One also has to take into consideration global dimming. The more pollutants in the atmosphere, the less solar gain the earth has. Thus the more populated (and assumed most polluted) regions of the world might find that, with the more particles released into the atmosphere, the more sunlight is reflected back into space, in turn cooling said areas. An excellent example of this is the Little Ice Age (1300's-1800's) that followed the period of extreme global warming known, coincidentally, as the Medieval Warming Period.
An article of interest- http://discovermagazine.com/2002/sep/cover/
Biting the hand that feeds IT
