Overview of the various models for ice ages in the recent past (3 MYBP to present) - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 212

century. This period is known as the Maunder Minimum and may be associated
with the unusually cold temperatures of the time (Rapp, 2008). In addition, the
length of the solar cycle has varied from about 9 to 13 years. Thus, the appear-
ance and activity of the Sun can be shown to pass though significant changes over
a mere few hundred years. However, it is not known whether these changes in
appearance are associated with changes in TSI over such time intervals.
Because past historical variation of the TSI is an important part of the science
of paleoclimatology, a number of attempts have been made to estimate historical
variations of TSI from approximate models. These models are described by Rapp
(2008). They include models based on (1) past sunspot activity, sunspot area or
sunspot cycle duration, (2) comparison with Sun-like stars, (3) models based on
coronal source flux, and (4) use of cosmogenic isotope proxies. Most of these
models only reach back about 100 to 300 years. A few go back as far as several
thousand years. None of the models provide estimates over a suciently long time
period to be compared with paleoclimatological data over hundreds of thousands
of years. Furthermore, the models all suffer from significant imperfections and
unsubstantiated approximations. Thus, it is a matter of pure conjecture whether
variations in TSI over the past few million years have contributed to glacial-
interglacial cycles, and there does not seem to be any way to check this
hypothesis. However, there are physical models of the Sun that suggest that such
variability is unlikely.
8.3 THE ASTRONOMICAL THEORY
The astronomical theory does not depend on any innate variability of the TSI but,
rather, depends on small quasi-periodic wobbles in the Earth's orbit about the Sun
which produce changes in solar input to high latitudes. Even though total solar
input falling on the Earth may not change, quasi-periodic changes in the distribu-
tion of solar input to high latitudes is thought to have a controlling effect on the
ability of high-latitude ice to persist through summer and, thereby, spread from
year to year. It is fairly straightforward to calculate changes in the Earth's orbit
over the past million years or so and, thereby, to estimate changes in solar input
to high latitudes.
While the astronomical theory is based on the general hypothesis that the
variability of solar input to high latitudes is the forcing function for glacial-
interglacial cycles, the specifics of how this variability produces glacial-interglacial
cycles are lacking. Although the simplistic notion is widely held that reduced
summer solar input at high latitudes allows ice and snow to survive the summer
and spread from year to year, it is not clear which distributions of ice and snow at
which locations survive to what degree from any quantitative reduction in solar
input above the atmosphere. The confounding effects of water vapor, cloud,
precipitation, wind, changing ocean and land albedo, ocean currents, aeolian dust,
ocean level, ice sheet formation, and exposure of continental shelves are not
included in this simple model. The fact that paleotemperature variations derived