Contact: Bob Nelson For immediate release
(212) 854-5573 Sept. 30, 1997
Solar Activity May Contribute to Global Warming,
Study by Columbia Researcher Finds
Satellite measurements of solar brightness analyzed by a Columbia
University researcher show an increase from one cycle of sunspot activity to the
next, indicating the Earth is absorbing more energy from the sun over the long
term. The finding could well mean the sun is contributing to global warming.
Solar radiation will not displace the dominant role of atmospheric carbon
dioxide in global warming, but could be a significant contributing factor,
according to the new report, by Richard C. Willson, senior research scientist at
Columbia's Center for Climate Systems Research, in the Sept. 26 issue of the
Greenhouse warming, in which gases created by human activity trap more
solar heat in the atmosphere, is expected to increase temperatures on Earth by
about 3.6 degrees Fahrenheit over the next 50 to 100 years. By contrast, according
to Dr. Willson, solar forcing - the sun's effect on long-term climate - might
account for between 0.7 and 1.4 degrees of warming over the next 100 years, if
sustained at the pace his observations suggest. The globe has already warmed by
about one degree since 1880, scientists say.
"Solar forcing would provide only about one-fourth as much warming, if the
solar trend persists over the same period," Dr. Willson said. "Solar forcing could
be significant, but not dominant."
Solar luminosity is closely linked to sunspot activity, which waxes and
wanes over a cycle that lasts about 10 to 11 years. The new research shows
dramatically that total solar irradiance, or the total radiant power received by the
Earth from the sun, was about 0.036 percent higher in 1996 than in 1986, a finding
that may mean that the luminosity of each new solar cycle is stronger than the
"The change that Willson measures is small," said James E. Hansen,
adjunct professor of earth and environmental science at Columbia and director of
the NASA Goddard Institute for Space Studies. "By itself, it would not be a
significant source of climate change, but the question is whether there are
changes on the century time scale. There, the changes may be significant. His
measurements are the first indication that there is long-term change."
Dr. Willson, who began the research at NASA's Jet Propulsion Laboratory
in Pasadena, Calif., and now pursues it under NASA auspices from his home in
Altadena, Calif., spliced together data from three probes in Earth orbit that
together have monitored the sun since 1978: ACRIM I (Active Cavity Radiometer
Irradiance Monitor) and ACRIM II, and the less precise Earth Radiation Budget
Satellite (ERBS). The two ACRIM satellites were able to calibrate the degradation
of their own sensors, a phenomenon that results from exposure to the sun's high-
energy, short-wavelength radiation; the ERBS was not. Comparisons in space are
essential in monitoring total solar irradiance precisely, Dr. Willson said.
ACRIM II was launched in September 1991, nearly two years behind
schedule and after ACRIM I had already ceased gathering data, preventing
comparisons between data from the two more sophisticated probes. To relate the
measurements from the two ACRIM experiments precisely, Dr. Willson had to
rely on ERBS data gathered during periods of overlap with the ACRIM sensors.
Because the Earth's atmosphere absorbs or reflects much solar radiation, direct
measurements of the sun's energy have only been possible since the late 1970s,
when solar probes were lofted into orbit above the atmosphere.
The satellite data show that total solar irradiance reached a low point in
1986, climbed to a peak in about 1991, then declined to another low in 1996. "The
significant finding is that the minimum total solar irradiance in 1996 is clearly
higher than the one ten years earlier," Dr. Willson said.
What determines how much energy the sun's fusion mechanism pumps
out is not well understood. Sunspots, the most visible evidence of solar dynamics,
are cooler, darker regions that emit only 30 to 80 percent as much light as the
normal sun, with temperatures ranging from 8,000 to 10,000 degrees F. Normal
surface temperatures on the sun are about 11,000 degrees F. One of the
discoveries of Dr. Willson's ACRIM I experiments was that periods of high
sunspot activity, counter to intuition, actually result in increased total solar
irradiance because sunspots are accompanied by regions of increased brightness
that overpower the dimming from sunspots.
The greatest correlation between sunspot activity and Earth's climate
occurred between 1640 and 1720, when solar activity in the form of sunspots and
other phenomena declined and temperatures in northern Europe fell by two
degrees Fahrenheit; the period is sometimes called the "Little Ice Age."
Dr. Willson holds a doctorate in atmospheric physics from the University of
California, Los Angeles, and was a scientist at the California Institute of
Technology until 1995, when he joined Columbia's Center for Climate Systems
Research. He has developed versions of the active cavity radiometer and used
them to measure total solar irradiance in balloon, rocket, space shuttle and
satellite experiments, and expects to launch his ACRIM III probe in 1999 as part
of the Earth Observation Program of NASA's Mission to Planet Earth. His work
is supported by the National Aeronautics and Space Administration.
The Center for Climate Systems Research, the University's partnership
with NASA Goddard Institute to understand and predict climate change, is a part
of the Columbia Earth Institute, launched in January of this year to promote wise
stewardship of our planet.
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