Here comes the sun-climate connection.

A study of millennia-old pine trees growing on the upper reaches of the Sierra Nevada hints that flickerings in the sun's brightness altered Earth's climate in the past - a finding that suggests scientists must consider solar changes when forecasting global warming.

Solar physicists and geoscientists long have wondered whether hot and cold spells in Earth's history could have resuited from changes in the sun. As concern over global warming has surged in recent years, the solar issue has attracted renewed interest, particularly because some skeptics of warming have suggested that the sun will ameliorate the predicted temperature increase from greenhouse gas pollution.

But clear evidence of a sun-climate connection has remained elusive.

The new analysis of Sierra Nevada pines adds weight to the suggestion that the sun plays a noticeable role in changing Earth's climate. "l think this is some of the best proof we have to date that the sun is actually forcing climate," says Louis A. Scuderi of Boston University.

Scuderi studied the annual growth rings of foxtail pine trees living at the 3,600-meter-high timberline on the harsh, windswept slopes of Cirque Peak. In these dry heights, foxtail pines can survive more than 1,500 years. By comparing the living trees with dead, fallen snags, Scuderi constructed a 2,000-yearlong record of tree-ring width, which he reports in the March 5 SCIENCE.

Because tree growth at such altitudes depends largely on temperatures during the growing season, Scuderi could translate the growth data into a temperature record for the site. To test whether the sun caused part of those temperature swings, Scuderi matched the data with a record of atmospheric carbon-14 concentrations, also determined from tree rings. Carbon-14 forms when solar radiation enters Earth's atmosphere, producing energetic neutrons that collide with nitrogen in the air. The carbon-14 concentration thus provides an indirect measure of the sun's strength.

Comparing the temperature and carbon-14 records, Scuderi found that they exhibit similar cycles, particularly one with a period of 125 years. He also discovered that both records show major changes at the same times, around the years 1050 A.D. and 1650 A.D. Such similarities lead Scuderi to suggest that periodic solar variations induced the Cirque Peak temperature cycles.

In the 15 years of satellite observations, the sun's brightness has changed only by 0.1 percent -- too little to alter Earth's climate substantially. But Scuderi's study and others suggest that longer term solar fluctuations have had the strength to change the climate.

Minze Stuiver, a geochemist at the University of Washington in Seattle, says Scuderi's study presents a good case for a sun-climate link: "It's one of the better studies." But Stuiver cautions that a simple correlation between two factors does not necessarily indicate that one caused the other. Proponents of the sun's influence on climate have yet to propose a mechanism whereby such minor changes in solar brightness can cause major swings in climate on Earth.

If the sun does wax and wane enough to warm and cool the Earth, then solar variations may have caused part of the global temperature increase since the late 1800s. In the past, climate experts have presumed that carbon dioxide pollution caused much of that warming. But if the sun played a substantial role, then carbon dioxide may affect the climate less than researchers have presumed, which might reduce the expected global warming, Scuderi suggests.

Don't look to the sun to eliminate global warming, however. Two studies in the Nov. 26, 1992 NATURE conclude that the warming power of greenhouse gases outweighs the climatic influence of the sun. No matter what the sun does, the greenhouse effect still dominates in the future;' says Tom M.L. Wigley of the University Corporation for Atmospheric Research in Boulder, Colo., who coauthored one of the papers.