Background
The authors note that in 1959 Edward Ney first suggested "cosmic rays could affect the weather (Ney, 1959), an idea revived by the positive correlation found between monthly galactic cosmic rays (GCRs) and satellite-retrieved low cloud amount from 1983 to 1994 (Marsh and Svensmark, 2000)," that "close associations have also been observed between cosmogenic isotopes and paleoclimate records, such as for the monsoon (Neff et al., 2001) and ocean temperatures (Bond et al., 2001)," and that "an increase in cosmogenic isotope production occurred during the Maunder Minimum in solar activity (Beer, 2000)." The basic idea behind these phenomena is that reduced solar activity leads to less magnetic shielding of the earth from ionizing GCRs, which leads to the production of more ion clusters in the atmosphere that grow to form more cloud condensation nuclei that lead to the creation of more low-level clouds that reflect more solar radiation back to space and thereby produce lower surface air temperatures, and vice versa.

What was done
Harrison and Stephenson explored the robustness of this chain of events by examining relationships between diffuse solar radiation and cloud amount measured at ten United Kingdom solar radiation-recording sites and "a globally representative measure of cosmic ray ion production, the daily average neutron count rate (Simpson, 2000)," which "has been measured by the University of Chicago at Climax, Colorado [USA] since 1951."

What was learned
The researchers report they "found a small yet statistically significant effect of cosmic rays on daily cloudiness regionally that supports the global results from satellite data (Marsh and Svensmark, 2000)," that "the decrease in the proportion of direct solar radiation associated with an increase in the diffuse fraction will lead to a local reduction in daytime surface temperature," and that "because the net global effect of cloud is cooling (Hartman, 1993), any widespread increase in the overcast days could also reduce temperature [regionally or globally]."

What it means
"In summary," in the words of Harrison and Stephenson, "our data analysis confirms the existence of a small, yet statistically robust, cosmic ray effect on clouds, that will emerge on long time scales with less variability than the considerable variability of daily cloudiness." This finding is important because it demonstrates the existence of an indirect warming effect of increased solar activity that operates in addition to the direct warming effect of enhanced solar radiation reception, which direct effect is generally considered to be too small to account for major observed and reconstructed changes in earth's climate on decadal, centennial and millennial time scales.

References
Beer, J. 2000. Long-term indirect indices of solar variability. Space Science Reviews 94: 53-66.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Hartman, D.L. 1993. Radiative effects of clouds on Earth's climate. In: Hobbs, P.V. (Ed.) Aerosol-Cloud-Climate Interactions. International Geophysics Series, Volume 54. Academic Press, New York, NY, USA.

Marsh, N.D. and Svensmark, H. 2000. Low cloud properties influenced by cosmic rays. Physical Review Letters 85: 5004-5007.

Neff, U., Burns, S.J., Mangini, A., Mudelsee, M., Fleitmann, D and Matter, A. 2001. Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago. Nature 411: 290-293.

Ney, E.P. 1959. Cosmic radiation and the weather. Nature 183: 451-452.

Simpson, J.A. 2000. The cosmic ray nucleonic component: the invention and scientific uses of the neutron monitor. Space Science Reviews 93: 11-32.