Air pollution may hamper water cycle

Air pollution could be weakening the world’s hydrological cycle by reducing the amount of sunlight that reaches the earth and suppressing rain formation, according to a paper published in Science this week. The paper’s authors hypothesize that a side effect may be less efficient removal of airborne pollutants.

The carbonaceous aerosol particles emitted when biomass is burned for fuel is the air pollution causing the most concern because of the black carbon it contains, according to V. Ramanathan of the Scripps Institution of Oceanography and his colleagues at Germany’s Max-Planck-Institute for Chemistry, the U.S. National Center for Atmospheric Research, and Israel’s Institute of Earth Sciences. Aerosols from other anthropogenic sources and the carbonaceous particles formed when fossil fuels are combusted for energy also play a role.

The carbonaceous aerosols both absorb and scatter solar radiation at the earth’s surface, and they also absorb the solar radiation reflected back from the earth. The scientists have determined that the effect on the atmosphere of these particles at the earth’s surface is more significant because it can decrease precipitation, according to the studies they have made as part of the Indian Ocean Experiment (INDOEX; watch quicktime video), which is being conducted by 150 scientists from around the world under the sponsorship of the National Science Foundation, the U.S. National Aeronautics and Space Administration (NASA), the U.S. Department of Energy, and the U.S. National Oceanic and Atmospheric Administration (NOAA).

“Through INDOEX, we found that aerosols are cutting down sunlight going into the ocean,” Ramanathan says. “The energy for the hydrological cycle comes from sunlight. As sunlight heats the ocean, water escapes into the atmosphere and falls out as rain. So as aerosols cut down sunlight by large amounts, they may be spinning down the hydrological cycle of the planet.”

The INDOEX experiment also suggests that these carbonaceous aerosols may be suppressing rain over polluted regions. By limiting the size of cloud droplets that can be formed, the aerosols may stifle the formation of raindrops.

To further investigate the role air pollution plays in the water cycle, the paper’s authors are calling for the creation of a reliable global inventory of aerosol emission rates, life times, and concentrations. —KELLYN S. BETTS