Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 256

spectrally and, to the extent that both have important frequency components that
agree, there is an implication that they are coupled. That belongs to the realm of
''circumstantial evidence'' (e.g., the suspect was known to be in the vicinity where
the crime was committed) but it does not lead to direct cause-effect conclusions.
The second approach involves comparing the detailed time series of isotope
data with solar data from hundreds of thousands (or even millions) of years to
determine whether there is any consonance between trends in the two datasets.
However, there is a diculty here because it is obvious that solar variability due
to precession of the equinoxes involves more rapid variations than the long-term
trends in global ice volume.
Assuming that the astronomical theory is fundamentally sound, it seems likely
that the buildup and decay of gigantic ice sheets is a slow process that depends on
the integral of solar variations over long periods—perhaps tens of thousands of
years. Therefore, an integrative model is needed to estimate the slow buildup and
decay of ice sheet volume as a function of more rapidly varying solar input. One
can then compare either the rate of change of ice volume with solar variations, or
an integral of solar variations with the measured time series for ice volume vs.
time. There does not seem to be a single a priori model based entirely on physics
with no adjustable parameters that allows unequivocal comparison of theory
with data. Unfortunately, all models developed to date have been limited by their
simplicity and the obvious bias of many scientists determined to validate the astro-
nomical theory via curve fitting. The combination of fixing the chronology of
isotope data by tuning to the astronomical theory and using adjustable parameters
to fit simplistic models to tuned data raises the question of circular reasoning.
However, it could be argued that the models provide a framework for connecting
theory with data, and the curve-fitting process fills in quantitative parameters that
are too dicult to estimate from fundamental principles in the real world. But the
degree of elasticity in the models creates doubt as to where such procedures fit
between the extremes of determining physical parameters and mere mathematical
curve fitting.
The bottom line seems to be that the astronomical theory may well be correct
to some extent in principle (i.e., there may be solar influence in glacial-interglacial
cycles), but translating this into a detailed quantitative comparison of theory and
experiment remains a dicult and elusive challenge.
9.6.1 Models for ice volume
Measurements of benthic d 18 O in foraminifera from ocean sediments are believed
to mainly represent ice volume, whereas planktic foraminifera may be more
sensitive to ocean temperature. Measurements of d 18 O and d D in ice cores are
believed to represent mainly local temperature at the core site.
In attempting to compare solar variability with isotope data representing ice
volume, one must formulate a model for how solar variations lead to changes in
local temperature and, ultimately, changes in ice sheet volume. Without such a