Aerosols represent one of the greatest sources of uncertainty in assessments of climate
change caused by humans. Modeling studies that include the interaction of aerosols with
cloud droplets render such assessments utterly uncertain. Estimates of the effects of
aerosols on clouds based on correlations of space-borne observations of aerosol burdens
and cloud properties are ambiguous. The correlations may be affected by a number of
physical processes which could have little to do with aerosol-cloud interactions. In
addition, clouds and aerosols respond to the thermodynamics of their environment,
further complicating the determination of the effects of aerosols on clouds. Biases in the
retrievals of cloud properties likewise enhance estimates of the aerosol indirect effect.
Nonetheless, observations of the response of clouds to aerosols obtained from ship tracks
off the west coast of the U.S. and from observations of summertime marine
stratocumulus in the northeastern Atlantic polluted by plumes from Europe show clouds
responding in ways predicted by large eddy simulation cloud models.