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

Svensmark and Friis-Christensen (1997, henceforth SFC) showed a strong corr elation between cosmic ray flux and ISCCP total cloudiness between 1984 and 1990. They concluded that ionisation by cosmic rays, more prevalent at tim es of lower solar activity, might explain apparent correlations between sol ar activity and climate through changes in cloud radiative forcing. We have extended SFC's approach with a study of the different cloud types, restric ting our analysis to the period 1985 to 1988 during which the ISCCP calibra tion is believed to be stable. We find no clear relationship between indivi dual cloud types and cosmic ray flux. Inclusion of data at high latitudes d ecreases the amplitude of the apparent correlation although ionisation by c osmic rays is greatest at high latitudes. Thin high cloud shows an increase throughout the period such that the combined effect of the changes in clou d types suggests an almost monotonic increase in cloud radiative forcing be tween 1985 and 1988 which is not related to cosmic ray activity.