I found this BBC article via kottke. It seems global warming would be much higher, if it were not for small particles of soot etc. which seed more clouds. Clouds in turn reflect heat away from earth. And although these clouds cause some nasty flooding in some monsoon regions perhaps all the filtering of exhaust particles might not be such a brilliant idea after all, unless CO2 production is reduced simultaneously.
What more can I say? Happy commute, everyone!