The first aerosol indirect effect occurs when increases in concentrations of aerosols that serve as cloud condensation nuclei lead to increases in cloud droplet number concentrations. Quantitative assessment of this microphysical effect at the global scale is challenging, and continues to be a major source of uncertainty in modeling the Earth’s climate. To reduce that uncertainty, a team of scientists, including a Department of Energy researcher at Pacific Northwest National Laboratory, compared aerosols, clouds, and signatures of their interactions in three global climate models to satellite observations. The team found that the three climate models did well in capturing many spatial patterns of aerosol optical depths around the globe, including South America, central and northern Africa, Southeast Asia, and the eastern continental United States. The analysis of aerosol-cloud interactions focused on near-coast marine areas near South Africa and Southeast Asia, which have persistent stratocumulus clouds and are subject to aerosol pollution from nearby land regions. Cloud droplet number concentrations were computed identically from satellite-simulated and MODIS-observed values of liquid cloud optical depth and droplet effective radius. Assessing the sensitivity of cloud droplet number concentration to aerosol optical depth in the three models, the study found two models were more sensitive than observations, while the sensitivity in another model was statistically insignificant. However, the calculated sensitivity could be subject to misinterpretation due to the influence of meteorology on both aerosols and clouds.