Estimating the change in low level cloud cover during the Brunhes-Matuyama magnetic field reversal: A first modelling approach

Geomagnetic variations are the perfect testbed to study the effect of Galactic Cosmic Rays (GCR) on climate, as they disentangle solar variability from direct GCR effects. We use the 3D chemical transport model GEOS-CHEM to simulate the change in the cloud condensation nuclei number density during the Brunhes-Matuyama magnetic field reversal assuming present day aerosol conditions. We then estimate the change in cloud condensation nuclei for low level clouds under both solar minimum and solar maximum ionisation conditions. We also test whether the effect of ion-enhanced growth significantly enhances the process. We find a cloud condensation nuclei enhancement for low level clouds throughout the magnetic field reversal of several percent, which supports the observational findings of a wetter and colder climate during the Brunhes-Matuyama magnetic field reversal.