Rising air pollution worsens drought and flooding

Increases in air pollution and other particulate matter in the atmosphere can strongly affect cloud development in ways that reduce precipitation in cool and relatively dry regions, such as Israel in winter, but also can increase rain and the intensity of severe storms in warm and moist regions or seasons, such as the eastern half of the US during summer, says a new study by researchers from the Hebrew University of Jerusalem and the University of Maryland.

The research provides clear evidence of how aerosols — soot, dust and other small particles in the atmosphere — can affect weather and climate. The findings have important implications for the availability, management and use of water resources in regions not only in Israel but around the world, say the researchers.

Using a 10-year dataset of extensive atmosphere measurements from the U.S. Southern Great Plains research facility in Oklahoma (run by the US Department of Energy’s Atmospheric Radiation Measurement program), the researchers uncovered, for the first time, the long-term, net impact of aerosols on cloud height and thickness and the resultant changes in precipitation frequency and intensity.

This study confirmed and showed the importance of the theory developed by Prof. Daniel Rosenfeld of the Hebrew University of Jerusalem in his previous studies. The authors of the new study, who published it, along with Prof. Rosenfeld, are Prof. Zhanqing Li and Feng Niu and Yanni Ding of the University of Maryland; Jiwen Fan of Pacific Northwest National Laboratory; and Yangang Liu of Brookhaven National Laboratory, Upton, NY, The article appeared in a recent issue of Nature Geoscience.

The findings have significant policy implications for sustainable development and water resources, especially for those developing regions susceptible to extreme events, such as drought and flood. Increases in manufacturing, building of power plants and other industrial developments, together with urbanization, are often accompanied with increases in pollution whose adverse impacts on weather and climate, as revealed in this study, can undercut economic gains, it was stressed.

The aerosols, tiny solid or liquid particles suspended in air, include soot, dust and sulfate particles and are what we commonly think of when we talk about air pollution. Aerosols come, for example, from the combustion of fossil fuels, industrial and agricultural processes, and the accidental or deliberate burning of fields and forests. They can be hazardous to both human health and the environment.

Aerosols also affect cloud microphysics because they serve as nuclei around which water droplets or ice particles form. Both processes can affect cloud properties and rainfall. Different processes may work in harmony or offset each other, leading to a complex yet inconclusive interpretation of their long-term net effect.

“When the air rises, the water vapor condenses on aerosol particles to form cloud drops. In cleaner air the cloud drops are larger due to fewer drops and have better chances of colliding to form large rain drops. In polluted air, more and smaller drops are formed. They float in the air and are slow to coalesce into rain drops,” says Prof. Rosenfeld.

“With a small amount of moisture, most cloud drops never become large enough for efficient precipitation, and hence rainfall is reduced. The rain that is withheld in moist, polluted, deep clouds freezes at higher altitudes to form ice crystals or even hail. The energy released by freezing fuels the clouds to grow taller and create larger ice particles that produce more intense precipitation. This explains why air pollution can exacerbate both drought and flood,” says Rosenfeld.

This may partially explain Rosenfeld’s finding in another study that there are more severe convective storms during summer on weekdays compared to weekends in the eastern United State, because more pollution is emitted during the working weekdays than during the weekend.

Support for this latest research was provided by the Department of Energy, NASA, the National Science Foundation and the Chinese Ministry of Science and Technology.

(Photo of Prof. Rosenfeld available via e-mail upon request)

For further information: Jerry Barach, Dept. of Media Relations, the Hebrew University,
Tel: 02-588-2904.
Orit Sulitzeanu, Hebrew University spokesperson, Tel: 054-8820016