...It is known that low-altitude clouds have an overall cooling effect on the Earth's surface. Hence, variations in cloud cover caused by cosmic rays can change the surface temperature. The existence of such a cosmic connection to Earth's climate might thus help to explain past and present variations in Earth's climate.H/T The Anchoress
Interestingly, during the 20th Century, the Sun's magnetic field which shields Earth from cosmic rays more than doubled, thereby reducing the average influx of cosmic rays.
The resulting reduction in cloudiness, especially of low-altitude clouds, may be a significant factor in the global warming Earth has undergone during the last century. However, until now, there has been no experimental evidence of how the causal mechanism linking cosmic rays and cloud formation may work.
"Many climate scientists have considered the linkages from cosmic rays to clouds to climate as unproven," comments Eigil Friis-Christensen, who is now Director of the Danish National Space Center.
"Some said there was no conceivable way in which cosmic rays could influence cloud cover. The SKY experiment now shows how they do so, and should help to put the cosmic-ray connection firmly onto the agenda of international climate research."