Scientists tracking smoke from agricultural burning warn that it's more than an air-pollution nuisance. By changing the way water condenses from the atmosphere, it can affect Earth's weather machine in what Meinrat Andreae calls "strange and unpredictable ways."
This adds a new piece to the puzzle of climate change one that Dr. Andreae, who heads up biogeochemistry for Germany's Max Planck Institute for Chemistry in Mainz, says may fit best in the tropics. That's where changes in rainmaking can directly affect what he calls "the steam engine" that powers global weather.
Solar heat warms the tropics more than any other zone on the planet. That heat evaporates vast amounts of water. The water vapor rises into the air, and some of it condenses into droplets. Some also moves north and south with the winds to condense at higher latitudes. Whenever the vapor turns back into water, it releases the heat that turned it into vapor in the first place. That heat energy then is available to power weather systems.
Earth's weather machine has run that way for millions of years. But over the past century, large-scale smoke pollution has increased in intensity and frequency. This may have begun to gum up the weather machine.
Smoke pollution can change the way the sun heats large areas of the tropical boiler. It floods the air with so many particles around which water vapor can form droplets that it changes the way rainfall works. This is especially important over major tropical regions such as the Amazon rain forest.
During a recent meeting of the American Geophysical Union in Washington, Zhanqing Li from the University of Maryland at College Park joined Andreae and several other scientists to emphasize this new environmental concern. He explained that, in clear air, rain can fall from almost any cloud. But when air is heavily polluted with small particles, it takes clouds with strong vertical development to make rain.
This is a significant, and probably widespread, change in tropical rainmaking. Burning to create new farms or to prepare existing farmland for new crops has increased as populations have grown. Andreae says that the smoke now can impact rainfall on a regional, even global, scale.
International wildfire- research that include satellite monitoring offers valuable data. Weather satellites and environmental monitors, such as NASA's Terra, constantly survey the global fire scene. Voram Kaufman with NASA's Goddard Space Flight Center in Greenbelt, Md., says satellite images show smoke that travels large distances to invade relatively unpolluted areas.
Andreae speculates that smoke pollution may help explain a 20th-century rainfall puzzle. While rainfall increased by 0.5 percent a decade in mid-latitudes, it declined by 0.35 percent a decade in the tropics.
The buildup of carbon dioxide and other heat-trapping gases isn't the only human influence on climate. Even if we do halt the greenhouse gas buildup, we will still have humanity's smoke pots to worry about.