An increasing number of aerosol particles released by human activity can cool the climate system. Analysis of data from field experiments show that the enhancement of aerosol number concentration increases the cloud droplet number and reduces the cloud droplet size, thus results in a higher cloud reflectivity (first indirect aerosol effect). The more numerous but smaller cloud droplets also decrease the precipitation efficiency, therefore prolonging cloud lifetime and increasing cloud reflectivity (second indirect aerosol effects). Additionally, anthropogenic aerosols modify the shape of...