The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 274

to the astronomical theory, one should be careful to take into account any circular
reasoning in the process of validating the astronomical theory.
Assuming for the sake of argument that orbitally induced changes in solar
intensity are the primary drivers for major climate change, a fundamental question
is whether the variability of solar intensity in the NH or the SH is most important.
While the major characteristic of ice ages is the expansion of ice sheets in the NH,
it is not obvious a priori whether this results from changes in solar intensity in the
NH, or whether it may be due to changes in solar intensity in the SH with conse-
quent changes in ocean currents that affect ice sheet formation in the NH, or some
combination of the two. While the overwhelming majority of researchers have
assumed that solar variability in the NH is the only relevant factor, a case can be
made for the contrary view (see Section 4.3). Some studies compare the isotope
time series from ice cores or ocean sediments with variable NH solar intensity,
using a time lag for the isotope time series (time lags of 9,000 to over 30,000 years
have been used). But, since NH and SH solar intensities are 11,000 years out of
phase, one could argue that solar intensity in the SH is the controlling factor and
the time lag is artificial.
Finally, instead of comparing the timing of variability in solar intensity with
that of the isotope time series, one might carry out spectral analysis of the
variability of the two datasets. To the extent that they have similar structures in
the frequency domain, that would suggest an underlying connection between the
two. M&M provided a very extensive and detailed discussion of spectral analysis
which is beyond the scope of this topic. A brief discussion of spectral analysis
results is given in Section 10.3.
10.2 DATA AND THE ASTRONOMICAL THEORY COMPARED
One of the stumbling blocks in the attempt to validate the astronomical theory
with sediment core data is the fact that over the past 2.7 million years there has
been a systematic change in the character of climate variations whereas the
astronomical theory does not predict such a change. As we showed in Figure 5.6 ,
glacial-interglacial cycles gradually became longer and gained amplitude in the
past million years or so. Whereas the period of oscillations in the early part of the
2.7-million-year era of ice ages tended to be near 41,000 years, the period length-
ened to roughly 100,000 years over the last million years. In fact, as Figure 7.1
shows, this period has increased by about 40,000 years during the past 800,000
years. This dichotomy between the early and late parts of the past 2.7 million
years has confounded scientists for many years, and many papers have been
written on the subject, mostly in a vain attempt to resolve the issue in favor of the
astronomical theory. In fact, it is rather common for scientists to refer to the early
period as the ''obliquity period'' and the more recent period as the ''eccentricity
period'', as if each of these orbital parameters acted independently on the Earth's
climate rather than by contributing to changes in solar intensity (which depends
on all three parameters: obliquity, eccentricity, and longitude of precession). There