Despite old evidence suggesting that greenhouse gases and pollution cause global warming, new research by two Duke physicists indicates that the sun may simply be getting hotter.

Inspired by research from Columbia University indicating that current data on solar output was erroneous, Nicola Scafetta, research associate in Duke's physics department, and Bruce West, adjunct physics professor, examined solar changes over the past 22 years to determine the sun's direct role in global warming.

What they found contradicted previous thoughts and studies regarding global warming trends. Since the 1980s scientists have believed that global warming was not influenced by increased heating from the sun.

"The sun may have minimally contributed about 10 to 30 percent of the 1980 to 2002 global surface warming," Scafetta and West said in their report, which was published Sept. 28 in Geophysical Research Letters, an online research journal. This contribution to global warming is higher than what researchers previously thought.

Scafetta and West said the 22-year interval they used was longer than the time studied by most current researchers, which allowed for more accurate results.

Scafetta and West introduced new statistical methods to test their hypothesis. The new methods better described the atmosphere's delayed response to solar heating and filtered out temperature-changing effects from the sun unassociated with global warming, they claimed.

"The actual role of solar variability is very contentious because the evidence is contentious," said Thomas Crowley, professor at the Nicholas School of Environment and Earth Sciences. "Sometimes you find lots of evidence and sometimes you don't."

Scafetta and West stressed that their findings do not completely contradict the previous evidence that global warming is caused by carbon dioxide and other greenhouse gases linked to human production.

Although these causes do play a role, their research indicates the sun's direct role in global warming has been underestimated, Scafetta and West said.

"If the solar activity increased during the last 25 years, this means that we cannot neglect the solar activity for evaluating the global warming phenomenon," Scafetta added.

Crowley said he does not believe the findings will radically revise the current thinking on global warming.

"It will require the climate modeling community to look at the way they configure and estimate the amount of global warming," he said.

Scafetta and West hope their findings will increase understanding of what has happened in terms of global warming and solar output during the last century.

"For now, if our analysis is correct, I think it is important to correct the climate models so that they include reliable sensitivity to solar activity," Scafetta said in the report.

Despite this new research, many unknowns regarding global warming still exist.

The future of global warming probably depends on natural effects, and the strength of these effects is unknown, Scafetta noted.

"If I were to make a guess, I think 10, 20, 30 years from now, the global temperature might still increase, even if the solar activity would decrease even a little bit," he said.