Hypothesis
A short-term drop in ion-induced nucleation, of the kind caused by a decrease in cosmic-ray ionisation, would measurably change atmospheric aerosol and cloud-condensation-nucleus properties — the proposed explanation for observed cosmic-ray/cloud correlations.
What failed
In a global aerosol microphysics model, a 10-day 15% reduction in the total nucleation rate (an upper limit on the ion-induced effect) lowered global mean surface cloud-condensation nuclei by only 0.15%, and against the model's own aerosol variability no statistically significant response could be detected in particles above 10 nm, CCN above 70 nm, or the Ångström exponent.
Broken assumption
Cosmic-ray-driven changes in ion-induced nucleation are large enough to alter cloud and aerosol properties.
Why
The modelled signal is far smaller than natural aerosol variability, so the authors conclude the observed cosmic-ray/cloud correlations cannot be explained by associated changes in the large-scale nucleation rate.
From the published paper: Dunne et al. Atmospheric chemistry and physics, 2012. ↗CC BY