Are sunspots prime suspects in global warming?

Climate-change 'optimists' say complex natural cycles may be at the heart of global warming.

from the September 27, 2007 edition

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Generally, peak periods of high sunspot activity deliver more sunlight to the top of the atmosphere than periods of minimum activity. Scientists measure this "total solar irradiance," which includes infrared and ultraviolet light as well as visible light.

In 1970, Russian researchers using high-altitude balloons to measure sunlight reported a 2 percent rise in the sun's output as the sun moved from periods of little sunspot activity to peak activity. Today, using better measurements from satellites over the past 28 years, the change in total solar irradiance is estimated to be much smaller, between 0.05 percent and 0.07 percent. The most important component for climate-change purposes – visible light – represents about half of this change, says Tom Woods, a researcher at the University of Colorado's Laboratory for Space and Atmospheric Physics, based in Boulder.

'Pesky' correlations with sunspots

Last fall, solar physicists and climate scientists in the US and Europe reviewed the latest studies of changes in total solar irradiance driven by the 11-year sunspot cycle. They concluded that those changes are unlikely to have had a "significant influence" on global warming since the 1600s. In particular, satellite measurements since the late 1970s showed changes too weak to have "contributed appreciably to accelerated warming over the past 30 years."

The effect "is really small, unless you can come up with ways to amplify it," says Tom Wigley, a senior scientist at the National Center for Atmospheric Research in Boulder, who took part in the study.

Other studies suggest that changes in sunlight – as well as the cooling effect of volcanic activity, which sends sunlight-reflecting particles high in the sky – probably played a major role in climate during preindustrial times and even into the early 20th century. But even these find that CO2 emissions have dominated the scene over the past half century.

Some pesky correlations – such as the one between sunspot cycles and cloud cover – linger. This has led some scientists to ask if some process in the atmosphere may be boosting those tiny changes.

One candidate is UV light. During swings in sunspot cycles, the largest fractional changes in the sun's output occur in the ultraviolet range, Shindell notes. But much of that is absorbed by ozone in the stratosphere – which may be the connection, he suggests. The rise and fall of UV light can alter the amount of heat-trapping ozone in the stratosphere, changing its circulation patterns. These changes can work their way into the layer below, the troposphere, where weather and people meet. Instead of warming the troposphere, changes in solar UV output appear to redistribute warmth, chill, rainfall, and other conditions already present.

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