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have utilized midsummer solar irradiance in the NH as a measure of solar
variability from year to year.
Alternatively, there is some evidence that suggests that the key site for solar-
induced climate change might be the SH where variations in oceanic transport
of heat link the two hemispheres with a time delay. A number of studies have
provided evidence that terminations of ice ages might originate in the SH.
In attempting to compare the astronomical theory with data, one must first
clarify what the data represent. Models suggest that isotope ratio data at Green-
land and Antarctica represent local temperatures. These interpretations of isotope
ratios are far from ironclad and involve a number of uncertainties. Nevertheless,
even if we accept these assumptions regarding the interpretation of ice core data,
we still face the problem of how to compare ice core data with the variability of
solar intensity from year to year. If increased solar intensity raises temperatures, is
there a time lag and does it depend on other factors as well? How much higher is
Greenland temperature increase than global temperature increase? If the main
driver for climate change is NH solar intensity, how does this relate to Antarctica
climate and temperature? If we ignore these legitimate concerns and merely
compare solar intensity variability with the temperature record from ice cores, the
results will be as shown in Figures 10.1 and 10.2 . These results are suggestive of a
solar influence. Solar intensity varies (as always) with a 22,000-year period due
to precession of the equinoxes. These oscillations vary in amplitude over long time
periods. The temperatures implied by ice core records do not oscillate with this
frequency. However, there seems to be a significant correlation between the ampli-
tude of solar oscillations and ice core temperatures. In many (but not all) cases,
time periods with higher amplitude solar oscillations appear to be associated with
increasing temperatures, and periods during which solar oscillations are weak seem
to be associated with decreasing temperatures. This would be the case if (1) there
were a fundamental tendency toward glaciation, and (2) ice sheets grow slowly
and disintegrate rapidly. In that case ice sheets would disintegrate and not recover
when solar oscillations were large, but would grow when solar oscillations were
small. The fact that the frequency spectrum shows frequencies for eccentricity and
obliquity—but not precession—suggests that it is the amplitude of solar oscilla-
tions that matters, that the precession frequency does not directly contribute to
climate change, and that it is the eccentricity and obliquity that determine the
amplitude of precession oscillations. There are problems with this interpretation:
(1) the change from 41,000-year spacing to 100,000-year spacing of ice ages,
and (2) the occurrence of an ice age around 400,000 ybp when solar oscillations
were minimal.
In a similar manner, if we accept the proposition that isotope ratios in benthic
sediments provide a measure of ice volume (V), how do we compare these data
with variable solar intensity? In order to compare ice sheet volume with variable
solar intensity at higher latitudes, we require models for ice sheet volume as a
function of variable solar intensity. A few models have been proposed. However,
they appear to this writer to be overly simplistic and fail to take account of
the complexities of the Earth's ocean and atmosphere systems. Comparison of the
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