Solar activity, cosmic rays, clouds and climate -: an update

Eighteen years of monthly averaged low cloud cover data from the International Satellite Cloud Climatology Project are correlated with both total solar irradiance and galactic cosmic ray flux from neutron monitors. When globally averaged low cloud cover is considered, consistently higher correlations (but with opposite sign) are found between low cloud variations and solar irradiance variations than between variations in cosmic ray flux and low cloud cover. The correlations are not significant at the 0.1 level, but it should be noted that non-solar effects such as El Nino and volcanic eruptions have not been removed. When spatial regression patterns between low cloud cover and total solar irradiance are studied, the Pacific Ocean exhibits patterns reminiscent of the Pacific Decadal Oscillation. A possible interpretation is that the solar signal interacts with variability modes in the ocean to give this kind of pattern. Correlating low cloud cover with its own global average shows that most of the variability is coming from the subtropical oceans, where the bulk of the Earth's low clouds are found. In conclusion, the updated analysis is not inconsistent with a modulation of marine low cloud cover due to variations in solar irradiance causing changes in lower tropospheric static stability, but many details are still missing. A cosmic ray modulation seems less likely, but can not be ruled out on the basis of the present analysis. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.