Aerosols that collect at the top of the atmosphere exert a greater influence on warming when there are clouds located beneath them, suggests a paper online in Nature Geoscience. Aerosols, such as those produced from biomass burning, can alter the radiative balance of the Earth by reflecting and absorbing solar radiation. Whether they exert a net warming or cooling effect depends upon the reflectivity of the underlying surface.
Duli Chand and colleagues used satellite data to quantify the warming effect of aerosols that were carried over the southeastern Atlantic Ocean. They found that the greater the cloud coverage below the aerosol layers, the more the aerosols warm the atmosphere. This relationship is nearly linear, making it possible to define a critical point where aerosols switch from exerting a net cooling effect to a net warming effect. They estimate that when this relationship between aerosols and underlying clouds is taken into account, regional warming is three times greater than otherwise predicted.