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Weather's longterm probability

With extreme weather hitting many places, people may wonder what scientists really know about climate change. Climatologist Jerry Mahlman heads the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., that runs computer-based climate simulations for the National Oceanic and Atmospheric Administration. He laid out the following scheme during a NASA-sponsored climate change briefing in Washington last month.

First, there are "virtually certain facts" with odds of 99 to 1 of being right:

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*Humans are pumping out greenhouse gases that warm Earth and will affect climate for many centuries to come. Cooling due to other changes in atmospheric substances and to aerosol pollution partly offset this warming.

*Predictions that cooling in the lower stratosphere will accompany the surface warming apparently are already coming true. Meanwhile, Earth's surface has warmed about a degree Fahrenheit in this century.

*Natural climate variability makes it hard to pin down human impact. Also, it will take more than a decade to significantly clear up key scientific uncertainties such as the effects of changes in clouds, atmospheric water vapor, ice, and ocean currents. It will also take at least a decade to figure out what may happen to local weather.

*No matter what we do to reduce emissions, atmospheric carbon dioxide "will eventually exceed a doubling of pre-industrial levels." Therefore, "significant climate change is almost guaranteed to occur."

Second, "very probable projections" with odds of 9 to 1:

*Computer simulations now reproduce the past century's global warming fairly well.

*Doubling atmospheric carbon dioxide should boost Earth's average annual temperature by about 2.5 to 8 degrees F. Sea level should rise by roughly 3 to 10 feet due to ocean warming alone. Global average precipitation should rise about 2 percent for every degree or so of warming. Temperature and precipitation changes will be greater in higher northern latitudes.

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Third, "probable projections" with odds of 2 to 1:

*Drought will increase over northern mid-latitudes.

*Tropical storms, once formed, may be more fierce.

*Regional effects will amplify summertime warming in humid subtropical climates.

Finally, "uncertain projections":

*The number of tropical storms will increase.

*Specific forecasts of El Nio changes.

*Specific forecasts of regional or local climate changes. While forecast changes may occur, "our ability to be very specific remains low." The bottom line, Mr. Mahlman said, is that humans will be hampered for some time by scientists' inability to say exactly what that will mean on the ground.

(c) Copyright 1999. The Christian Science Publishing Society