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March 20, 2003
NASA Study Finds Increasing Solar Trend That Can Change Climate
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 NASA's Nimbus 7 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 Nimbus 7/ERB and ERBS measurements overlapped the
ACRIM 'gap.' Using Nimbus 7/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.
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Contacts:
Elvia H. Thompson
Headquarters, Washington
(Phone: 202/358-1696)
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-3026)
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 Current Solar Cycle 23
During Maximum
This image from the Solar and Heliospheric Observatory
(SOHO) Extreme ultraviolet Imaging Telescope (EIT) image shows large
magnetically active regions and a pair of curving erupting prominences
on June 28, 2000 during the current solar cycle 23 maximum. Prominences
are huge clouds of relatively cool dense plasma suspended in the Sun's
hot, thin corona. Magnetically active regions cause the principal total
solar irradiance variations during each solar cycle. The hottest areas
appear almost white, while the darker red areas indicate cooler temperatures.
Credit: NASA & European Space Agency (ESA)
 Largest Sunspot Group
Observed During Current Solar Cycle
Active region 9393 as seen by the Michelson Doppler
Imager (MDI) instrument on SOHO hosted the largest sunspot group observed
so far during the current solar cycle. On March 30, 2001, the sunspot
area within the group spanned an area more than 13 times the entire surface
of the Earth! Sunspots like these can cause temporary decreases in total
solar irradiance of the Earth of ~ 0.2 % as they pass across our side
or the Sun during a week or ten days. This amount of missing irradiance
is insignificant compared to the Sun?s total energy output but equivalent
to all that man produces and consumes in a year! Credit: ESA & NASA
 New Sun/Old Sun
This footage from the SOHO Extreme ultraviolet Imaging
Telescope (EIT) depicts a solar minimum (December 1996, on left) and
a solar maximum (January 2000, on right). The file shows three sequences;
the first with solar minimum and maximum side by side with dates; the
second shows the same sequence without dates; and the third depicts a
solar minimum with a transition to a solar maximum. The total solar irradiance
of Earth varies by about 0.1 percent over each solar cycle with more
occurring during solar maxima. Credit: NASA & ESA
 ACRIM Composite Showing
Total Solar Irradiance Trend Between Solar Minima
This graphic shows the recent trend of a .05 percent
per decade increase in Total Solar Irradiance (TSI) in watts per meter
squared, or the amount of solar energy that falls upon a square meter
outside the Earth?s atmosphere. The trend was measured between successive
solar minima that occur approximately every 11 years. At the bottom,
the timeline of the many different datasets that contributed to this
finding can be seen from 1978 to present. Credit: ACRIM3 Science Team
(R. Willson)
 Comparison Between Total
Solar Irradiance Trend and Greenwich Sunspot Numbers
By comparing the two plots, it can be seen that the
total solar irradiance is directly proportional to solar activity, a
fact discovered by NASA?s SMM/ACRIM1 experiment. The sunspot numbers
are representative of the general level of solar activity. The solar
cycle occurs approximately every 11 years. The peaks in the Sunspot graph
show the period of increased magnetic and sunspot activity called the "solar
maximum," and the valleys show a quiet period called the "solar minimum." Credit:
ACRIM3 Science Team (R. Willson)
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