]]]]]]]]]]]]        THE SUN IN THE "GREENHOUSE"      [[[[[[[[[[[
                        By Albert Arking

[``Albert Arking is  head of the climate  and radiation branch of
the National Aeronautic and  Space Administration's Goddard Space
Flight Center.'']

        [From The New York Times, 23 May 1989, p. A29:2]

             [Kindly uploaded by Freeman 10602PANC]

                                                   Greenbelt, Md.
   Stories in the news media give the impression that a consensus
exists among scientists, and Government scientists in particular,
that the global warming of the  1980's is primarily the result of
increased ``greenhouse'' gases in the atmosphere.
   There is no  question that these  gases are increasing rapidly
and that  their overall effect,  other things being  equal, is to
increase global surface temperatures.   However, the magnitude of
their impact  is the  subject of  considerable debate  within the
scientific community.   Moreover, there  is strong  evidence that
another factor is  having an important  influence on our climate:
solar activity.
   The recent  upward trend in  global temperatures  has been the
subject of intense scrutiny.  The current warming trend goes back
only 12 years, however. Before that, temperatures were decreasing
or unvarying  between 1940 and  the late 1970's  -- although this
period was one  of strong growth in  world energy consumption and
fossil-fuel burning.
   Furthermore, the 50 years  prior to that --  from 1890 to 1940
-- was  a period of  significantly less  fossil-fuel burning, yet
the  Earth warmed  up  by more  than  1 degree  Fahrenheit.  That
represents about twice the amount of the recent warming.
   These changes in temperature do  not at all follow the pattern
of the changes in the greenhouse gases.
   Indeed, while these gases  are a potentially important factor,
there are other possibilities to consider.
   Volcanic eruptions, for example,  have long been recognized as
influencing  changes  in  climate.   Volcanic  eruptions  produce
sulfur dioxide,  which becomes droplets  of sulfuric  acid in the
stratosphere, forming a global haze.
   This haze  partly screens the  Earth from the  sun's rays, and
cools the planet.  To effect a warming trend, there would have to
be a lengthy period  in which the rate  and intensity of volcanic
eruptions is substantially below  normal.  This has certainly not
been the case in recent years.
   Another factor  -- variations in  the sun's  output of radiant
energy --  has been  identified in  many studies  over the years.
But these  variations have  not been  given the  recognition they
deserve because  it was not  fashionable to think  that the sun's
power could vary.  (Indeed, it is still common to call the energy
from the sun ``the solar constant.'')
   However, satellite  measurements over  the past  10 years have
confirmed  that such  solar variations  occur.  They  reveal that
during the last solar  cycle -- the 11-year  period of growth and
decline  of   solar  activity,  typically   manifested  by  large
variations in sunspots,  solar flares and  other phenomena -- the
sun's power varied correspondingly.
   The amplitude of  the observed variation  was small (about 0.1
percent  during the  course of  one solar  cycle).  But  once the
principle is established that the  sun's output can vary over the
course of one sunspot cycle, the possibility that over decades or
centuries the variations  can be much more  than 0.1 percent must
be given serious consideration.
   In fact, there have been substantial changes in solar activity
over the centuries,  and they correlate  with temperature changes
on Earth.
   The facts are striking:
   First, the  period of  lowest solar  activity occurred between
1600  and 1700,  when  sunspots practically  vanished  during the
later half of the  century.  That was also  the coldest period in
the last thousand years, sometimes called ``the little ice age.''
   Second, more or  less simultaneous with  the cooling that took
place from the 1940's to the late 1970's, there was a substantial
reduction of  solar activity.   And from  the late  1970's to the
present,  as the  temperature of  the  Earth rose  sharply, solar
activity also reversed and started back up again.
   This pattern  of changes  in global  surface temperatures over
the  centuries  suggests  a   strong  solar  influence.   Perhaps
increased  greenhouse  gases also  played  a role  in  the recent
warming  trend,  but it  is  too  early to  judge  their relative
importance and draw conclusions about the future.

      [The following is not part of the original article.]


Lean, J.  ``Contribution of Ultraviolet Irradiance Variations to
   Changes in the Sun's  Total Irradiance''.  Science 244:197-200
   (14 April 1989).

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