When Co-Variability Mimics Causal Aerosol-Cloud Interactions in Satellite Data

Relationships between variables derived from satellite observations can be interpreted as causal connections between explanatory and response variables. When physical processes support the observed relationship, it appears more likely to represent a true causal link. A notable example is the observed relationship between liquid water path and droplet number concentration in marine low clouds, which aligns with the physical mechanisms involved. However, a closer examination reveals that the observed relationships may actually be driven by co-variability between meteorological conditions and aerosol levels, reflecting the climatological evolution of stratocumulus clouds. We therefore suggest that the aerosol influence on marine low clouds should be separated into two pathways: (1) the influence of background aerosol levels on the clouds' climatology, which overlays the co-variability, and (2) the causal response, as seen in the case of ship tracks.