Clouds and cosmic rays
There is a lot of discussion on the correlation of GCR (cosmic ray) modulation of the climate during quiet sun periods. As a simplistic statement during periods of high solar activity the magnetosphere sheath protects the earth from galactic rays.
As we observed here
Low-altitude clouds are significant because they especially shield the Earth from the sun to keep us cool. Low cloud cover can vary by 2% in five years, affecting the Earth's surface by as much as 1.2 watts per square metre during that same period. "That figure can be compared with about 1.4 watts per square metre estimated by the Intergovernmental Panel on Climate Change for the greenhouse effect of all the increase in carbon dioxide in the air since the Industrial Revolution," Dr. Svensmark explained.
There is also an interesting paper
Empirical evidence for a nonlinear effect
of galactic cosmic rays on clouds
BY R. GILES HARRISON* AND DAVID B. STEPHENSON
Galactic cosmic ray (GCR) changes have been suggested to affect weather and climate, and new evidence is presented here directly linking GCRs with clouds. Clouds increase the diffuse solar radiation, measured continuously at UK surface meteorological sites since 1947. The ratio of diffuse to total solar radiation—the diffuse fraction (DF)—is used to infer cloud, and is compared with the daily mean neutron count rate measured at Climax, Colorado from 1951–2000, which provides a globally representative indicator of cosmic rays. Across the UK, on days of high cosmic ray flux (above 3600!102 neutron counts hK1, which occur 87% of the time on average) compared with low cosmic ray flux, (i) the chance of an overcast day increases by (19G4) %, and (ii) the diffuse fraction increases by (2G0.3) %. During sudden transient reductions in cosmic rays (e.g. Forbush events), simultaneous decreases occur in the diffuse fraction. The diffuse radiation changes are, therefore, unambiguously due to cosmic rays. Although the statistically significant nonlinear cosmic ray effect is small, it will have a considerably larger aggregate effect on longertimescale (e.g. centennial) climate variations when day-to-day variability averages out.
Graph above Stott et al science 2001