Rehovot, Israel -- July 17, 2006 A scientist at the Weizmann Institute of Science and his colleagues caused a storm in the atmospheric community when they suggested a few years back that tiny airborne particles, known as aerosols, may be one of the main culprits causing climate change having, on a local scale, an even greater impact than the greenhouse gases effect. Attempts to understand how these particles influence clouds have generated many uncertainties. A new paper by Dr. Ilan Koren of the Weizmann Institute Environmental Studies and Energy Research Department and Dr. Yoram Kauffman of the NASA/Goddard Space Flight Center, USA,* published in Science Express online, weaves together two opposing effects of atmospheric aerosols to provide a comprehensive picture of how they may be affecting our climate.
Cloud formation is dependent upon the presence of small amounts of aerosols such as sea salt and desert dust. These tiny particles serve as the seeds around which water vapor in the air condenses, forming tiny water droplets that rise as they release heat. As the small droplets rise, they collide and merge with larger droplets. When the droplets reach a critical size, gravity takes over, causing them to fall from the cloud in the form of rain.
One of the controversies surrounding the extent of aerosol impact on climate change is the duality of their influence. On the one hand, Koren and his colleagues previously found evidence to suggest that the extra seeds planted in the atmosphere by the emission of man-made aerosols (pollution, forest fires, and fuel combustion) lead to more, but smaller-sized, water droplets. The formation of larger water droplets by the collision process is less efficient and, therefore, rainfall is suppressed. The smaller droplets are lifted higher up into the atmosphere, creating larger and taller clouds that will persist longer. Not only does this alter the whole water cycle, but the increased cloud cover reflect
Contact: Jeffrey Sussman
American Committee for the Weizmann Institute of Science