Dr. Yu, of UAlbany�s Atmospheric Sciences Research Center, believes the secret is in the clouds.

With an advanced computer code of particle microphysics and a pioneering ion-mediated nucleation theory, Yu has been investigating the roles of ions, sulfuric acid, and organics in aerosol formation and evolution. He concluded that atmospheric nanoparticles, which react to changes of cosmic ray flux associated with the sun�s activity, may explain why the fraction of sky covered by low clouds is closely related to solar activity. Low clouds, similar to the house roof that keeps your floor cool by sheltering it from the sunlight but heats up the air around it due to sunlight (and longwave radiation) absorption, tend to cool the Earth�s surface, but warm the low atmosphere.

Yu suggests that a decrease in cosmic ray flux during the last two decades might have led to a slight decrease in the global mean low cloud amount, which in turn might have warmed the Earth�s surface, but cooled the low atmosphere. By superimposing this effect of cosmic ray-induced cloud changes (warming the surface but cooling the atmosphere) on the effect of greenhouse gases (warming both the surface and the atmosphere), Yu can explain why the Earth�s surface has warmed, but the atmosphere has not.