As prize-winning science writer Nigel Calder and climate physicist Henrik Svensmark explain, an interplay of the clouds, the Sun and cosmic rays - sub-atomic particles from exploding stars - seems to have more effect on the climate than man-made carbon dioxide.
This conclusion stems from Svensmark's research at the Danish National Space Center which has recently shown that cosmic rays play an unsuspected role in making our everyday clouds. And during the last 100 years cosmic rays became scarcer because unusually vigorous action by the Sun batted many of them away. Fewer cosmic rays meant fewer clouds and a warmer world.
http://hnn.us/blogs/entries/35373.html and click on the link for an interesting
book review on climate change.