Few subjects in the history of science have been more infamously politicized than global warming.

Those who back the theory say the science is settled and opposition is from right-wing "denialists." Doubters say the theory is a left-wing attempt to cripple the free enterprise system.

But in the scientific media, the debate over global warming continues. The great majority of scientists say humans are causing global warming by burning fossil fuels. But a minority of scientists publishing in scientific journals say other factors probably play a role.

Cosmic rays and solar activity provide at least part of the explanation for recent warming, according to scientists such as Henrik Svensmark, director of the Center for Sun-Climate Research at the Danish Space Research Institute.

Svenskmark's 13-year-old hypothesis is that cosmic rays play an important role in cloud formation, and that high levels of solar activity interfere with cosmic rays reaching Earth. So when the sun is active, there is less cloud cover, hence, more warming.

Svenskmark has published this hypothesis in scientific journals. Scientists who promote global warming theory have replied with their own rebuttals in the journals. Delving into politics, they also say discussion of alternatives is dangerous, because it could deter governments from action to reduce greenhouse gas levels.

This spring, scientists led by Peter Adams, a professor of civil and environmental engineering at Carnegie Mellon University, tried to drive a stake through the cosmic ray hypothesis. In a paper published in the journal Geophysical Research Letters, "Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates?" the team reported results of a computer model of cosmic ray interaction with clouds.

The computer model showed that the cosmic ray effect was 100 times too small to alter the climate, according to a Carnegie Mellon news release on the study. The release also called the effect a "troubling hypothesis," now proven to be a "myth" that should be "laid to rest."

But Svenskmark refused to cooperate.

This summer, he and his colleagues published another paper backing the linkage, also in Geophysical Research Letters: "Cosmic ray decreases affect atmospheric aerosols and clouds."

Svenskmark's team measured the level of cloud cover after especially large, sudden decreases in cosmic rays, called Forbush decreases. The team concentrated on low-level clouds, which previous research indicated would be most affected by cosmic ray levels.

For the five strongest Forbush decreases, from 2001 to 2005, the team found a 7 percent decrease in the liquid water content of clouds. The vanishing water remained in the air as water vapor, but unlike liquid water, it doesn't block sunlight. And satellite measurements of the area of cloud cover found a 5 percent decrease.

"A loss of clouds of 4 to 5 percent may not sound like very much, but it briefly increases the sunlight reaching the oceans by about 2 watts per square meter, and that's equivalent to all the global warming during the 20th century," Svensmark said in a news release about his study.

Of course, this is far from the last word about the cosmic ray warming hypothesis, nor is it necessarily incompatible with conventional global warming theory. Both could be at work.

Inevitably, supporters of conventional global warming theory will reply with their own debunking. Their critics will reply with their own studies. And so on until (one hopes), the politics can be laid to rest and a firm consensus based on scientific fact is established.

Call staff writer Bradley J. Fikes at 760-739-6641. Read his blogs at bizblogs.nctimes.com.