There's a good explanation of this, as well as the possibility that climate change is affected by cosmic rays, in this article in the Christian Science Monitor:
http://www.csmonitor.com/2007/0927/p....html/(page)/2
It appears that neither contributes significantly to it, but "the phenomenon is worth exploring."
From the article, "Dr. Kirkby (a physicist at Europe's center for high-energy physics research in Geneva) and colleagues at several institutions aim to do just that. They've designed an aerosol chamber to test how cosmic rays might affect cloud formation and how significant the effect might be. "You really can't settle the issue by more heated debate," he says. "You need experimental data."
Sounds like a good approach.