Some doubts concerning a link between cosmic ray fluxes and global cloudiness

Abstract

Svensmark and Friis‐Christensen (1997, henceforth SFC) showed a strong correlation between cosmic ray flux and ISCCP total cloudiness between 1984 and 1990. They concluded that ionisation by cosmic rays, more prevalent at times of lower solar activity, might explain apparent correlations between solar activity and climate through changes in cloud radiative forcing. We have extended SFC's approach with a study of the different cloud types, restricting our analysis to the period 1985 to 1988 during which the ISCCP calibration is believed to be stable. We find no clear relationship between individual cloud types and cosmic ray flux. Inclusion of data at high latitudes decreases the amplitude of the apparent correlation although ionisation by cosmic rays is greatest at high latitudes. Thin high cloud shows an increase throughout the period such that the combined effect of the changes in cloud types suggests an almost monotonic increase in cloud radiative forcing between 1985 and 1988 which is not related to cosmic ray activity.