Anne Jolis has written an very nice, level-headed review of Svensmark and the CLOUD experiment.
But a few physicists weren't worrying about Al Gore in the 1990s. They were theorizing about another possible factor in climate change: charged subatomic particles from outer space, or "cosmic rays," whose atmospheric levels appear to rise and fall with the weakness or strength of solar winds that deflect them from the earth. These shifts might significantly impact the type and quantity of clouds covering the earth, providing a clue to one of the least-understood but most important questions about climate. Heavenly bodies might be driving long-term weather trends.
The theory has now moved from the corners of climate skepticism to the center of the physical-science universe: CERN, also known as the European Organization for Nuclear Research. At the Franco-Swiss home of the world's most powerful particle accelerator, scientists have been shooting simulated cosmic rays into a cloud chamber to isolate and measure their contribution to cloud formation. CERN's researchers reported last month that in the conditions they've observed so far, these rays appear to be enhancing the formation rates of pre-cloud seeds by up to a factor of 10. Current climate models do not consider any impact of cosmic rays on clouds.