Elvia H. Thompson
Headquarters, Washington
(Phone: 202/358-1696)

Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-3026)

Since the late 1970s, the amount of solar radiation the
sun emits, during times of quiet sunspot activity, has
increased by nearly .05 percent per decade, according to a
NASA funded study.

“This trend is important because, if sustained over many
decades, it could cause significant climate change,” said
Richard Willson, a researcher affiliated with NASA’s Goddard
Institute for Space Studies and Columbia University’s Earth
Institute, New York. He is the lead author of the study
recently published in Geophysical Research Letters.

“Historical records of solar activity indicate that solar
radiation has been increasing since the late 19th century.
If a trend, comparable to the one found in this study,
persisted throughout the 20th century, it would have
provided a significant component of the global warming the
Intergovernmental Panel on Climate Change reports to have
occurred over the past 100 years,” he said.

NASA’s Earth Science Enterprise funded this research as part
of its mission to understand and protect our home planet by
studying the primary causes of climate variability,
including trends in solar radiation that may be a factor in
global climate change.

The solar cycle occurs approximately every 11 years when the
sun undergoes a period of increased magnetic and sunspot
activity called the “solar maximum,” followed by a quiet
period called the “solar minimum.”

Although the inferred increase of solar irradiance in 24
years, about 0.1 percent, is not enough to cause notable
climate change, the trend would be important if maintained
for a century or more. Satellite observations of total solar
irradiance have obtained a long enough record (over 24
years) to begin looking for this effect.

Total Solar Irradiance (TSI) is the radiant energy received
by the Earth from the sun, over all wavelengths, outside the
atmosphere. TSI interaction with the Earth’s atmosphere,
oceans and landmasses is the biggest factor determining our
climate. To put it into perspective, decreases in TSI of 0.2
percent occur during the weeklong passage of large sunspot
groups across our side of the sun. These changes are
relatively insignificant compared to the sun’s total output
of energy, yet equivalent to all the energy that mankind
uses in a year. According to Willson, small variations, like
the one found in this study, if sustained over many decades,
could have significant climate effects.

In order to investigate the possibility of a solar trend,
Willson needed to put together a long-term dataset of the
sun’s total output. Six overlapping satellite experiments
have monitored TSI since late 1978. The first record came
from the National Oceanic and Atmospheric Administration’s
(NOAA) Nimbus7 Earth Radiation Budget (ERB) experiment (1978
– 1993). Other records came from NASA’s Active Cavity
Radiometer Irradiance Monitors: ACRIM1 on the Solar Maximum
Mission (1980 – 1989), ACRIM2 on the Upper Atmosphere
Research Satellite (1991 – 2001) and ACRIM3 on the ACRIMSAT
satellite (2000 to present). Also, NASA launched its own
Earth Radiation Budget Experiment on its Earth Radiation
Budget Satellite (ERBS) in 1984. The European Space Agency’s
(ESA) SOHO/VIRGO experiment also provided an independent
data set (1996 to 1998).

In this study, Willson, who is also Principal Investigator
of NASA’s ACRIM experiments, compiled a TSI record of over
24 years by carefully piecing together the overlapping
records. In order to construct a long-term dataset, he
needed to bridge a two-year gap (1989 to 1991) between
ACRIM1 and ACRIM2. Both the Nimbus7/ERB and ERBS
measurements overlapped the ACRIM ‘gap.’ Using Nimbus7/ERB
results produced a 0.05 percent per decade upward trend
between solar minima, while ERBS results produced no trend.
Until this study, the cause of this difference, and hence
the validity of the TSI trend, was uncertain. Willson has
identified specific errors in the ERBS data responsible for
the difference. The accurate long-term dataset, therefore,
shows a significant positive trend (.05 percent per decade)
in TSI between the solar minima of solar cycles 21 to 23
(1978 to present). This major finding may help
climatologists to distinguish between solar and man-made
influences on climate.

NASA’s ACRIMSAT/ACRIM3 experiment began in 2000 and will
extend the long-term solar observations into the future for
at least a five-year minimum mission.

For more information on the Internet, visit:
http://www.gsfc.nasa.gov/topstory/2003/0313irradiance.html

For more information about ACRIM on the Internet, visit:
http://www.acrim.com