Neither rain, nor sleet, nor snow falls heavily in extremely polluted areas, suggests a new study. Scientists modeling cloud systems have debated whether air pollution tends to increase or decrease precipitation. New evidence from satellites now indicates that aerosols—pollution made up of fine, airborne particles—have a dampening effect on rainfall.
Previous satellite pictures showed that the particle-laden exhaust that oceangoing ships pump out makes visible trails in low clouds. The particles change the structure of clouds by breaking large water drops into droplets that reflect more light into space, creating a shiny trail. The polluted clouds in a ship’s track produce less rain than pristine clouds, but scientists didn’t know how massive pollution would affect precipitation, says Yoram J. Kaufman of NASA’s Goddard Space Flight Center in Greenbelt, Md.
To tackle the bigger problem, cloud physicist Daniel Rosenfeld of the Hebrew University of Jerusalem traced pollution plumes from cities in Australia, Canada, and Turkey. For his study, published in the March 10 Science, Rosenfeld peered into the clouds with two different types of satellites. Using the Advanced Very High Resolution Radiometer on satellites operated by the National Oceanic and Atmospheric Administration, Rosenfeld saw shiny pollution trails in clouds that had floated over the cities.
Rosenfeld picked up pollution tracks extending out of Adelaide, Australia, from a power plant, a lead smelter, oil refineries, and the port. He traced one strong plume—arching across Canadian clouds—to the Hudson Bay Mining and Smelting Co. in Flin-Flon, Manitoba.
He then used sensors onboard NASA’s Tropical Rainfall Measuring Mission satellite to determine how much rain the clouds in the pollution plume produced. No rain fell from these clouds, while similar clouds in nearby unpolluted areas produced normal amounts of precipitation.
Although he suspected that aerosol pollution would suppress some rainfall, “I was actually stunned to see that it was shut off all together,” says Rosenfeld.
The black-and-white nature of his evidence is particularly striking because conflicting models have clouded the connection between pollution and rain, Rosenfeld says. Some studies have indicated that both pollution and the heat associated with cities might increase rainfall (SN: 3/27/99, p. 198). Rosenfeld’s data suggest that any heat effect could be canceled out by aerosol pollution, says Owen B. Toon, an atmospheric scientist at the University of Colorado at Boulder.
Aerosol pollution in the tropics may reach far beyond suppressing local rainfall, says Hans F. Graf of the Max Planck Institute for Meteorology in Hamburg, Germany. Tropical rain provides much of the energy needed for worldwide movement of air in the atmosphere. “Any change in precipitation in the tropics will affect global climate,” says Graf.
Air pollution more easily stifles rain from short-lived tropical clouds than from the longerlasting clouds common in northern latitudes, says Rosenfeld. This may explain why rainfall in the tropics has decreased despite predictions that global warming would make the area wetter, he says.
The shininess that aerosol pollution generates on clouds may counteract some of the green house effect by reflecting sunlight away from Earth, says Toon. However, aerosol pollution carries health hazards. “You can’t beat the greenhouse effect by polluting more and more,” he says.
