Background
When solar activity is high, the magnetic field that is carried by the solar wind intensifies, providing more shielding of the earth from low-energy galactic cosmic rays. This effect may lead to a decrease in ion production in the lower atmosphere, possibly resulting in the creation of fewer cloud condensation nuclei and less low-level cloud cover; and this phenomenon, in turn, may allow more solar radiation to impinge upon the earth and drive global warming.

What was done
In an important test of a critical portion of this hypothesis, the authors compared the spatial distributions of low cloud amount (LCA) and cosmic ray-induced ionization (CRII) over the globe for the period 1984-2000. In doing so, they used observed LCA data obtained from the ISCCP-D2 database limited to IR radiances, while they employed CRII values calculated by Usoskin et al. (2004a) at 3 km altitude, which corresponds roughly to the limiting altitude below which low clouds form.

What was learned
Usoskin et al. determined that "the LCA time series can be decomposed into a long-term slow trend and inter-annual variations, the latter depicting a clear 11-year cycle in phase with CRII." In addition, they say there is "a one-to-one relation between the relative variations of LCA and CRII over the latitude range 20-55°S and 10-70°N," and that "the amplitude of relative variations in LCA was found to increase polewards, in accordance with the amplitude of CRII variations."

What it means
The findings of the five-member team of Finnish, Danish and Russian scientists provide substantial evidence for a solar-cosmic ray linkage (the 11-year cycle of CRII) and a cosmic ray-cloud linkage (the in-phase cycles of CRII and CLA), making the full solar activity/cosmic ray/low cloud/climate change hypothesis appear to be rather robust.

Reference
Usoskin, I.G., Gladysheva, O.G. and Kovaltsov, G.A. 2004a. Cosmic ray-induced ionization in the atmosphere: spatial and temporal changes. Journal of Atmospheric and Solar-Terrestrial Physics 66: 1791-1796.