 2)

This ''uncharacteristically bad'' prediction, said Dr. Chapman, would not have been issued had it been subjected to normal review by the forecaster's scientific peers. But, he said, there was no peer-review apparatus set up to make sure that ''off-the-wall predictions don't get out.'' (Such a committee has since been established by NASA.)

Most sins committed in the name of prediction, however, appear to stem from the uncertainty inherent in almost all forecasts. ''People don't understand error bars,'' said one scientist, referring to margins of error. Global climate change and the Red River flood offer two cases in point.

Computer models of the climate system are the major instruments used by scientists to project changes in climate that might result from increasing atmospheric concentrations of heat-trapping gases, like carbon dioxide, emitted by the burning of fossil fuels.

Basing its forecast on the models, a panel of scientists set up by the United Nations has projected that the average surface temperature of the globe will rise by 2 to 6 degrees Fahrenheit, with a best estimate of 3.5 degrees, in the next century, and more after that. This compares with a rise of 5 to 9 degrees since the depths of the last ice age. The temperature has increased by about 1 degree over the last century.

But the magnitude and nature of any climate changes produced by any given amount of carbon dioxide are uncertain. Moreover, it is unclear how much of the gas will be emitted over the next few years, said Dr. Jerry D. Mahlman, a workshop participant who directs the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory at Princeton, N.J. The laboratory is one of the world's major climate modeling centers, and the oldest.

This uncertainty opens the way for two equal and opposite sins of misinterpretation. ''The uncertainty is used as a reason for governments not to act,'' in the words of Dr. Ronald D. Brunner, a political scientist at the University of Colorado at Boulder. On the other hand, people often put too much reliance on the precise numbers.

In the debate over climate change, the tendency is to state all the uncertainties and caveats associated with the climate model projections -- and then forget about them, said Dr. Steve Rayner, a specialist in global climate change in the District of Columbia office of the Pacific Northwest National Laboratory. This creates a ''fallacy of misplaced confidence,'' he said, explaining that the specific numbers in the model forecasts ''take on a validity not allowed by the caveats.'' This tendency to focus unwisely on specific numbers was termed ''fallacious quantification'' by Dr. Naomi Oreskes, a historian at the University of California at San Diego.

Where uncertainty rules, many at the workshop said, it might be better to stay away from specific numbers altogether and issue a more generalized forecast. In climate change, this might mean using the models as a general indication of the direction in which the climate is going (whether it is warming, for instance) and of the approximate magnitude of the change, while taking the numbers with a grain of salt.

None of which means that the models are not a helpful guide to public policy, said Dr. Mahlman and other experts. For example, the models say that a warming atmosphere, like today's, will produce heavier rains and snows, and some evidence suggests that this is already happening in the United States, possibly contributing to damaging floods. Local planners might be well advised to consider this, Dr. Mahlman said.

One problem in Grand Forks was that lack of experience with such a damaging flood aggravated the uncertainty of the flood forecast. Because the river had never before been observed at the 54-foot level, the models on which the prediction was based were ''flying blind,'' said Dr. Pielke; there was no historical basis on which to produce a reliable forecast.

But this was apparently lost on local officials and the public, who focused on the specific forecast of a 49-foot crest. This number was repeated so often, according to the report of an inquiry by the National Weather Service, that it ''contributed to an impression of certainty.'' Actually, the report said, the 49-foot figure ''created a sense of complacency,'' because it was only a fraction of a foot higher than the record flood of 1979, which the city had survived.

''They came down with this number and people fixated on it,'' Tom Mulhern, the Grand Forks communications officer, said in an interview. The dikes protecting the city had been built up with sandbags to contain a 52-foot crest, and everyone figured the town was safe, he said.

It is difficult to know what might have happened had the uncertainty of the forecast been better communicated. But it is possible, said Mr. Mulhern, that the dikes might have been sufficiently enlarged and people might have taken more steps to preserve their possessions. As it was, he said, ''some people didn't leave till the water was coming down the street.''

Photo: Petty Officer Tim Harris patroled an area of Grand Forks, N.D., in April 1997, where the Red River flooded the houses up to the second story. Residents, relying on the precision of forecasts, were forced to flee quickly. (Reuters)(pg. F6)