Harvard Gazette: Brightening Sun is Warming Earth


Richard Moore

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Brightening Sun is Warming Earth
May account for major part of global warming
By William J. Cromie
Harvard Gazette Staff

There is a better explanation for global warming than air pollution, two Harvard
researchers say: the Sun is increasing in brightness and radiance.

"Changes in the Sun can account for major climate changes on Earth for the past 
300 years, including part of the recent surge of global warming," claims Sallie 
Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics 

"We're not saying that variations in solar activity account for all of the 
global rise in temperature that we are experiencing," cautions her CfA 
colleague, astrophysicist Willie Soon. "But we believe these variations are the 
major driving force. Heat-trapping gases emitted by smokestacks and vehicles -- 
the so-called greenhouse effect -- appear to be secondary."

If that conclusion proves true, it promises a huge economic and political impact
on the "third rock from the Sun." The Clinton Administration is trying to 
negotiate an international treaty to gradually reduce greenhouse pollutants 
without bringing economic havoc to industries that satisfy our enormous appetite
for the energy that comes from burning oil, coal, and gas.

Other world leaders and environmentalists are pushing for immediate action, but 
Baliunas thinks there is time to carefully consider what action to take. "The 
best models of global warming call for a very slow temperature rise of less than
two degrees in the next 100 years," she has told various congressional 
committees and briefings. "There is time for more research and a measured 
response because the penalty you pay in increased temperatures from greenhouse 
warming is small."

Anything that's cost-effective to cut emissions can be done right away, Baliunas
says. Dramatic cuts with high economic penalties might be postponed in the 
expectation that more effective and affordable technologies will become 
available in the next 25 years or so.

To ease the economic burdens, President Clinton has proposed various incentives.
These include offering $5 billion in tax breaks for businesses to conserve 
energy and to develop new technologies, such as efficient electric cars and fuel
cells that burn clean hydrogen. Vice President Al Gore described these 
incentives last Friday in a talk at the Kennedy School of Government.

A Bright Connection

Baliunas and Soon base their ideas about the cause of global warming on 
irrefutable evidence that sunlight is getting stronger. Since the late 1970s, 
three Sun-watching satellites recorded surprising changes in heat, ultraviolet 
radiation, and solar wind. The radiation alters the paths of winter storms; 
solar winds affect cloudiness and rainfall.

The increased activity, everyone agrees, is tied to a cycle that sees the Sun 
dimming, then brightening, every 11 years or so. From the late 1970s to 
mid-1980s, activity on Earth's star declined. Since then it has risen, declined,
then risen again. The satellites measured an increase in brightness of as much 
as 0.14 percent on the latest rise.

Two unknowns, however, prevent Sun-watchers from making any useable forecasts 
about the next five years. No one knows why the Sun cycles like it does, or when
it will reach its next maximum. The best guess is the year 2000.

Also, a 0.14 percent jump in brightness is not enough to account for the 
approximately 1 degree F rise in temperature on Earth in the past 100 years. 
What's more, various observations show that our planet is almost 2 degrees F 
warmer than it was around the year 1700.

Baliunas quickly points out that the satellite measurements apply to only one 
cycle, and evidence exists that the estimated jump in brightness over several 
previous cycles was almost four times as much -- 0.5 percent.

Also, looking elsewhere in the Milky Way reveals larger shifts in brightness of 
other Sunlike stars. Twenty years ago, when still a Harvard graduate student, 
Baliunas took over a project that has been recording brightness changes in such 
stars of between 0.1 and 0.7 percent.

"A change of 0.5 percent in brightness sustained over several past cycles could 
account for the 2 degree change in climate we have experienced since the 
beginning of the 18th century," Baliunas maintains. "We don't know if this 
actually happened, but it indicates that the Sun is a major driver of climate 
change. We cannot ignore its variations when accounting for the present global 

Sun Spots and Storms

What is more, these Baliunas-Soon assumptions consider only brightness changes. 
Also increasing during the maximum part of solar cycles are invisible but potent
ultraviolet rays which heat up Earth's atmosphere and change the paths of winter

This radiation hits oxygen molecules in the upper stratosphere and converts them
to ozone. Some 25 miles above our heads, the ozone layer is best known for 
screening out ultraviolet radiation implicated in skin cancer, cataracts, and 
crop damage. However, researchers at Harvard's Department of Earth and Planetary
Sciences have found that increased amounts of ozone interfere with movement of 
heat from the equator to the poles. That, in turn, shifts the pattern of jet 
streams that steer the storms around the planet.

Exactly how this contributes to warming Earth during maximum solar activity, and
to cooling it during minimums, remains a mystery. "Our uncertainty is enormous,"
Soon admits, "but we can't omit ultraviolet forcing as a factor in global 

The most striking markers of the Sun's waxings and wanings are the coming and 
going of black spots on its face. Sunspots mark areas where strong magnetic 
fields exit and enter the surface of the Sun. They are about a thousand degrees 
cooler than the bright areas that surround them, but are still incandescently 

These spots not only follow an 11-year cycle; they also cycle through longer 
periods of high and low magnetic activity. When the Sun boasts a maximum of 
spots, cycle after cycle, Earth tends to be warmer than when its face is clear.

During the years from 1640 to 1720, for example, observers counted abnormally 
few sunspots and Earth's climate entered a period of unusually cold weather. 
Since the mid-1960s, solar magnetism has been increasing along with global 

At such maximums, the wind of magnetic fields and charged particles that 
normally wafts across the 93 million miles from Sun to Earth blows harder. These
gusts can trigger colorful displays of auroral lights during long polar nights. 
The strongest winds may also disrupt long-range radio communications, cause 
power outages, and disturb the operation of satellites.

Solar winds also produce radioactive carbon atoms in the atmosphere that 
eventually rain down and become assimilated into tree rings. High solar winds 
lead to rings with fewer radioactive atoms and vice versa. Changing levels of 
radiocarbon provide a natural record of magnetic changes on the Sun that can be 
matched with weather records of coldings and warmings.

"There have been 19 cold periods in the past 10,000 years and a decrease in 
solar magnetic activity can be linked to 17 of them," Baliunas notes.

Exactly how this happens remains unknown. It probably involves both changes in 
energy and variations in electrical charges on drops of water in the atmosphere.
The drops provide seeds for the formation of clouds which add to natural and 
greenhouse warmings.

Neither Baliunas nor Soon ties these changes to El Niño, the periodic warming of
the tropical Pacific Ocean that brings mostly unwanted weather changes from 
India to Indiana. "There is no solar cycle with the same 4-to-7-year period and 
no known direct connection with changes on the Sun," Baliunas says.

"Over longer periods, both the ultraviolet radiation and the particles in solar 
winds alter the balance of energy in the atmosphere, affecting the movement of 
winds," Soon points out. "Together with changes in brightness, these mechanisms 
must affect longer-term changes in climate. All the records we have of climate 
tie it to variations in the Sun. It is reasonable to assume that that effect 
persists at the present time."

No one doubts this; but the magnitude of its influence on global warming remains
in question. However, a significant number of researchers insist that solar 
changes are not great enough to produce the warming we are experiencing. They 
maintain that human activity is the main cause of rising temperatures that 
threaten widespread flooding, increased storminess, and potentially disruptive 
shifts in croplands. It is this group that wants to take immediate action to 
reduce heat-trapping air pollutants.

Baliunas and Soon maintain that interest in and understanding of solar effects 
will increase faster than rising temperatures, allowing time to study the 
Sun-climate relationships.

"But," Baliunas admits, "I am addressing scientific issues. Economic, political,
and environmental considerations are quite another story."

Copyright 1998 President and Fellows of Harvard College

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