there was this phenomenon after 9/11, where there were no planes flying for 2 days, in the following few days, temperature dropped about 2 degree. Another one is global dimming, where the particles that come from burning fossil fuels dim the sunlight. When you get rid of the excess co², the dimming effect should cool down the earth - not sure that's a good idea though. I think it would be much better to phase out fossil fuels alltogether, but that will take a long time and the alternatives aren't as readily available yet.