Is there a link between galactic cosmic rays and clouds? 

The CLOUD experiment at CERN

Urs Baltensperger
Laboratory of Atmospheric Chemistry, Paul Scherrer Institut,
5232 Villigen PSI, Switzerland

Globally, a significant source of cloud condensation nuclei for
cloud formation is thought to originate from new particle
formation (aerosol nucleation). Despite an extensive research effort,
many questions remain about the dominant nucleation mechanisms.
The potential influence of ions from galactic cosmic rays on
the atmospheric aerosol nucleation processes may play an important
role relevant for aerosol production, cloud formation and climate. 
Variability of galactic cosmic rays due to modulating influences from
the sun therefore may affect (regional) climate on various time scales. 
Similarly, a quantitative understanding of the dependence of the
nucleation rate on the concentration of the nucleating substances such
as gaseous sulfuric acid, ammonia, water vapor and others has not
been reached. This is of relevance for climate as the
atmospheric concentrations of sulfuric acid, ammonia and other
nucleating agents are strongly influenced by anthropogenic emissions.
In this presentation first an overview is given of the potential
processes by which cosmic rays may affect climate and examples of
the correlations of atmospheric and paleo-climatic parameters
suggesting this connection are briefly reviewed.
Then, the first results from the CLOUD experiment in the new
aerosol chamber facility at CERN are presented, which recently have
been published in Nature (Kirkby et al., 2011).

Kirkby J. et al. (2011) Role of sulphuric acid,
ammonia and galactic cosmic rays in atmospheric aerosol nucleation.
Nature, 476, 429-433, doi: 10.1038/nature10343