What This project sets out to gauge the atmospheric relevance of a potential cloud production mechanism that has recently been demonstrated in the laboratory. If important, this mechanism links galactic cosmic rays, originating from supernova explosions in the Milky Way, to Earth's climate. Correlations between cosmic rays and clouds have been observed, but scientific convergence on their implications has been missing, as no direct microphysical mechanism linking cosmic rays to clouds was known. This has recently changed as laboratory experiments uncovered a physical mechanism, that ties ionization from cosmic rays to cloud formation: Ions have been shown to accelerate the production and growth of nanometer size aerosols towards sizes that allow them to act as seed particles for cloud droplets. Why The largest uncertainty in estimates of Earth's energy budget stems from clouds and aerosols. Clouds regulate the albedo of our planet, and any systematic modulation of atmospheric aerosol and clouds must be taken into account for us to build a complete picture of climate variability. If cosmic rays affect aerosols through ion production, then that is completely unaccounted for in our current climate models. The atmospheric relevance of this new ion-based aerosol growth mechanism should therefore be quantified. My project aims precisely to investigate wether the ion induced aerosol production and growth mechanism observed in the laboratory can function in the real atmosphere. How The most practical way to conduct experiments on a global scale is through atmospheric computer models. I will implement the newly found aerosol growth mechanism into ECHAM-HAM, a global circulation aerosol model developed at University of Oxford, and investigate wether it is able to affect aerosol concentrations on a global scale. Cosmic rays and thus atmospheric ionization exhibits natural variation on a range of time scales. These variations may be imposed upon the model, and their consequences for aerosol concentrations monitored. We plan to target weeklong Forbush decreases of the cosmic ray flux, the 11-year solar cycle and millennial scale variations, and describe the response in global and local aerosol concentrations. SSR This projects addresses clouds, the most uncertain area of climate science, by testing a new potentially important cloud forming process. Constraints from this project could reinforce the current assessment from the IPCC that a cosmic-ray cloud connection is weak. Alternatively, it could uncover climate dynamics that are currently not accounted for. Either way, this adds constraints to our understanding of terrestrial climate. So many decisions across the globe rest on the premise that we understand the current and future state of our atmosphere. Therefore, a precise climate understanding is imperative for our society to meet future challenges.