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 211

provide the forcing function for glacial-interglacial cycles. Within this group of
models there are those that are based on:
(a) variability of solar intensity in the NH summer;
(b) variability of solar intensity in the SH summer;
(c) variability of individual parameters such as eccentricity, obliquity,
precession of the equinoxes, one at a time or in pairs.
3. Volcanism The occurrence or absence of high levels of volcanism cause
temporary changes in the Earth's response to the Sun which may trigger the
initiation of longer term glacial-interglacial cycles.
4. Greenhouse gases Variability of concentrations of greenhouse gases
(particularly CO 2 and CH 4 ) induced by unspecified forcings have been conjec-
tured to be a cause of glacial-interglacial cycles via changes in the greenhouse
effect. However, the change in greenhouse gases appears to be a secondary effect
of other primary geological or biological processes.
5. The oceans Variability in the thermohaline circulation of the oceans producing
large changes in the heat delivered to higher latitudes has been hypothesized to
provide the forcing function for glacial-interglacial cycles.
6. Extraterrestrial accretion Several models are based on the effects due to quasi-
periodic accretion of extraterrestrial dust in the Earth's atmosphere as the
primary forcing that induces changes in cloud cover, which in turn affects the
climate.
7. Ocean-atmosphere interactions In this model, the primary factor that controls
large-scale variations in the Earth's climate is the Earth's albedo, which in turn is
controlled by the degree of cloudiness which is subject to repetitive cycles due to
ocean-atmosphere interactions.

The greatest effort and attention has been addressed to the variability of
Earth's orbital parameters as the forcing function for glacial-interglacial cycles,
and this model is widely accepted among scientists. Thus, we devote all of
Chapter 9 to this model.

8.2 VARIABILITY OF THE SUN

Rapp (2008) provided a lengthy and detailed review of the literature on variations
in power emitted by the Sun. Total solar irradiance (TSI) above the atmosphere
has only been measured since 1978, so there are no long-term measurements of
solar intensity. The Sun currently passes through repeated sunspot cycles that
range from solar maximum to solar minimum with a period of about 11 years. At
solar maximum there are typically > 100 sunspots whereas at solar minimum there

are typically 10 sunspots. Total solar irradiance at solar maximum is about 0.1%

higher than at solar minimum. The Sun has been the subject of visual observation
since the 17th century, and it is known that the number of sunspots at solar
maximum has varied widely over the years. There is evidence that sunspots essen-
tially disappeared altogether for a 70-year period toward the end of the 17th

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