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.
He wasn't the first scientist to have the idea, but he was the first to try to demonstrate it. He got in touch with Mr. Friis-Christensen, and they used satellite data to show a close correlation among solar activity, cloud cover and cosmic-ray levels since 1979.
They announced their findings, and the possible climatic implications, at a 1996 space conference in Birmingham, England. Then, as Mr. Svensmark recalls, "everything went completely crazy. . . . It turned out it was very, very sensitive to say these things already at that time." He returned to Copenhagen to find his local daily leading with a quote from the then-chair of the U.N. Intergovernmental Panel on Climate Change (IPCC): "I find the move from this pair scientifically extremely naïve and irresponsible."
Mr. Svensmark had been, at the very least, politically naïve. "Before 1995 I was doing things related to quantum fluctuations. Nobody was interested, it was just me sitting in my office. It was really an eye-opener, that baptism into climate science." He says his work was "very much ignored" by the climate-science establishment—but not by CERN physicist Jasper Kirkby, who is leading today's ongoing cloud-chamber experiment.
On the phone from Geneva, Mr. Kirkby says that Mr. Svensmark's hypothesis "started me thinking: There's good evidence that pre-industrial climate has frequently varied on 100-year timescales, and what's been found is that often these variations correlate with changes in solar activity, solar wind. You see correlations in the atmosphere between cosmic rays and clouds—that's what Svensmark reported. But these correlations don't prove cause and effect, and it's very difficult to isolate what's due to cosmic rays and what's due to other things."