A star in the greenhouse; can the sun dampen the predicted global warming? - use of solar observation in research

Science News, Oct 24, 1992 by Richard Monastersky

At the time of his article, Eddy did not know whether the solar constant could change at all. Although Abbot and other astronomers had tried to measure the sun's overall radiation using instruments on the ground, the measurements lacked sufficient accuracy because Earth's atmosphere obscures the minute changes that researchers seek. Only satellites could provide a definitive answer.

Two years after Eddy's paper came out, the United States launched the Nimbus 7 satellite, which carried a radiometer that could measure the sun's brightness from high above the interfering effects of Earth's atmosphere. Two years later, a second satellite, called the Solar Maximum Mission, went up carrying a radiometer specifically designed to measure total solar irradiance.

The satellite data clearly showed the term "solar constant" to be a misnomer: Over a period of a few weeks, the sun's total brightness varied by a much as 0.2 percent. Yet it took several years before scientists could detect any long-term waxing and waning of the constant. Watching from the peak of the solar cycle in 1980 to the minimum in 1986, researchers saw the sun's average brightness drop by 0.1 percent.

According to climate theory, such flickerings are too puny to cause a major warming or cooling on Earth. At most, solar variations of that order could change the planet's average temperature by about 0.2 [degree] C, says David Rind, a climate modeler at NASA's Goddard Institute for Space Studies in New York City. To explain the 1 [degree] C cooling of the little Ice Age relative to today, the sun would have to dim by five times the amount seen during the 1980s.

To an astronomer like Baliunas, that type of cooling is not out of the question. She and Robert Jastrow of Dartmouth College in Hanover, N.H., have addressed the solar variability issue indirectly, by looking at the behavior of other stars that belong to the same class as the sun. Observations of 13 well-studied solar siblings reveal that the stars split into two distinct groups: the cyclers and the noncyclers. In about two-thirds of the sun-like stars, magnetic activity waxes and wanes over a 7- to 15- year cycle very similar to the sunspot cycle of our own sun. The rest of the stars appear to lack any cycle of magnetic activity.

Baliunas and Jastrow proposed in the Dec. 6, 1990 NATURE that an individual star could go through both phases, sometimes cycling and other times not. At present, the sun ranks among the most active of the cycling stars, but Baliunas and Jastrow suggest that in the past it may have spent time as a noncycling star with little magnetic activity. According to the researchers, this scenario might explain what happened during the Maunder minimum activity recorded in tree rings and ice cores.

If so, the cycles of more distant stars might provide a sense of how much the sun could vary over the long term. "We've seen changes of up to 0.6 percent [in visible light] over one cycle," Baliunas says. "One of the ways to understand those results is to imagine that there is some longer-term change on the sun that isn't uncovered by looking at only one cycle. The other stars can be viewed as random snapshots of some longer term activity and, in time, the sun will also do what these stars are doing."