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1993-04-23
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PUMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 9ll09. TELEPHONE (213) 354-5011
FOR RELEASE A.M. TUESDAY APRIL 20, l982
A marked l8-month decrease in the Sun's energy
output,
recently detected by a NASA satellite experiment, may have
been a
factor in this year's unusually harsh winter, according to a
JPL
scientist.
Results from JPL's Active Cavity Radiometer
Irradiance
Monitor (ACRIM) experiment on the Solar Maximum Mission (SMM)
satellite, when viewed in the context of the severe U.S.
winter
of l98l-82, may be the first direct observation of the
cause-and-
effect relationship between variability in the Sun's energy
output and changes in Earth's weather and climate.
The instrument detected a persistent decrease of a
tenth of a percent in the total amount of solar energy
reaching
Earth (called solar irradiance) over an l8-month period from
February l980 to August l98l, according to physicist Dr.
Richard
C. Willson of Caltech's JPL, principal investigator and
designer
of the experiment.
"This is a small change in the total energy outputof the
Sun, but has great potential significance for the Earth's
fragile
ecosystem," Willson said. Results of the experiment are
being
studied by climatologists, and will be correlated with global
climate
indicators, like average temperatures, ice coverage and sea
level
to evaluate the effects of the solar irradiance decrease.
A systematic increase or decrease in the Sun's
release
of energy -- as little as one half percent per century -- can
pro-
duce vast changes in Earth's climate. Scientists believe
that a
one percent decrease would lower Earth's mean global
temperature
by more than l Kelvin (2 degrees Fahrenheit). According to
some
models, a decrease in solar energy of less than l0 percent
would
effectively freeze Earth's entire surface.
Nearly all life forms on Earth exist within the l0
kilometers (6.2 miles) of atmosphere or ocean above and below
the mean sea level. The temperatures within this thin
environ-
mental shell, called the biosphere, are determined by the
amount
of energy received by the Sun and delicate interactions
between
the atmosphere, ocean and land masses.
The climatic effects of short-term variations in
solar î
irradiance are moderated by the heat capacity of the ocean
and
atmosphere. A long-term increase or decrease, however, can
eventually change the temperature of the ocean and atmosphere
sufficiently to cause change in the weather and climate.
"These kinds of small but persistent trends in
solar
irradiance are believed to have been causes of climatic
changes
in the past," Willson said.
Solar magnetic activity reaches a maximum
approximately
every ll years. The peak of the current solar cycle (called
solar
cycle 2l), occured in early l980, about the time SMM was
launched.
The irradiance decrease detected by ACRIM may be related to
the
general decline in solar activity since then. The decrease
might,
however, be an indication of a longer-term trend in the Sun's
irradiance. Years of careful measurements would be required
to
identify such a trend.
During its two years of operation, ACRIM also
observed
short-term increases and decreases, lasting from days to
weeks,
in the amount of solar energy that reaches Earth. Analysis
indicates the decreases are the effects of sunspots, dark,
cooler patches on the Sun, while increases are caused by
faculae,
îwhich are bright, extra-hot solar regions.
ACRIM also detected evidence of solar oscillatory
phenomena -- global pulsations whose effects extend deep into
the Sun. The oscillations were found to occur with a
five-minute
periodicity. ACRIM's observations of five-minute
ocsillations
match similar results of ground-based observers who
discovered
the effect in the l970s. Study of this phenomenon, so-called
"solar seismology," will provide new information on the inner
workings of the Sun that cannot be obtained by other means.
During most of Earth's history, the climate appears
to
have been considerably warmer, with average global
temperatures
about 25 degrees C (77 degrees F) The current average global
temperature is l5 degrees C (59 degrees F).
A gradual trend to a cooler climate began about l00
million years ago, resulting in the glacial climate of the
last 20 million years. At least four major glacial epochs,
each
lasting nearly l00 million years, have occured in the last
bil-
lion years. The last epoch ended 250 million years ago. The
present glacial period may yet prove to be another major
epoch.
There have been two particulaly cold periods during
î
the last l00 million years. The coldest period was about 25
million
years ago when Earth's average temperature was as cold as 5
degrees
C (4l degrees F). Sixty-five million years ago the average
temperature was about l0 degrees C (50 degrees F). Large
numbers
of living creatures that had flourished in the warmer climate
of
the previous 200 million years became extinct during this
cold
period.
At the peak of the last major ice age l8,000 years
ago,
the global average temperature was about 5 degrees C (9
degrees F)
colder than today. Massive ice sheets covered much of the
continents
in the northern hemisphere.
The climate of the last l0,000 years has been a
relatively
warm one for a glacial epoch. During this period, several
so-called
"little ice ages" have occured. The most recent occured
during
the l5th through l7th centuries, when the average global
temperature
decreased by l.5 degrees C (about 3 degrees F), shortening
the
agricultural growing seasons and causing glacial ice to
increase
in volume.
The existence of climate changes in the past are
well-known,
but their specific causes are more difficult to ascertain. During
the last billion years, the most likely causes are changes in
the
composition of Earth's atmosphere, continental drift (which
changed the circulation of the ocean and atmosphere), and
changes
in solar irradiance. Some climate variations have been tied
to
specific episodes of continetal drift, to atmospheric
chemistry
change, and to known cyclic changes in the orbit of the Earth
and
its orientation to the Sun. Many of the major climate
variations
were probably the result of a combination of these factors,
including
solar variability.
In December l980, after l0 months of normal
operation,
the SMM satellite's attitude control system lost its
capability
to point precisely at the Sun. A less precise pointing
technique
was subsequently achieved by spinning the spacecraft so that
it
rotates every six minutes. In this configuration, three of
the
satellite's seven instruments continue to acquire useful
data.
The Sun crosses ACRIM's field of view several times
per orbit, providing an adequate quality and quantity of data
for the experiment's primary mission objectives, though the
data are deficient in some solar physics information (like
the î
observation of solar oscillations).
While the spin-stabilized pointing allows SMM to
continue studying the Sun, the satellite's 550 kilometer-
(340
mile-) orbit is slowly decaying due to atmospheric drag.
At the present rate of decay, the SMM satellite
will
reenter Earth's atmosphere in l984. Its demise will leave at
least a three-year gap in the precise record of solar
irradiance
observations made for the National Climate Program. The
earliest
successor experiment to ACRIM is planned for deployment on
NASA's
Upper Atmospheric Research Satellite (UARS) in l987.
SMM was the first NASA satellite designed to be
retrieved by the space shuttle. An effort to retrieve the
satel-
lite, repair it in orbit and redeploy it is being planned
by NASA for late l983. The rejuvenated satellite would allow
scientists to observe a wide range of solar phenomena in a
different
part of the solar activity cycle and sustain solar irradiance
monitoring with the precision required for climate studies.
Authorization for the proposed SMM repair mission
is
currently under consideration by Congress.
The ACRIM experiment is conducted by JPL as part of
the weather and climate program of NASA's Office of Space
Science î
and Applications. SMM is managed by NASA's Goddard Space
Flight
Center.
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