Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 242

variability, there is also the question of which measure to use for solar input to
any latitude. We could use peak insolation using total daily insolation (or, almost
equivalently, solar intensity at noon) reached in midsummer as our measure for
any year. Alternatively, we could use the integral of insolation over the summer
season (nearly equal to the yearly integral for high latitudes). Because the Earth's
orbit is elliptical, these two measures are somewhat different. When the Earth is
farthest from the Sun, the peak insolation reached in midsummer is lower, but the
summer lasts longer. When the Earth is nearest to the Sun, the peak insolation
reached in midsummer is higher, but the summer does not last as long. Thus, over
many years, the extremes of variability of integrated insolation are much smaller
than those of peak insolation in summer.
In the following sections, we derive the variability of peak solar intensity over
the past few hundred thousand years. The next question that arises is how should
one compare the historical variability of solar intensity with isotope records of the
past climate? There is no immediately obvious connection between the time
sequences of solar variability and isotope variability. The simplest version of the
astronomical theory suggests that solar variability is the forcing function that
causes glaciation-deglaciation cycles. However, the astronomical theory (in its
simple form) does not predict the timing and intensity of such cycles based on the
solar variability record. Some isotope records are believed to mainly represent ice
volume in ice sheets, while others are believed to represent temperatures prevailing
at the region where samples were taken. Temperature records show wide vari-
ability vs. time whereas ice volume measurements show a more gradual variability.
It is also widely believed that the time constant for the buildup of major ice sheets
is greater than 10,000 years. Therefore, if solar variability is the main factor
producing glaciation-deglaciation cycles, the ice volume curve is likely to lag
behind the solar irradiance curve significantly. Testing the astronomical theory by
comparing historical solar variability with isotope variability is far from straight-
forward. While several investigators have claimed to have done this and, thereby,
proclaimed validation of the astronomical theory, closer examination raises some
doubts as to the veracity of such comparisons. This topic is discussed further in
the following sections.
While almost all discussions of the astronomical theory in the literature deal
mainly with the initiation and termination of ice ages via variable solar input to
high latitudes, it should be borne in mind that an ice age is a global phenomenon,
and a full explanation of an ice age (or a termination) requires a global
description that goes well beyond solar input to high latitudes. While solar input
to high latitudes may well be the trigger that initiates an ice age, the increased
albedo of growing ice sheets and sea ice, changes in vegetation over widening
areas, lowered oceans and expanded coastlines, ocean currents circulating between
high latitudes and the tropics, dust in the atmosphere, and changes in greenhouse
gas concentrations all contribute to ice age-interglacial cycles. Nevertheless, it is
widely believed that all these other factors are like orchestra musicians waiting for
the conductor (solar input to high latitudes) to ascend the podium before they
play their roles.