How Background Aerosol Conditions Shape the Indirect Radiative Forcing of Volcanic Eruptions

Aerosol‐cloud interactions contribute substantially to global climate forcing, yet their quantification is hindered by the diverse responses of clouds to aerosol perturbations. In this Southwest Pacific case study, we use volcanic plumes as opportunistic experiments to isolate aerosol effects from meteorological confounders, and show that although the Twomey effect dominates the aerosol indirect forcing (53%–57%, −0.60 to −0.63 W m −2 ), adjustments in cloud fraction (32%–35%, −0.36 to −0.40 W m −2 ) and liquid water path (11%–12%, −0.12 to −0.14 W m −2 ) remain substantial. Regime‐specific analysis indicates that cloud susceptibility is most pronounced in clean conditions ( N d < 55 cm −3 ). These results underscore that accurate assessment of aerosol forcing must comprehensively account for cloud adjustments modulated by background aerosol conditions, particularly across varying cloud regimes.