Until now, climate scientists have generally assumed that aerosols such as soot from forest fires and industrial pollution lead to cooling by increasing the size and lifespan of clouds as water droplets form around the soot particles. However, soot can also absorb the sun’s radiation, reducing cloud formation.

Without an accurate model for which of these effects will be most influential, global climate models will be less accurate.

Now, a team of scientists from the Weizmann Institute in Israel, the University of Maryland, Baltimore County and NASA have developed an analytical model that reveals the complexity of the cloud–aerosol interaction. They found that adding small quantities of aerosols into a clean environment produces a net cooling effect, but as the number of particles entering the cloud layer increases, the effect reverses.

October 2008