Sun's Magnetic Activity Varies In 100,000-Year Cycles
Thanks to new calculations by a Dartmouth geochemist, scientists are now
looking at the earth's climate history in a new light.
Mukul Sharma, Assistant Professor of Earth Sciences at Dartmouth,
examined existing sets of geophysical data and noticed something
remarkable: the sun's magnetic activity is varying in 100,000-year cycles, a
much longer time span than previously thought, and this solar activity, in turn,
may likely cause the 100,000-year climate cycles on earth.
This research helps scientists understand past climate trends and prepare
for future ones.
Published in the June 10 issue of Earth and Planetary Science Letters
(Elsevier, volume 199, issues 3-4), Sharma's study combined data on the
varying production rates of beryllium 10, an isotope found on earth
produced when high-energy galactic cosmic rays bombard our atmosphere,
and data on the past variations in the earth's magnetic field intensity.
With this information, Sharma calculated variations in solar magnetic
activity going back 200,000 years, and he noticed a pattern.
Over the last 1 million years, the earth's climate record has revealed a
100,000-year cycle oscillating between relatively cold and warm conditions,
and Sharma's data on the sun's magnetic activity corresponded to the
earth's ice age history.
"Surprisingly, it looks like solar activity is varying in longer time spans than
we realized," says Sharma. "We knew about the shorter cycles of solar
activity, so maybe these are just little cycles within a larger cycle. Even more
surprising is the fact that the glacial and interglacial periods on earth during
the last 200,000 years appear to be strongly linked to solar activity."
Sharma's calculations suggest that when the sun is magnetically more
active, the earth experiences a warmer climate, and vice versa, when the
sun is magnetically less active, there is a glacial period. Right now, the
earth is in an interglacial period (in between ice ages) that began about
11,000 years ago, and as expected, this is also a time when the estimated
solar activity appears to be high.
Beryllium 10 is useful for studying the geology from hundreds of thousands
of years ago mainly because it has a half-life of about one and a half million
years.
In addition, there are two key factors that have affected beryllium 10
production over the last 200,000 years: the earth's magnetic field and the
sun's magnetic activity. When there are high-intensity solar magnetic
storms, more charged particles are interacting with cosmic rays, and less
beryllium 10 is produced. Likewise, the earth's magnetic field changes the
flux of cosmic rays into and out of the atmosphere.
Since the production rate of beryllium 10 and earth's magnetic field intensity
are known for the last 200,000 years, Sharma could calculate solar
magnetic activity for this time period.
"I took sets of existing, independent data and made new comparisons and
calculations," says Sharma. Then he went a step further to make a
connection with the history of ice ages by looking at oxygen isotopes in the
oceans, which reveal the history of how much ice was at the poles and are
therefore a measure of average global surface temperature.
"I compared the estimated past variations in the solar activity with those of
the oxygen isotopes in the ocean. Although there is a strong relationship
between solar activity and oxygen isotopic variations, it is too early to say
exactly what is the mechanism through which the sun is influencing the
terrestrial climate."
One explanation of the 100,000-year cycle was offered by the Milankovitch
Theory of Ice Ages in the 1940s, which suggested that the cyclical
variations in the earth's orbit around the sun result in the earth receiving
varying amounts of solar radiation that, in turn, control the climate.
This explanation is under dispute because the variations of the solar energy
in relation to the changes in orbit are very small. Other current research
focuses on past variations in the sun's irradiance, or heat intensity (as
opposed to the magnetic activity).
Sharma notes that more analysis is needed to test his theory.
"I've only looked at 200,000 years. My calculations need to be verified for a
million years, for instance. Plus, regarding the current global warming
debate, it still needs to be examined if the role of solar activity will
exacerbate the rising temperatures that result from carbon dioxide buildup
in the atmosphere."
This work was supported by Dartmouth College, the Max Planck Institute
and by a grant from the National Science Foundation. - By Susan E. Knapp
[Contact: Susan E. Knapp]
06-Jun-2002 |
