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 mean less reflection of sunlight; less reflection of sunlight and absorption of radiation lead to warming.
Policy makers have argued that, in the bottom line, the warming effect of the greenhouse gases and the (mainly 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.
Also controversial is the question of how such tiny localized particles affect weather systems thousands of kilometers away from their sources. There is no doubt that aerosols do play a role, but the skeptics believe it is negligible compared to meteorological key players such as temperature, pressure, the amount of water vapor in the air, and wind strength.
What Koren needed was a way to separate meteorological from aerosol influences ?something which was lacking in his previous studies. Together with Kauffman, he used a network of ground sensors (AERONE
Source:American Committee for the Weizmann Institute of Science