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Weizmann Institute of Science researchers have developed a comprehensive picture of how natural and man-made aerosols - minuscule particles in the atmosphere such as sea salt, desert dust, pollution and elements of forest fire smoke and exhaust from fuel combustion - cause greater changes in local climates than the "greenhouse gas effects" of global warming.
A few years ago, Dr. Ilan Koren of the Rehovot institute's environmental studies and energy research department, raised a storm in the atmospheric sciences community when he and colleagues suggested that aerosols may be one of the main culprits causing climate change. Attempts to understand how these particles influence clouds have generated many uncertainties. Now a new paper by Koren and Dr. Yoram Kauffman of the NASA/Goddard Space Flight Center in the US, just published in Science Express online, weaves together two opposing effects of atmospheric aerosols to provide an exhaustive picture of how they affect our climate.
"We hope that this study has finally provided closure," said Koren. "It is hoped that policymakers will start to tackle the issue of climate change from a different perspective, taking into account not only the global impact of aerosols and greenhouse gases but local effects, too."
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 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 leads to more, but smaller, water droplets. The formation of larger water droplets by the collision process is less efficient; therefore, rainfall is suppressed. The smaller droplets are lifted higher up into the atmosphere, creating larger and taller clouds that exist for longer durations. Not only does this alter the whole water cycle, but the increased cloud cover reflects more of the sun's radiation back into space, creating a local cooling effect on Earth.
But to complicate matters, Koren, in another study, showed that certain types of aerosols - those containing black carbon - can also decrease cloud cover, ultimately leading to a warming effect. This occurs as black carbon absorbs part of the sun's radiation, warming the surrounding atmosphere and reducing the difference in temperature between the Earth's surface and the upper atmosphere. This combination prevents atmospheric instability - the condition needed to form clouds and rain. A stable atmosphere means fewer clouds; fewer clouds means less reflection of sunlight; less reflection of sunlight and absorption of radiation lead to warming.
Policymakers have argued that the warming effect of the greenhouse gases and the mostly cooling aerosol effect may balance each other out so that the net global climate change will be small. Koren argues that it is the local climate change that is problematic: Clouds may persist without releasing their rain over regions where they would normally precipitate, such as rainforests, and move to precipitate over regions where rain is not needed, such as oceans - or the effect could lead to the warming up of cold and the cooling down of hot regions. These additional effects to the already problematic warming by greenhouse gases could have disastrous repercussions in the long run.
Koren also dealt with the controversial question of how such tiny localized particles affect weather systems thousands of kilometers away from their sources. Lengthy analysis showed that aerosols, surprisingly, have more influence than metereological factors.
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