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

After years spent in a frustrating search, renowned astronomer Giovanni Domenico Cassini must have wondered if he would die before seeing a legendary sunspot grace the heavens again. These dark blemishes on the sun's face, so common during the days of Cassini's youth, had virtually disappeared by the time he started observing the sky in the mid-17th century. So rare were sunspots, in fact, that any reported sighting caused a celebration among European astronomers of the day. When Cassini finally spied one in 1671, the editor of the PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON felt obliged to describe the last recorded sighting in 1660, in case some readers had forgotten what a sunspot was during the intervening years.

Today, the idea of partying over a sunspot observation seems ridiculous. When the sun reaches its most active state, every 11 years or so, astronomers can count more than 200 in a single year. Even during the quiet part of the solar cycle, a few dark marks will march across the surface of the sun each year.

The dearth of sunspots during Cassini's time is far from just a footnote to history, for it ties into the hot '90s issue of climate change here on Earth. If the cycle of sunspots could change so dramatically since the 17th century, solar scientists wonder, could the sun's overall brightness wax or wane enough to alter Earth's climate?

Some skeptics of greenhouse warming push that idea even further, predicting that the sun will weaken significantly in the next century -- an effect that would mitigate the predicted global warming caused by greenhouse gas pollution. That idea -- voiced in 1989 by the Marshall Institute of Washington, D.C., a science policy organization -- reportedly echoed all the way to the White House, where it fit with the President's policy of stressing the uncertainties in the climate change arena.

All these suggestions, however, inhabit the realm of speculation. Because reliable observations of the sun's brightness go back only 14 years, it remains unclear how much the sun can vary over a few decades or longer. Yet new studies are beginning to take a more serious look at whether the sun has the muscle to battle the greenhouse warming.

"The tendency has been to say that the sun doesn't have a role [in climate change] because there is no information. But there is a hint that the sun does have a role....I think it's pretty important to resolve this issue," says Sallie Baliunas, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

For the better part of 200 years, theories linking solar variations to conditions on Earth's surface have bounced in and out of vogue almost as often as necktie fashions. In the middle of this century, solar physicists shied away from this field of research because it still bore a stigma from the days when Charles Greeley Abbot, director of the Smithsonian Institution's Astrophysical Observatory, maintained he could predict the weather at various sites on Earth on the basis of solar variations.

"When I got into this work in the early 1970s, you could count on one hand the number of solar physicists who were willing to stand up as practitioners of the study of the sun's effect on climate," says Peter V. Foukal of Cambridge (Mass.) Research and Instrumentation, Inc.

But the black cloud over this field started to fade after solar researcher John A. Eddy published a now famous paper in SCIENCE in 1976 documenting the sunspot drought Cassini had witnessed.

Although astronomers had long remarked on the peculiar lack of sunspot sightings in the late 17th and early 18th centuries, many researchers had written this off as a problem stemming from inadequate telescopes. Eddy presented convincing evidence that the 11-year sunspot cycle did indeed die out between 1645 and 1715 -- a time he called the Maunder minimum after a 19th-century astronomer who first drew attention to the strange period. Besides sunspot sightings, Eddy also examined records of auroras, the solar corona, and carbon-14 levels stored in tree rings -- all of which indicate that the magnetic activity of the sun decreased substantially during this period.

Because the sun provides the energy that determines Earth's climate, many scientists have wondered whether the solar changes in the Maunder minimum altered conditions here on our own planet. After all, this time corresponds to the coldest part of the so-called Little Ice Age, which lasted from the 16th century to the mid-19th century. With average global temperatures roughly 1 [degree] C lower than today's, the decades of the Maunder minimum were among the coldest periods Earth has seen since the end of the last true ice age 11,000 years ago.

A disappearance of sunspots would not in itself exert much effect on the overall warmth of the sun, so it shouldn't have had a major impact on Earth's climate. But the sunspot deficiency could have been symptomatic of some change in the sun's total brightness -- the so-called solar constant. Eddy suggested that the frigid conditions could have developed if the sun's total radiation had dimmed by only 1 percent.