"radiative forcing"
is a measure of the influence that a factor has in alternating the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. Positive forcing tends to warm the surface while negative forcing tends to cool it. (from the below link)
Here is the 2007 IPCC (Intergovernmental Panel on Climate Change) summary for policy makers; it is 18 pages and a relatively-easy read:
http://www.ipcc.ch/SPM2feb07.pdf