Dirty air brings rain -- then again,...( An international team of scientists ...)

An international team of scientists, headed by Prof. Daniel Rosenfeld of the Institute of Earth Sciences at the Hebrew University of Jerusalem, has come up with a surprising finding to the disputed issue of whether air pollution increases or decreases rainfall. The conclusion: both can be true, depending on local environmental conditions.

The determination of this issue is one with significant consequences in an era of climate change and specifically in areas suffering from manmade pollution and water shortages, including Israel.

In an article appearing in the Sept 5 issue of the journal Science, the scientific team, which included researchers from Germany, has published the results of its research untangling the contradictions surrounding the conundrum. They do this by following the energy flow through the atmosphere and the ways it is influenced by aerosol (airborne) particles. This allows the development of more exact predictions of how air pollution affects weather, water resources and future climates.

Mankind releases huge amounts of particles into the air that are so tiny that they float. Before being influenced by man, air above land contained up to twice as many of these so called aerosol particles as air above oceans. Nowadays, this ratio has increased to as much as a hundredfold.

Natural and manmade aerosols influence our climate that much is agreed. But which way do they push it? They produce more clouds and more rain, some say. They produce fewer clouds and less rain, say others. This disputed role of aerosols has been the greatest source of uncertainties in our understanding of the climate system, including the question of global warming.

"Both camps are right", says Prof. Meinrat O. Andreae, director of the Max Planck Institute for Chemistry in Germany, a coauthor of the publication. "But you have to consider how many aerosol particles there are." The lead author, Prof. Rosenfeld of the H
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