The astronomical theory and data compared - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 315

accepted for a number of years) to convert Greenland ice core isotope ratio data
to Greenland temperatures appear to be off by a factor of 2 according to borehole
temperature models. Nevertheless, Greenland isotope ratio data are believed to
represent regional temperature conditions even if the absolute conversion to
temperature is uncertain. Similarly, Antarctica isotope ratio data are believed
to represent global temperatures accurately. Ocean sediment data from benthic
sources are believed to represent mainly global ice volume, although Lisiecki et al.
(2008) recently concluded: ''Generating a robust age model for benthic d 18 O or ice
volume without the assumptions of orbital tuning remains an important, unsolved
problem.''
The astronomical theory of ice age cycles originated in the 19th century and
has evolved over the past century and a half. Quasi-periodic variations in the
Earth's orbital parameters change solar energy input to higher latitudes with
periods of multiple tens of thousands of years. The fact that solar inputs to high
latitudes and data on past climate variations are both subject to quasi-periodic
variations over similar time periods suggests that the two may be coupled. Spectral
analysis supports this viewpoint. According to the astronomical theory, this
variability of solar input to higher latitudes has a significant effect on the ability
of surface and sea ice at higher northern latitudes to withstand the onslaught of
summer. It has been theorized that during time periods when peak solar energy
input in summer to higher northern latitudes is lower than average, the lower
solar input may trigger feedback processes that lead to the spread of ice cover and
the start of ice ages. Conversely, time periods with high peak solar energy input in
summer to higher northern latitudes might trigger feedback processes that cause
melting, leading to deglaciation. Thus, according to this theory, variability of the
Earth's orbit about the Sun is a primary factor in determining the timing of
glacial-interglacial cycles.
M&M asserted that a persuasive reason to think that astronomy is responsible
(at least to some extent) for the observed glacial-interglacial variations is that over
long periods ( 800,000 years) these oscillations remain coherent (i.e., they main-
tain a relatively constant phase). However, as Figure 7.1 shows, the coherence is
only approximate and there has been a systematic increase in the spacing of ice
ages over the past 800,000 years. Even more important is the fact that coherence
is still worse over a 2.7-million-year period. M&M further argued that the narrow-
ness of the spectral peaks implies that glacial cycles are driven by a quasi-periodic
astronomical force, regardless of the details of the actual driving mechanism—and
that appears to be a strong argument.
It is not immediately obvious which measure of solar intensity is of greatest
relevance in the astronomical theory. There is some reason to believe that ice ages
originate at high latitudes in the Northern Hemisphere (NH) because that is where
the great ice sheets grew during ice ages and that land (rather than water) occurs
at high northerly latitudes, providing a base for ice sheet formation. It also seems
reasonable to guess that the onset of widespread glaciation at high northern lati-
tudes would be enhanced if a greater preponderance of ice could survive the effects
of higher regional peak solar irradiance in summer. Hence, most investigators