The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 275

does not seem to be any credible mechanism by which one of these parameters
could influence climate independently, so this description of action by independent
parameters on climate seems to lack foundation.
Imbrie and Imbrie (1980) have been leaders for many years in the quest to
find support for the astronomical theory from isotope data. However, even they
admitted that:

''One of the remaining major problems is the origin and history of the
100,000-year climatic cycle. At least over the past 600,000 years, almost all
climatic records are dominated by variance components in a narrow frequency
band centered near a 100,000-year cycle. Yet a climatic response at these
frequencies is not predicted by the [conventional] astronomical theory—or any
other version that involves a linear response . Another problem is that most

published climatic records that are more than 600,000 years old do not exhibit a
strong 100,000-year cycle.''

...

Nevertheless, in a rather remarkable twist of logic, they then went on to say:

''Whatever the outcome of future research on the 100,000-year problem may
be—and whatever stochastic or deterministic processes may operate in addition
to the astronomical causes—the conclusion seems inescapable that for at least
the past 730,000 years, the climate system has responded to orbital forcing at the
frequencies of variation in obliquity and precession. Therefore, we argue that the
time has come to make a fundamental shift in research strategy: instead of using
numerical models of climate to test the astronomical theory, we should use
the geological record as a criterion against which to judge the performance of
physically motivated models of climate.''

This paragraph summarizes the approach taken by the Imbries and, under
their influence, by others over the years. The fact that the frequency spectrum of
ocean sediment isotope data includes peaks corresponding to obliquity and preces-
sion makes it a fait accompli in their minds that the astronomical theory is correct,
and now the issue is no longer validating the astronomical theory but rather
testing climate models that relate solar variability to ice volume to determine
which parameters in models agree best with ocean sediment isotope data. Thus,
the task consists of curve fitting and parameter adjustment to seek a best fit.
However, like George W. Bush in Iraq, they may have declared victory for the
astronomical theory prematurely.
In the simplest approach, one could plot solar intensity at some high latitude
(north or south?) vs. time on the same axes as long-term isotope data, perhaps
with a time offset between the two. This has been done in several instances. Alter-
natively, one may attempt to develop models for how the variability of solar
intensity at high latitudes affects the climate, in general, and the growth of ice
sheets, in particular. Such processes are likely to be complex and may introduce
nonlinearity, which makes direct comparison of the variability of solar intensity