Will We Control The Weather?

By J. MADELEINE NASH

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.

This creates an additional conundrum. Because a polluted cloud does not rain itself out, notes University of Colorado atmospheric scientist Brian Toon, it tends to grow larger and last longer, providing a shiny white surface that bounces sunlight out to space. Indeed, one reason the earth has not yet warmed up as much as many anticipated may be due to the tug-of-war between industrial aerosols like sulfuric acid (which reflect heat) and greenhouse gases like carbon dioxide (which trap it). Ironically, then, the cost of reducing one kind of pollution may come at the price of intensifying the effects of the other.

Deforestation has a similarly broad range of impact. One thing trees do is lock up a lot of carbon in their woody tissues, thereby preventing it from escaping into the atmosphere. Trees are also important recyclers of moisture to the atmosphere. In some parts of the Amazon basin, deforestation has reached the point where it is altering precipitation patterns. This is because so much of the moisture entrained by clouds comes from the canopy of the forest below; as large tracts of trees disappear, so do portions of the aqueous reservoir that feeds the local rainmaking machine.

Shrubs, grasses and other vegetative covers act in much the same way, trapping water, feeding moisture into the atmosphere and providing shade that shields the surface of the land from the drying rays of the sun. Large-scale land-clearing efforts under way around the world wipe all that out. The ongoing development of South Florida, for instance, has filled in and paved over much of the Everglades wetlands, which have long served as an important source of atmospheric moisture. As a consequence, says Colorado State University atmospheric scientist Roger Pielke Sr., South Florida in July and August has become significantly dryer and hotter than it would have been a century ago under the same set of climatic conditions.

To complicate matters further, we are changing the landscape in ways that increase our exposure to meteorological extremes, so that even if weather patterns in coming decades were to turn out to be identical to those of the past century, the damage inflicted would be far worse. To appreciate what happens when vegetative cover is removed, one need look no further than the 1930s Dust Bowl in the U.S. and the 1970s famine in Africa's Sahel. In both cases, a meteorological drought was exacerbated by agricultural and pastoral practices that stripped land bare, exposing it to the not so tender mercies of sun and wind.

Removal of vegetative cover also worsens the flooding that occurs during periods of torrential rain. Riverine forests serve as sponges that soak up excess water, preventing it from rushing all at once into rivers and tributaries. In similar fashion, estuarine wetlands and mangrove forests help shield human settlements from the storm surges that accompany tropical cyclones and hurricanes. Biologists estimate that 50% of the world's mangrove forests have already been replaced by everything from shantytowns to cement plants and shrimp farms. Stir in the expectation that rising temperatures will trigger a rise in sea level, and you have a recipe for unprecedented disaster.

Scientists are just beginning to disentangle the myriad levels on which human beings and the natural climate system interact, which only increases the potential for surprise. For example, we now realize that not all the aerosols we are pumping into the atmosphere exert a cooling effect. A notable exception is soot, which is produced by wood fires and incomplete industrial combustion. Because of its dark color, soot absorbs solar energy rather than reflecting it. So when a recent scientific excursion to the Indian Ocean established that big soot clouds were circulating through the atmosphere, a number of scientists speculated that their presence might be raising sea-surface temperatures, potentially affecting the strength of the monsoon.

The monsoon is not the only climate cycle that human activity could alter. Atmospheric scientist John M. Wallace of the University of Washington believes that rising concentrations of greenhouse gases are already beginning to have an impact on another important cycle, known as the North Atlantic or Arctic Oscillation. In this case it's not the warming these gases create in the lower atmosphere that is key, but the cooling they cause in the stratosphere, where molecules of carbon dioxide and the like emit heat to space rather than trapping it in the upper atmosphere. This stratospheric cooling, Wallace and others speculate, may have biased prevailing wind patterns in ways that favor a wintertime influx of mild marine air into Northern--as opposed to Southern--Europe.

Is Wallace right about this? No one yet knows. We are tampering with systems that are so complex that scientists are struggling to understand them. Climatologist Tom Wigley of the National Center for Atmospheric Research, for one, fervently believes the answer to our problems lies not just in improved knowledge of the climate system but in technological advances that could counter--and perhaps reverse--present trends. In other words, the farfetched dreams that prominent scientists like Von Neumann once harbored have not died. Rather they have been transformed and, in the process, become more urgent.