Aerosol Pollution Could Drain Earth's Water Cycle

By Cat Lazaroff

SAN DIEGO, California, December 7, 2001 (ENS) - Pollution produced by humans may be seriously weakening the Earth's water cycle - reducing rainfall and threatening fresh water supplies. A new study by researchers at the Scripps Institution of Oceanography suggests that tiny particles of soot and other pollutants are having a far greater effect on the planet's hydrological cycle than previously realized.


Particles of soot and other pollutants may dissipate cumulus clouds and reduce rainfall (Photo courtesy NOAA)
A United Nations Population Fund report released November 7 found that water use has grown six-fold over the past 70 years. "Water may be the resource that defines the limits of sustainable development," that report noted.

But pollution may be slowly draining the water supplies on which humans and other species depend, suggests the study by Scripps researchers at the University of California, San Diego. Tiny aerosols, primarily made up of black carbon, can lead to a weaker hydrological cycle, which directly affects fresh water availability and quality, the authors argue.

The paper, based on results obtained during the international Indian Ocean Experiment (INDOEX), appears in today's issue of the journal "Science."

"Initially we were seeing aerosols as mainly a cooling agent, offsetting global warming. In this article we are saying that perhaps an even bigger impact of aerosols is on the water budget of the planet," said Scripps professor V. Ramanathan, who along with professor Paul Crutzen, a coauthor of the new study, led the INDOEX science team as co-chief scientists.


Professor Paul Crutzen (left) and Professor V. Ramanathan (Photo courtesy Scripps)
"Through INDOEX we found that aerosols are cutting down sunlight going into the ocean," added Ramanathan. "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 fourth co-author of the paper, Daniel Rosenfeld, also notes that these aerosol particulates may be suppressing rain over polluted regions. Within clouds, aerosols can limit the size of cloud droplets, stifling the development of the larger droplets required for raindrops.

If pollutants cut back on rain and snowfall, it could directly affect the replenishment of the world's major stores of freshwater, including lakes, groundwater supplies, glaciers and high elevation snowpack. If humans continue to draw down these stores at a faster rate than they are replenished, access to fresh water could become the most crucial problem facing civilization.

The INDOEX project involved more than 150 scientists across several disciplines from Austria, France, Germany, India, Maldives, the Netherlands, Sweden and the United States. The $25 million project, funded in part by the National Aeronautics and Space Administration (NASA), focused on the Indian Ocean region in a multiplatform analysis using satellites, aircraft, ships, surface stations and balloons.

The project was designed to assess the nature and magnitude of the chemical pollution over the tropical Indian Ocean and to evaluate the environmental significance of the region's aerosols.


Water pumped from shrinking underground aquifers is used for everything from agricultural irrigation to drinking water (Photo courtesy California Department of Water Resources)
Early in the project, INDOEX researchers documented a human produced brownish gray haze layer of about 10 million square kilometers over the Indian-Asian region. The particles within the haze, the researchers discovered, were causing a three-fold decrease in solar radiation reaching the earth's surface as compared with the top of the atmosphere.

The aerosols, typically in the submicrometer- to micrometer-size range, were a mixture of sulfates, nitrates, organic particles, fly ash, and mineral dust, formed by fossil fuel combustion and burning of forests and other biomass.

"One of the key revelations from INDOEX is that air pollution is not only an industrial phenomenon," said Scripps professor Crutzen, a 1995 Nobel Laureate. "The part of the atmosphere that you would expect to be the cleanest - the areas without a lot of industrialization - in fact can be very highly polluted, especially during the dry season."

In the new "Science" article, Ramanathan, Crutzen, J.T. Kiehl of the National Center for Atmospheric Research, and Rosenfeld of the Hebrew University of Jerusalem, say the aerosol issues raised by INDOEX are a "major environmental concern."


The INDOEX project uses ground, ship and airplane based instruments to gather information in the Indian Ocean (Photo courtesy INDOEX)
The team not only questions the role aerosols are playing on the regional and global water cycle, but they also suggest that aerosol pollution increases the solar heating of the atmosphere, and reduces the solar heating of the surface of the planet. These effects maybe comparable to the global warming effects of greenhouse gases, the team argues.

"At present these effects are not generally accounted for in climate model prediction studies, but we will need to include the absorbing aerosols in future model predictions," said Kiehl.

The immediate next step, the authors say, is to develop a reliable global inventory of aerosol emission rates, life times and concentrations. Integrating new satellite observations, field experiments and laboratory studies with computer models will pave the way for breakthroughs in scientific understanding of aerosols and how they are modifying the environment, they say.

water pail

Access to clean water can mean the difference between health and disease for millions of children (Photo courtesy United Nations Development Programme - World Bank Water and Sanitation Program)
"Part of these results are important for creating awareness," said Crutzen. "The biomass burning in the countries that are producing this pollution cannot go on."

The "Science" study was funded by the U.S. Department of Energy, the National Oceanic and Atmospheric Administration (NOAA), NASA and the National Science Foundation.