How Strongly Does the Sun Influence the Global Climate? (Forwarded)
From: Andrew Yee (ayee_at_nova.astro.utoronto.ca)
Date: 08/06/04
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Date: Fri, 06 Aug 2004 13:50:18 -0400
Press and Public Relations Department
Max Planck Society for the Advancement of Science
Munich, Germany
Contact:
Prof. Sami K. Solanki
Max Planck Institute for Solar System Research, Katlenburg-Lindau
Tel.: +49 5556 979-325
Fax: +49 5556 979-190
E-mail: solanki-office@mps.mpg.de
Prof. Manfred Schüssler
Max Planck Institute for Solar System Research, Katlenburg-Lindau
Tel.: +49 5556 979-469
Fax: +49 5556 979-190
E-mail: schuessler@mps.mpg.de
August 2nd, 2004
SP / C / 2004 (26)
How Strongly Does the Sun Influence the Global Climate?
Studies at the Max Planck Institute for Solar System Research reveal: solar
activity affects the climate but plays only a minor role in the current global
warming
Since the middle of the last century, the Sun is in a phase of unusually high
activity, as indicated by frequent occurrences of sunspots, gas eruptions, and
radiation storms. Researchers at the Max Planck Institute for Solar System
Research (MPS) in Katlenburg-Lindau (Germany) and at the University of Oulu
(Finland) have come to this conclusion after they have succeeded in
reconstructing the solar activity based on the sunspot frequency since 850 AD.
To this end, they have combined historical sunspot records with measurements of
the frequency of radioactive isotopes in ice cores from Greenland and the
Antarctic. As the scientists have reported in the renowned scientific journal,
Physical Review Letters, since 1940 the mean sunspot number is higher than it
has ever been in the last thousand years and two and a half times higher than
the long term average. The temporal variation in the solar activity displays a
similarity to that of the mean temperature of the Earth. These scientific
results therefore bring the influence of the Sun on the terrestrial climate, and
in particular its contribution to the global warming of the 20th century, into
the forefront of current interest.
However, researchers at the MPS have shown that the Sun can be responsible for,
at most, only a small part of the warming over the last 20-30 years. They took
the measured and calculated variations in the solar brightness over the last 150
years and compared them to the temperature of the Earth. Although the changes in
the two values tend to follow each other for roughly the first 120 years, the
Earth's temperature has risen dramatically in the last 30 years while the solar
brightness has not appreciably increased in this time.
Astronomers have regularly observed sunspots since the invention of the
telescope in the early 17th century. These are areas on the surface of the Sun
where energy flow from the interior is reduced due to the strong magnetic fields
that they exhibit. As a result, these regions cool by about 1500 deg and thus
appear relatively darker than their surroundings at 5800 deg. The number of
sunspots varies over an 11-year activity period, which in turn is subject to
longer term variations. For example, in the second half of the 17th century,
there were hardly any sunspots at all.
The German-Finnish research team has now applied a new method to obtain insight
into the development of the sunspot number from before the beginning of direct
records. In addition, these experts have analyzed measured abundances of
beryllium-10 in ice cores from Greenland and the Antarctic. This radioactive
isotope is created when energetic particles in cosmic rays enter the Earth's
atmosphere and split atomic nuclei of nitrogen and oxygen. Beryllium-10
(half-life 1.6 million years) is a product of this decay process, which is then
washed out of the atmosphere by precipitation and then deposited in layers in
the polar ice fields. Since the cosmic rays are partially deflected by the solar
magnetic field filling interplanetary space, the production rate of Beryllium-10
in the atmosphere varies with the strength of this magnetic field, which in turn
is associated with the number of sunspots.
A comparison of the Beryllium-10 data with the historical records of sunspot
numbers reveals a high degree of correlation. Thus it was possible for the
researchers to test and calibrate this new reconstruction method. The solar
research team has managed, for the first time, to substantiate with consistent
physical models every link in the complex chain, from the isotope abundance in
the ice back to the sunspot number. This includes the creation of Beryllium-10
by cosmic rays, the modulation of the cosmic rays by the interplanetary magnetic
field, and finally the relationship between the solar magnetic field and the
number of sunspots. In this way it was possible for the scientists to obtain,
for the first time, a reliable, quantitative determination of the sunspot
numbers even for times before direct measurements were made.
These data show clearly that the Sun is in a state of unusually high activity,
for about the last 60 years. The time interval for which this statement can be
made has been tripled by these new investigations, for now the reconstructed
sunspot numbers extend back to 850 AD. Another period of enhanced solar
activity, but with substantially fewer sunspots than now, occurred in the Middle
Ages from 1100 to 1250. At that time, a warm period reigned over the Earth, as
the Vikings established flourishing settlements in Greenland.
The Sun affects the climate through several physical processes: For one thing,
the total radiation, particularly that in the ultraviolet range, varies with
solar activity. When many sunspots are visible, the Sun is somewhat brighter
than in "quiet" times and radiates considerably more in the ultraviolet. On the
other hand, the cosmic ray intensity entering the Earth's atmosphere varies
opposite to the solar activity, since the cosmic ray particles are deflected by
the Sun's magnetic field to a greater or lesser degree. According to a much
discussed model proposed by Danish researchers, the ions produced by cosmic rays
act as condensation nuclei for larger suspension particles and thus contribute
to cloud formation. With increased solar activity (and stronger magnetic
fields), the cosmic ray intensity decreases, and with it the amount of cloud
coverage, resulting in a rise of temperatures on the Earth. Conversely, a
reduction in solar activity produces lower temperatures.
Two scientists from the MPI for Solar System Research have calculated for the
last 150 years the Sun's main parameters affecting climate, using current
measurements and the newest models: the total radiation, the ultraviolet output,
and the Sun's magnetic field (which modulates the cosmic ray intensity). They
come to the conclusion that the variations on the Sun run parallel to climate
changes for most of that time, indicating that the Sun has indeed influenced the
climate in the past. Just how large this influence is, is subject to further
investigation. However, it is also clear that since about 1980, while the total
solar radiation, its ultraviolet component, and the cosmic ray intensity all
exhibit the 11-year solar periodicity, there has otherwise been no significant
increase in their values. In contrast, the Earth has warmed up considerably
within this time period. This means that the Sun is not the cause of the present
global warming.
These findings bring the question as to what is the connection between
variations in solar activity and the terrestrial climate into the focal point of
current research. The influence of the Sun on the Earth is seen increasingly as
one cause of the observed global warming since 1900, along with the emission of
the greenhouse gas, carbon dioxide, from the combustion of coal, gas, and oil.
"Just how large this role is, must still be investigated, since, according to
our latest knowledge on the variations of the solar magnetic field, the
significant increase in the Earth's temperature since 1980 is indeed to be
ascribed to the greenhouse effect caused by carbon dioxide," says Prof. Sami K.
Solanki, solar physicist and director at the Max Planck Institute for Solar
System Research.
Original work:
Krivova N.A., Solanki S.K.
Solar Variability and Global Warming: A Statistical Comparison Since 1850
Adv. Space Res. 34, 361-364 (2004)
Ilya G. Usoskin, Sami K. Solanki, Manfred Schüssler, Kalevi Mursula, Katja Alanko
A Millennium Scale Sunspot Reconstruction: Evidence For an Unusually Active Sun
Since the 1940's
Physical Review Letters 91, 211101-1--211101-4 (2003)
Sami K. Solanki, Natalie A. Krivova
Can Solar Variability Explain Global Warming Since 1970?
Journal of Geophysical Research 108, 1200 (2003)
IMAGE CAPTIONS:
[Fig.1:
http://www.mpg.de/bilderBerichteDokumente/multimedial/galerie/bilderWissenschaft/2003/10/schuessler1/Web_Zoom.jpeg
(185KB)]
Extended group of sunspots visible in the southern hemisphere of the Sun in
September 1998. These sunspots are fascinating not only due to their large size,
but also due to their detailed structure that reveals a dynamic complexity. The
smallest visible features are about 350 km across, the entire area covers about
200,000 km in the horizontal direction. The sub-pictures in this mosaic were
obtained with the German Vacuum Tower Telescope at the Observatorio del Teide
(Teneriffe).
Image: Kiepenheuer Institute for Solar Physics, Freiburg im Breisgau
[Fig. 2:
http://www.mpg.de/bilderBerichteDokumente/multimedial/galerie/bilderWissenschaft/2003/10/schuessler2/Web_Zoom.jpeg
(135KB)]
A large sunspot visible on the Sun in June 2000. The picture shows a section of
the Sun's surface 80,000 by 80,000 km across. Sunspots appear darker than their
surroundings because their strong magnetic fields suppress the energy
transported by gas flows. This picture was taken with the German Vacuum Tower
Telescope at the Observatorio del Teide (Teneriffe).
Image: Kiepenheuer Institute for Solar Physics, Freiburg im Breisgau
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