Sun-like stars may offer clues to climate.

Sun-like stars may offer clues to climate

Satellite measurements show that the sun's brightness has varied by about 0.1 percent during the current 11-year solar cycle, but astronomers lack precise records of earlier cycles. Two researchers now propose a way to extend the record back for centuries by comparing solar activity with observations of other stars. Their approach may yield clues not only to past trends in solar activity but also to future climate trends on Earth.

The key, says Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lies in comparing changes in the sun's surface activity with those of other stars of similar age and mass. Some of these solar-type stars show variations as great as 0.4 percent, up to four times those observed for the sun. These changes presumably reflect the coming and going of "starspots," say Baliunas and Robert Jastrow of Dartmouth College in Hanover, N.H. They described their work this week at a meeting of the American Astronomical Society in Arlington, Va.

Though representing only the current solar cycle, data from the Earth-orbiting Solar Maximum Mission and Nimbus 7 satellites suggest the sun's activity rises and falls with changes in its magnetic field, Baliunas reports. Solar magnetic variations leave a long-term record in the form of carbon-14 in tree rings, she notes, because when the magnetism is stronger, less carbon-14 is produced in Earth's upper atmosphere and less finds its way into the tree rings. "The amount of carbon-14 in trees over time maps the history of solar magnetism," she says, and "the tree rings thus provide a timeline of the magnetic history of the sun." One well-known period of low solar magnetism identified from tree-ring studies is the Maunder minimum, which lasted from about A.D. 1640 to 1710. Researchers have remained uncertain, however, whether solar magnetism correlates with brightness from solar surface events.

For the last 23 years, scientists working at Mt. Wilson Observatory near Loss Angeles have tracked the magnetic activity of 13 solar-type stars by measuring the intensity of their calcium emissions. Baliunas, who is involved in the effort and who now draws upon its data, says variations in these emissions show that nine of the stars go through activity cycles ranging from about eight to 13 years. Activity levels of the remaining four have remained almost flat, as though they are in the process of undergoing Maunder minima of their own.

"Our sun has spent about a third of its time over the last millennium in this relatively inactive state," Baliunas says, calling the Mt. Wilson project "a portrait of what the sun would do over time, if we waited long enough."

If the Maunder minimum on Earth indeed reflects a lower solar brightness during that 70-year period -- a possibility the Mt. Wilson observations support -- the correlation "has implications for the explanation of climate changes over time scales of decades to centuries," Baliunas and Jastrow say. Climate forecasters and others have long espoused the notion of a "solar constant," in which the long-term brightness remains unchanged despite short-term cyclic fluctuations. however, the researchers say, "confirmation of these tentative results would call into question the assumption of a constant sun in current [long-range] climate forecasts."