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 210

Today, 112 years later, Wright's observations are still appropriate. Wright
(1896) went on to say:

''It is easily seen that a glacier is the combined product of cold and moisture.
A simple lowering of the temperature will not produce an Ice Age. Before an area
can maintain a glacier, it must first get the clouds to drop down a sucient
amount of snow upon it. A climate that is cold and dry may not be so favorable
to the production of glaciers as one which is temperate, but whose climatic
conditions are such that there is a large snowfall. For example, on the steppes
of Asia, and over the Rocky Mountain plateau of our Western States and
Territories, the average temperature is low enough to permit the formation of
extensive glaciers, but the snowfall is so light that even the short summers in high
latitudes cause it all to disappear; whereas, on the southwestern coast of South
America, and in southeastern Alaska, where the temperature is moderate, but the
snowfall is large, great glaciers push down to the sea even in low latitudes. The
circumstances, then, pre-eminently favoring the production of glaciers, are abun-
dance of moisture in the atmosphere, and climatic conditions favorable to the
precipitation of this moisture as snow rather than as rain. Heavy rains produce
floods, which speedily transport the water to the ocean level; but heavy snows
lock up, as it were, the capital upon dry land, where, like all other capital, it
becomes conservative, and resists with great tenacity both the action of gravity
and of heat. Under the action of gravity, glaciers move, indeed, but they move
very slowly. Under the influence of heat ice melts, but in melting it consumes an
enormous amount of heat.''

In his day, Wright (1896) identified nine possible causes of ice age cycles.
These included shifting of the polar axis, a former period of greater moisture in
the atmosphere at higher latitudes, depletion of carbon dioxide in the atmosphere,
variations in the heat radiated by the Sun, changes in the Earth's orbit, and
changes in the distribution of land and water as well as changes in land elevation.

8.1 INTRODUCTION

A number of models have been proposed to explain the alternating cycles of
glaciation and interglacial cycles. We can classify the various models for the
occurrence of ice ages and interglacial periods as follows:

1. Solar Variability of the Sun—it has been hypothesized that variations in the
innate solar intensity due to structural variations within the Sun may have
provided the forcing function for glacial-interglacial cycles.
2. Orbit Variability of Earth's orbital parameters—quasi-periodic variations in
eccentricity, obliquity, and precession of the equinoxes produce changes in the
sequencing of solar intensity to higher latitudes which have been hypothesized to