I just watching Nova, Dimming the Sun while having my breakfast. The program is about global dimming which is:
…a term describing the gradual reduction in the amount of global hemispherical irradiance (or total solar irradiance) at the Earth’s surface since the 1950s. – from Wikipedia
I found the program really eye opening and disturbing. In short, the effects of green house gases on global warming might be much worse than we though.
Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise. – from Wikipedia
Here is a link to a preview of the show: http://www.pbs.org/wgbh/nova/preview/i_3310.html