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

Judging from these other stars, Baliunas thinks the sun could brighten or dim by 0.6 percent over a period of several decades to a century. If so, the solar changes could give Earth's climate a considerable push.

In the minds of some scientists, there are hints that the sun has nudged the climate during the past century. Just as the global average temperature has risen about 0.5 [degrees] C over this period, the number of sunspots has increased with each cycle. Researchers have long noted this relationship and wondered whether to consider it cause and effect or just coincidence.

Last year, two Danish researchers pushedd the connection one step further when they compared the length of each sunspot cycle with the average temperature of the northern hemisphere (SN: 12/7/91, p. 380). The two curves look astoundingly alike, prompting the researchers to suggest that the global warming over the last century resulted mostly from solar variations rather than from the buildup of greenhouse gases.

While that correlation has intregued researchers, few give it much weight because there is no evidence that solar output did indeed change dramatically over this period. "That's just another one of those mathematical correlations with no physics behind it," says james Hansen of NASA's Goddard Institute for Space Studies.

To show how farfetched correlations could be, Eddy once noted that the period of the Maunder minimum coincided almost exactly with the reign of the Sun King, Louisd XIV, who ruled France between 1643 and 1715. Should one conclude from that correlation that the solar variations produced le Roi Soleil?

From their comparison of the sun with other stars, Baliunas and Jastrow suggest that the sun could dim enough to explain the unusually cold conditions during the peak of the Little Ice Age. Yet not all stars were created equal, and this leads many scientists to question whether the sun behaves just like these other stellar bodies.

To address th sun's potential for change, solar physicist Judith Lean of the Naval Research Laboratory in Washington, D.C., and her colleagues have looked directly at the subject in question. Like Baliunas and Jastrow, Lean's group took an approach that revolves around observations of th sun's calcium emmissions, which relate to the vigor of the magnetic activityd on the sun's surface.

This magnetic energy creates many of the patterns that appear on the sun. During times when magnetic disturbances grow stranger, at the peak of the solar cycle, dark sunspots and bright regions called faculae show up on the sun's surface. In the background, there is an ever-present pattern of bright lines, called the network, that also stems from magnetic activity.

When sunspots and faculae all but disappeared during the low part of the last solar cycle, total solar radiation dropped by only 0.1 percent. So Lean and her colleagues decided to explore even more dramatic solar changes. How much further would the sun's output dim if the network, as well as sunspots and faculae, vanished?