Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0987XX.PR

Text File | 1993-04-23 | 9KB | 372 lines

PUMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 9ll09. TELEPHONE [phone]


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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