Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 270

1 million years ago, climate oscillations had moderate amplitudes and periods near
41,000 years, in more recent times the amplitude of cycles increased considerably and
the dominant period extended to roughly 100,000 years. This transition is known as
the ''Mid-Pleistocene Transition'' (MPT) (Clark et al., 2006). Since variations in the
Earth's orbit did not change character over the 2.7-million-year time period, the
astronomical theory does not predict the MPT. Thus, the MPT poses a challenge
to the astronomical theory, and a number of attempts have been made to resolve this
without much success.
There are a fair few journal articles that seek the separate effects of orbital
parameters (precession, obliquity, and eccentricity) one at a time on historical
climates. This makes no sense because the Earth does not react to these orbital
parameters directly, at least according to the astronomical theory. The Earth
reacts to solar insolation—not to orbital parameters. Because there appears to
have been a transition from 41,000-year cycles before a million years ago or so
to 100,000-year cycles for the past million years or so, some scientists have
referred to the earlier epoch as the ''obliquity era'' and the more recent epoch as
the ''eccentricity era''. This seems strange because both obliquity and eccentricity
constantly varied during both eras. No one understands why this transition took
place although many have proposed explanations, none of which seem credible to
this writer. As we pointed out in Section 3.2.10, Wunsch (1999) emphasized that
''the purely random behavior of a rigorously stationary process often appears
visually interesting, particularly over brief time intervals, and creates the tempta-
tion to interpret it as arising from specific and exciting deterministic causes.'' For
example, Maslin and Ridgwell (2005) discussed the MPT by placing considerable
emphasis on the separate roles of individual orbital parameters and their spectral
periods.
Huybers (2009a) suggested ''that Pleistocene glacial variability is chaotic and
that transitions from 40KY to 100KY modes of variability occur spontaneously.''
However, there is no physical explanation why this might be so.
Clark et al. (2006) discussed the MPT at some length. They pointed out that
this transition occurred over a time period from about 1.25 mybp to about
700 kybp . Figure 7.1 shows that on balance the cycles tended to increase some-
what in amplitude and period even after the MPT. As we pointed out at the end
of Section 5.1, Clark et al. (2006) estimated that there was a ''60/40 ice volume/
deep-water temperature contribution to the global d 18 O signal over the last several
glacial cycles.'' They then addressed the question of how this affects the MPT:
''how much of the increase in amplitude of the d 18 O cycles represents an increase
in ice volume relative to additional global cooling of deep-water during glacia-
tions?'' They argued that if they assumed contributions of 60% ice volume and
40% deepwater temperature to d 18 O cycles over the past 2.8 million years, then
cycles prior to the MPT ''indicate significantly smaller changes in ice volume
than'' cycles after the MPT. Alternatively, had they assumed that much of the
increase in d 18 O across the MPT reflects mainly decreasing glacial deepwater
temperatures, then similar changes in ice volume would have occurred prior to
and after the MPT. They examined a range of data and concluded: