Will We Control The Weather?

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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.