A tropical storm quickly takes shape over the Atlantic Ocean, a furiously whirling dervish with a skirt of thunderstorms. But just as quickly the storm is challenged by dozens of National Weather Service planes, which sally forth from East Coast airstrips like fighters on the tail of an enemy bomber. Attacking from above and below, the planes fire off a barrage of esoteric weapons that sap the strength of the raging winds in the developing eye wall.

Ammunition expended, the lead pilot flashes a thumbs-up, confident that once again she and her team of veteran storm chasers have prevented a hurricane from forming.

Could something like this really happen? Probably not. Such fanciful scenarios are period pieces. They belong to the 1950s and '60s, when scientists harbored an almost naive faith in the ability of modern technology to end droughts, banish hail and improve meteorological conditions in countless other ways. At one point, pioneering chemist Irving Langmuir suggested that it would prove easier to change the weather to our liking than to predict its duplicitous twists and turns. The great mathematician John von Neumann even calculated what mounting an effective weather-modification effort would cost the U.S.—about as much as building the railroads, he figured, and worth incalculably more.

At the start of the 21st century, alas, all that remains of these happy visions are a few scattered cloud-seeding programs, whose modest successes, while real, have proved less than earthshaking. In fact, yesterday's sunny hopes that we could somehow change the weather for the better have given way to the gloomy knowledge that we are only making things worse. It is now clear that what the world's cleverest scientists could not achieve by design, ordinary people are on the verge of accomplishing by accident. Human beings not only have the ability to alter weather patterns on local, regional and global scales, but they are already doing it—in ways that are potentially catastrophic.

Consider the billions of tons of carbon dioxide that are emitted every year in the course of our daily life. Driving a car, switching on a light, working in a factory, fertilizing a field all contribute to the atmosphere's growing burden of heat-trapping gases. Unless we start to control emissions of CO2 and similar compounds, global mean temperatures will probably rise somewhere between 2[degrees]F and 7[degrees]F by the end of the next century; even the low end of that spectrum could set the stage for a lot of meteorological mischief. Among other things, the higher the temperature, the more rapidly moisture can evaporate from the earth's surface and condense as rain droplets in clouds, substantially increasing the risk of both drought and torrential rain. There could also be a rise in the number of severe storms, such as the tornado-spawning monsters that hit Texas last week.

Human activity is modifying precipitation in other dramatic ways. Satellite images show that industrial aerosols—sulfuric acid and the like—emitted by steel mills, oil refineries and power plants are suppressing rainfall downwind of major industrial centers. In Australia, Canada and Turkey, according to one study, these pollution patterns perfectly coincide with corridors within which precipitation is virtually nil. Reason: the aerosols interfere with the mechanism by which the water vapor in clouds condenses and grows into raindrops big enough to reach the ground.

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