The theory of 'Global Dimming"; ( http://rst.gsfc.nasa.gov/Sect16/Sect16_2.html )...
The cause of global dimming is largely due to increased soot, ash, and sulphur particles released to the atmosphere by industrial activity and automobile use. The particles serve as nuclei for water droplets that make up clouds. The clouds in turn become more reflective, sending a greater percentage of incoming irradiation back into space. Much of the particulate matter is carbon-rich, being soot caused in part by regional burning of forests and grasslands (both controlled crop-management burning and wildfires). MODIS on Terra and Aqua produced this global map of carbon soot in the atmosphere; wildfires in northwest Siberia account for the high readings there.
Carbon distribution in the world's atmosphere for one day in July, 2006.
Another factor which seems important in moderating temperatures is the large number of contrails from airplanes, as seen here:
Contrails from airplanes, seen looking up from the surface.
While the contrails may seem insignificant, their role was indirectly verified as a side effect of 9/11 (the Sept. 11, 2001 terrorist attacks in the U.S.). For almost 4 days after the government grounded all commercial flights in the U.S., the amount of measured irradiation increased enough to raise average temperatures by a degree Celsius. Upon resuming flights, this increase was reversed