DURHAM, N.C. -- At least 10 to 30 percent of global warming measuredduring the past two decades may be due to increased solar output ratherthan factors such as increased heat-absorbing carbon dioxide gasreleased by various human activities, two Duke University physicistsreport.
The physicists said that their findings indicate that climate modelsof global warming need to be corrected for the effects of changes insolar activity. However, they emphasized that their findings do notargue against the basic theory that significant global warming isoccurring because of carbon dioxide and other "greenhouse" gases.
Nicola Scafetta, an associate research scientist working at Duke'sphysics department, and Bruce West, a Duke adjunct physics professor,published their findings online Sept. 28, 2005, in the research journalGeophysical Research Letters.
West is also chief scientist in the mathematical andinformation sciences directorate of the Army Research Office inResearch Triangle Park.
Scafetta's and West's study follows a Columbia Universityresearcher's report of previous errors in the interpretation of data onsolar brightness collected by sun-observing satellites.
The Duke physicists also introduce new statistical methods that theyassert more accurately describe the atmosphere's delayed response tosolar heating. In addition, these new methods filter outtemperature-changing effects not tied to global warming, they write intheir paper.
According to Scafetta, records of sunspot activity suggest thatsolar output has been rising slightly for about 100 years. However,only measurements of what is known as total solar irradiance gatheredby satellites orbiting since 1978 are considered scientificallyreliable, he said.
But observations over those years were flawed by the spaceshuttle Challenger disaster, which prevented the launching of a newsolar output detecting satellite called ACRIM 2 to replace a previousone called ACRIM 1.
That resulted in a two-year data gap that scientists had torely on other satellites to try to bridge. "But those data were not asprecise as those from ACRIM 1 and ACRIM 2," Scafetta said in aninterview.
Nevertheless, several research groups used the combinedsatellite data to conclude that that there was no increased heatingfrom the Sun to contribute to the global surface warming observedbetween 1980 and 2002, the authors wrote in their paper.
Lacking a standardized, uninterrupted data stream measuring anyrising solar influence, those groups thus surmised that all globaltemperature increases measured during those years had to be caused bysolar heat-trapping "greenhouse" gases such as carbon dioxide,introduced into Earth's atmosphere by human activities, their paperadded.
But a 2003 study by a group headed by Columbia's RichardWillson, principal investigator of the ACRIM experiments, challengedthe previous satellite interpretations of solar output. Willson and hiscolleagues concluded, rather that their analysis revealed a significantupward trend in average solar luminosity during the period.
Using the Columbia findings as the starting point for theirstudy, Scafetta and West then statistically analyzed how Earth'satmosphere would respond to slightly stronger solar heating.Importantly, they used an analytical method that could detect thesubtle, complex relationships between solar output and terrestrialtemperature patterns.
The Duke analyses examined solar changes over a period twice aslong -- 22 versus 11 years -- as was previously covered by anothergroup employing a different statistical approach.
"The problem is that Earth's atmosphere is not in thermodynamicequilibrium with the sun," Scafetta said. "The longer the time periodthe stronger the effect will be on the atmosphere, because it takestime to adapt."
Using a longer 22 year interval also allowed the Dukephysicists to filter out shorter range effects that can influencesurface temperatures but are not related to global warming, their papersaid. Examples include volcanic eruptions, which can temporarily coolthe climate, and ocean current changes such as el Nino that affectglobal weather patterns.
Applying their analytical method to the solar output estimatesby the Columbia group, Scafetta's and West's paper concludes that "thesun may have minimally contributed about 10 to 30 percent of the1980-2002 global surface warming."
This study does not discount that human-linked greenhouse gasescontribute to global warming, they stressed. "Those gases would stillgive a contribution, but not so strong as was thought," Scafetta said.
"We don't know what the Sun will do in the future," Scafettaadded. "For now, if our analysis is correct, I think it is important tocorrect the climate models so that they include reliable sensitivity tosolar activity.
"Once that is done, then it will be possible to better understand what has happened during the past hundred years."
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