The Role of Aerosol Types in Mediating the Impact of Galactic Cosmic Rays on Climate Variability Over the Past Two Decades

Galactic Cosmic Rays (GCRs), high-energy particles originating from supernovas, have been hypothesized to influence Earth's climate by ionizing atmospheric aerosols and accelerating the formation of cloud condensation nuclei (CCN). This mechanism leads to increasing the cloud cover and enhances the cooling effect at the Earth’s surface. However, the magnitude of this natural forcing remains a subject of debate. This study proposes the use of multivariate linear regression to model monthly anomalies in near-surface air temperatures as a function of anomalies in GCR flux and other solar and climate variables, including sunspot number, geomagnetic indices, greenhouse gas concentrations (CO₂ and CH₄), cloud effective radius (CER), cloud liquid water, radiation, and aerosol optical depth (AOD) across different latitudes. Monthly data  collected over the past 20 years from a variety of instruments, surface-based and satellite on board, and networks monitoring the atmosphere and from three neutron monitoring stations at different latitudes:  in Hermanus (South Africa, low-latitude), Newark (USA, mid-latitude), and Oulu (Finland, high-latitude) have been considered, being the location of three neutron monitor stations. CER and AOD emerged as the most significant predictors across all stations. Incorporating GCR flux as a covariate for AOD improved model performance, with adjusted R-squared values increasing from 0.22 to 0.31 in Oulu, 0.37 to 0.52 in Newark, and 0.69 to 0.78 in Hermanus. Further analysis using ECMWF atmospheric composition reanalysis indicated that sea salt aerosols, particularly in the 5–20 µm size range, dominate across all locations, suggesting their potential role to the mechanisms enhanced by the GCRs ionization power, such as CCN formation and particle aggregation. A next step would be to investigate the impact of GCRs on cloud characteristics, such as cloud cover, cloud fraction and cloud top properties like pressure and temperature, to gain a clearer understanding of their influence on climate variability.

Keywords: galactic cosmic ray, near surface temperature, aerosol type, sea salt aerosol, cloud condensation nuclei, climate natural variability