In April 1990, Al Gore published an open letter in the New York Times "To Skeptics on Global Warming" in which he compared them to medieval flat-Earthers. He soon became vice president and his conviction that climate change was dominated by man-made emissions went mainstream. Western governments embarked on a new era of anti-emission regulation and poured billions into research that might justify it. As far as the average Western politician was concerned, the debate was over.

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: the European Organization for Nuclear Research, also known as CERN. 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.

Scientists have been speculating on the relationship among cosmic rays, solar activity and clouds since at least the 1970s. But the notion didn't get a workout until 1995, when Danish physicist Henrik Svensmark came across a 1991 paper by Eigil Friis-Christensen and Knud Lassen, who had charted a close relationship between solar variations and changes in the earth's surface temperature since 1860.

"I had this idea that the real link could be between cloud cover and cosmic rays, and I wanted to try to figure out if it was a good idea or a bad idea," Mr. Svensmark told me from Copenhagen, where he leads sun-climate research at the Danish National Space Institute.