This was on PBS's NOVA a few months back. A by-product of greenhouse gas emissions is what I am going to term "macro particles." Thanks to their size and composition, these particles have been reflecting sunlight and therefore reducing global warming, although on balance they could not reverse it. In the absence of these particles, the greenhouse effect would have been more severe. A scientist on NOVA claimed that these particles have reduced the warming effect by about 20%. Are there studies of how this information can be used to further reduce or even reverse GW?