Opportunistic experiments to constrain aerosol effective radiative forcing

Aerosol–cloud interactions (ACIs) are considered to be the most uncertain driver of present-day radiative
forcing due to human activities. The nonlinearity of cloud-state changes to aerosol perturbations make
it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to
infer causality can be challenging when meteorological variability also drives both aerosol and cloud changes
independently. Natural and anthropogenic aerosol perturbations from well-defined sources provide “opportunistic
experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more
confidently inferred. These perturbations cover a wide range of locations and spatiotemporal scales, including
point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and
tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct
a synthesis of the available satellite datasets and field campaigns to place these opportunistic experiments on
a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative
forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong
liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments.
Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear
how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper
investigation in order to improve assessments of aerosol radiative forcing and climate change.