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Solar activity and climatic change
N.O. Weiss (DAMTP, Cambridge)
Although greenhouse gases released by burning fossil fuel appear to
be responsible for the current rate of global warming, there have been
many previous episodes of climatic change that cannot be ascribed to the
same cause. There is, moreover, increasing evidence that the climate
responds to variations of magnetic activity in the Sun. Solar activity
varies cyclically with a mean period of eleven years. These cycles are
aperiodic and apparently chaotic. They are associated not only with
sunspots but also with slight variations in solar luminosity and with
modulation of the solar wind. That in turn affects galactic cosmic rays
and hence the production of cosmogenic isotopes such as 10Be and
14C. The sunspot cycle was interrupted by the Maunder Minimum in
the seventeenth century, though cyclic activity continued at a weaker
level. The 10Be and 14C records show that similar grand
minima have recurred over the past 10,000 years, and that the basic cycle is
modulated chaotically with a characteristic timescale of 200 years.
Comparison between records of solar activity and climatic records suggests
that the Sun's magnetic activity has been a major influence on climate
until recently, when greenhouse gases have taken over. However there is,
as yet, no compelling evidence that a direct forcing is sufficiently
effective to drive changes in the climate. Changes in total irradiance
are fairly small and the effects of ultraviolet emission, or those
of cosmic rays on cloud formation, are hard to estimate. On the other
hand, the earth's atmosphere and oceans support oscillations with many
different frequencies and resonant forcing provides a powerful mechanism
for amplifying the climatic response.
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