Marsden, D. and Lingenfelter, R.E. 2003. Solar activity and cloud opacity variations: A modulated cosmic ray ionization model. Journal of the Atmospheric Sciences 60: 626-636.
When solar activity increases and the weak magnetic field that is carried by the solar wind intensifies, providing more shielding of the earth from low-energy galactic cosmic rays, there is believed to be a reduction in ion production in the lower atmosphere that results in the creation of fewer cloud condensation nuclei there and, hence, less low-level cloud cover, which consequently allows more solar radiation to impinge upon the earth and increase surface air temperature. Many people believe this scenario explains much of the global warming that has occurred since the temperature minimum of the Little Ice Age. Support for this hypothesis is provided by the studies of Svensmark and Friis-Christensen (1997), Marsh and Svensmark (2000) and Palle Bago and Butler (2000), who derived a positive relationship between global cosmic ray intensity and low-cloud amount from infrared (IR) cloud measurements contained in the International Satellite Cloud Climatology Project (ISCCP) database for the years 1983-1993.
What was done
Among other things, the authors of the current paper used the ISCCP database for the expanded period 1983-1999 to study this latter relationship as it may or may not be expressed via cloud amount measurements made in the visible spectrum.
What was learned
With respect to the hypothesized relationship between global cosmic ray intensity and low-cloud amount, the authors report that they found "a positive correlation at low altitudes, which is consistent with the positive correlation between global low clouds and cosmic ray rate seen in the infrared."
What it means
The results of this study provide further evidence for the robustness of the solar forcing of climate theory.
Marsh, N.D. and Svensmark, H. 2000. Low cloud properties influenced by cosmic rays. Physical Review Letters 85: 5004-5007.
Palle Bago, E. and Butler, C.J. 2000. The influence of cosmic rays on terrestrial clouds and global warming. Astronomy & Geophysics 41: 4.18-4.22.
Svensmark, H. and Friis-Christensen, E. 1997. Variation of cosmic ray flux and global cloud coverage - A missing link in solar-climate relationships. Journal of Atmospheric and Solar-Terrestrial Physics 59: 1225-1232.
Reviewed 16 April 2003