The blame for the global temperature rise in the last century is often laid on civilization and its genius for generating clouds of greenhouse gases that trap sunlight in the atmosphere. But another possible culprit is fast emerging.
For centuries, scientists have known that the Sun is less steady than sunbathing and casual observation suggest. It has seasons and storms and rhythms of activity, its sunspots and flares appearing in cycles roughly 11 years long. But only in the last decade or so have these and other kinds of solar variations begun to be tied to climate shifts on Earth -- first tenuously, and more solidly of late.
Today, a growing number of scientists contend that the Sun's fickleness might rival human pollution as a factor in climatic change. And some research, though sketchy and much debated, suggests that the Sun's variability could account for virtually all of the global warming measured to date.
Other experts dismiss the possibility of such an influence, even while saying the solar factor should be studied more thoroughly.
The Sun is now approaching a stormy period in its 11-year cycle, promising a wealth of new data (and possibly hot weather) that might help answer the riddle.
In recent years, Sun-Earth climate detectives have focused mainly on discovering statistical links between things like long-term fluctuations in the world's temperature and the sunspot cycle's length, which varies from eight to 15 years and averages 11 years.
Those links are now seen as firm. Increasingly, scientists are trying to pin down the physical mechanisms that might be at work. This is considered a crucial step because correlations can occur by chance, as is perhaps the case with crimes of violence and the phases of the Moon. But if scientists can discover a "why" that links the phenomena, they may be able to turn speculation into verifiable fact.
Experts are now scrutinizing three solar variables as likely agents of terrestrial change: the Sun's overall brightness, which is seen as affecting temperatures; the Sun's ultraviolet rays, which are seen as affecting winds and ozone production high in the atmosphere, and the Sun's storms of magnetic fields and subatomic particles, which are seen as affecting rainfall and the amount of cloud cover.
Many pieces of the puzzle, said Dr. Brian Tinsley, an atmospheric physicist at the University of Texas who works on the problem, are rapidly being "fitted together to suggest a fascinating picture."
Dr. Sallie L. Baliunas, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., said the breakthrough insight might occur tomorrow "or we could struggle for decades." She added: "It's maddening. I wish I had a crystal ball."
Interest in the field is rising fast. At its next annual meeting, the American Association for the Advancement of Science, the world's largest general scientific society, is holding a session titled "New Frontiers in the Sun-Earth Connection."
Experts say much of the ferment derives from a rush of physicists, astronomers and astrophysicists onto turf customarily trod by meteorologists and atmospheric scientists. In the past, such interdisciplinary forays have been quite fruitful, as when geologists and astronomers seized on cosmic bombardments to explain mass extinctions on Earth over the ages, going far beyond the theorizing of paleontologists.
"A lot more people are crossing lines, and that's very healthy," Dr. Albert Arking, an atmospheric physicist at Johns Hopkins University in Baltimore who studies Sun-Earth interactions, said in an interview.
Scientists are "looking over each other's shoulders instead of putting on blinders," he added. "That's tremendous. It's providing new insights."
The central fact that commands the attention of all scientists investigating climate change is that average global temperatures appear to be rising gradually, going up roughly one degree Fahrenheit between 1880 and the present. The question is why.
Most mainstream scientists say they are unsure about how much of the observed warming is due to natural causes and how much stems from human activities.
Conventional wisdom points to a significant role for heat-trapping gases like carbon dioxide, which is a natural part of the atmosphere but is also emitted by cars, factories and the burning of oil, coal and wood around the globe. Levels of this and other greenhouse gases are rising and are thought to be warming the planet by trapping sunlight that otherwise would be reflected back into space.
Heeding this logic, the nations of the world are to meet in December in Kyoto, Japan, to try to negotiate cuts in emissions of the heat-trapping gases. Not surprisingly, given the sketchiness of the science and the political difficulty of trying to curb industrial growth, debate is rising on whether such cuts are warranted.
Some experts contend that the majority of the century's warming occurred before 1940 and that most of the gas buildup occurred after that. Thus, they argue, only part of the temperature rise -- perhaps a small part -- can be attributed to human activity.
Clues to the Sun's effect on climate can be found in the many intriguing links between terrestrial shifts and solar activity. For instance, the snow layers of glaciers show curious variations in dust and chemical residues that appear to echo the solar cycle, as do records of tree growth and storm paths.
The biggest correlation of all occurred centuries ago, when the number of sunspots fell sharply between 1640 and 1720 and the Earth cooled about two degrees Fahrenheit. Northern Europe was hit especially hard, as its glaciers and winters lengthened.
At first, this chill made no sense to modern science. After all, sunspots are magnetic darkenings of the Sun whose temperatures are relatively low. It would seem that their disappearance for almost a century would mark a time of greater solar brightness and thus, if anything, higher temperatures on Earth, not the reverse.
The mystery began to lift in the late 1970s as satellites started to fly above the obscuring atmosphere to study the Sun directly. A decade later, the verdict was in. Contrary to intuition, the Sun was found to be brighter when sunspots abounded and dimmer when they vanished.
The reason was simple, at least in retrospect. It turned out that bright patches known as faculae, which accompany sunspots in the 11-year solar cycle, overpower the dimming effect of the dark blemishes. This brightening of the Sun has now been observed twice by satellite, and all the evidence suggests that the rhythm is ancient and variable.
The discovery toppled centuries of assumptions about the Sun's steadiness and overturned the modern dogma that its radiated energy represents a "solar constant." It also led scientists to study how the variable brightness might affect the Earth.
"We figure half the climate change from 1850 to now can be accounted for by the Sun," said Dr. Judith Lean of the Naval Research Laboratory in Washington, who is prominent in such studies.
She and her colleagues seek out historical temperature records to probe the likelihood that fluctuations in solar brightness heat and cool the Earth directly.
Others say this mechanism is stronger. Dr. Baliunas, of the Harvard-Smithsonian Center, and her co-workers studied records of the past 120 years and found the Sun responsible for up to 71 percent of the Earth's temperature shifts. When other factors were added to their research model, that figure rose to 94 percent.
Many experts say that is too high. They note that the Sun's overall radiance appears to ebb by only about one-tenth of 1 percent during the solar cycle -- which is too small to account for dramatic change on Earth, at least by direct heating.
So investigators are looking elsewhere. One suspect mechanism focuses on sunlight in the ultraviolet range. Short, potent and invisible, these rays account for most of the brightness change and have significant effects on Earth in addition to direct heating.
For instance, ultraviolet rays that strike the upper atmosphere break apart molecular oxygen (O subscript 2) to form ozone (O subscript 3), a three-atom type of oxygen that has wide atmospheric repercussions.
Dr. Joanna Haigh of Imperial College in London has found that increases in heat and ozone at solar maximums might account not only for temperature shifts but for changes in the paths of winter storms. Her study predicted that storms sweeping across the Mediterranean would move northward, on average, about 400 miles during a solar maximum, a shift that would be consistent with observed changes in real weather patterns.
As with direct heating, however, the ultraviolet mechanism is often found wanting.
The predicted effects "are still not quite as large as the observed changes" on Earth during the ups and downs of the solar cycle, Dr. Alan Robock, a meteorologist at University of Maryland, wrote in the May 17, 1996, issue of Science, the weekly journal of the American Association for the Advancement of Science.
Another possible mechanism is not light from the Sun but rather its winds of particles and magnetic fields, which change dramatically in strength during the solar cycle.
On Earth, the solar wind is well known to produce the aurora borealis, or northern lights. When it is blowing strong, it can also cause transformers at power plants to burst into flames, radio communications to fail and satellites to stop working or even fall from the sky.
Now the solar wind is getting increased attention as a possible engine of climate change, since its severity appears responsible for a number of terrestrial shifts on time scales from days to centuries. The envisioned mechanism is indirect but intriguing.
During peaks of the solar cycle, the Sun's magnetism is perturbed as it stretches past the Earth, blocking some of the cosmic rays that stream in from deep space. Conversely, times of solar quiet allow more cosmic rays to come through.
This rhythm is unambiguously reflected in the production of radioactive atoms high in Earth's atmosphere. For instance, times of low solar winds and high cosmic rays coincide with an increase in the amount of carbon-14 that rains down and is eventually incorporated by trees. The analysis of carbon-14 in tree rings gives a detailed history of solar magnetism.
Experts suspect that this same mechanism affects the production of clouds and rain, which are major factors in climate change.
Tinsley, of the University of Texas at Dallas, and his colleague Dr. Kenneth Beard of the University of Illinois at Urbana-Champaign have found evidence that a high flux of cosmic rays produces high electrical conductivity in the atmosphere and high electrical charges on tiny droplets of water at the tops of clouds, causing rain.
Tinsley found clear links between this mechanism and the paths and intensity of severe winter storms during the solar cycle. His work was intriguing enough to prompt a study of the electrical effect in June 1996 at the Los Alamos National Laboratory in New Mexico, where 14 papers were presented at a special workshop.
Skeptics say the solar ebb and flow is interesting but insufficient to explain satisfactorily the drama of climate change, which they insist is mainly driven by the rise in carbon dioxide.
Dr. James Hansen of the Goddard Institute for Space Studies in New York, a top proponent of the view that humans cause global warming, said in an interview that the effect of solar brightening on climate might be significant "but smaller than other mechanisms we already know about."
Sun researchers, on the other hand, say the solar mechanisms are just starting to be understood so much work is needed to understand how they interact and perhaps amplify one another. They say the knowledge of the Sun's role is destined to grow in the years ahead with the rise of new analyses and evidence, especially as a new solar maximum is reached around 2000.
Dr. Baliunas said that "our goal is to make the best model possible" so scientists can better understand the true role of human pollution in climate change.
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