Watered down

aerosols may be the culprits weakening the Earth's water cycle by reducing rainfall. This was discovered during a study conducted by researchers at the Scripps Institution of Oceanography, University of California, San Diego, usa. According to the researchers, the tiny aerosols that are primarily made up of black carbon could be leading to a weaker hydrological cycle, which directly affects the fresh water availability and quality.

Their study is based on results obtained during the Indian Ocean Experiment (indoex), an international field experiment in the Indian Ocean. "Earlier aerosols were thought to be cooling agents,offsetting global warming. But perhaps, their even bigger impact is on the water budget of the planet,' said V Ramanathan, one of the co-chief scientists of the indoex science team (www.ens.com, December 7, 2001).

"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. Since aerosols cut down sunlight by large amounts, they may be spinning down the hydrological cycle of the planet,' the Scripps researchers opine.

Another 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 and glaciers. 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 humanity.

According to the researchers, aerosol pollution may also be increasing the solar heating of the atmosphere and reducing 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 considered by researchers in climate model prediction studies, but we will need to include the absorbing aerosols in future model predictions for combating global warming,' said J T Kiehl of the National Centre for Atmospheric Research, Boulder, usa.

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