By Dan Whipple

Source: ScienceNOW Daily News

09/13/06 

The sun has gone out--at least as a source of 20th century global warming. A paper in tomorrow's issue of Nature concludes that changes in the sun's brightness over the past 100 years have been too small to significantly impact Earth's climate.

Picture of sun
A color-enhanced view of the sun in which the faculae (white regions), which leak heat, are hotter than sunspots (red-black regions), which divert heat from the solar surface. Photo Credit: UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH

Whereas most studies have blamed greenhouse gases for increasing temperatures over the last century, some people say the sun should also bear some responsibility. At the peak of the its 11-year energy cycle--called the solar maximum--the sun's heat output increases 0.07%. Some climatologists and solar physicists have also speculated that there is another cycle that leads to longer-term increases in the sun's total solar irradiance (TSI).

Are these bursts in solar energy enough to explain Earth's warming over the last 100 years? Peter Foukal, a solar physicist at Heliophysics Inc., a small private research firm based in Nahant, Massachusetts, looked at variations in sun activity since 1978. He and others scanned historical records of sunspots, which are indicative of solar fluctuations, and examined radioisotopes from Greenland, Antarctic ice cores, and Earth's atmosphere, which provide a record of past solar activity. The researchers also ran a blend of seven reconstructions of paleoclimate data in a climate model used by the Intergovernmental Panel on Climate Change to test the impact of TSI. They then calculated how much the solar brightness contributed to the observed temperature changes compared to climate impacts from other sources such as volcanoes and human-caused greenhouse gases.

Although solar brightness has increased over the past 400 years, the team concludes that the amount is too small to explain the 20th century warming. Furthermore, no known physical mechanism in the sun can account for longer-term variations in solar activity, the researchers report.

Asked whether total solar irradiance can be discounted as a driver of the current warming, climatologist Thomas Wigley of the National Center for Atmospheric Research in Boulder, Colorado, one of the paper's authors, says simply, "Yes." After a pregnant pause, he adds, "I'm very categorical about that final answer. I feel very confident about that." Still, Wigley says other solar processes, such as cosmic rays or ultraviolet radiation, could still have an affect on Earth's climate. The current study did not examine these phenomena.

"I think that this is an important contribution," says paleoclimatologist Thomas Crowley of Duke University in Durham, North Carolina. "This makes it much more unlikely that solar irradiance can be invoked as a primary cause of warming."