Figure 1.12 shows the ¿18O of benthic forams in a tropical Pacific core over the past 4 million years. The short-period fluctuation represents fluctuations in both ice volume and benthic temperature, and in addition there is a downward trend in temperature and increasing trend in ice volume over the earlier two million years of the period. By 2 million years ago, the flucutations have settled into a fairly regular pattern, with a dominant period of about 40,000 years. The period may be crudely estimated by counting peaks in the isotope record. This says that major ice advances in the early Pleistocene occur roughly every 40,000 years. About 800,000 years ago, there is a major transition in which the amplitude of the glacial-interglacial cycle becomes markedly larger, and the periodicity lengthens to about 100,000 years. During this period, an asymmetry between glaciation and deglaciation becomes readily apparent: the climate cools and ice builds up over long periods of time, but deglaciation occurs rather precipitously.
The periodicity of the ice ages, and the reason for the transition to a dominant 100,000 year cycle later in the Pleistocene is another of the Big Questions of climate science. We will learn in Chapter 7 that the periodicities are almost certainly connected with the quasiperiodic variations of the Earth's orbital characteristics - namely the tilt of its rotation axis and the departure of the orbit from circularity. The cycles are known as Milankovic cycles, after the scientist who first formulated a detailed theory connecting orbital parameters with the coming and going of ice ages. Milankovic's theory was largely ignored for decades, because not enough was known about the pattern of ice ages to give the theory a fair test. It was only revived in the 1970's, when data of the sort given in Figure 1.12 first became available. Even today, the means by which cycles in orbital characteristics are expressed in the climate record are far from clear, and remain a subject of active research.
This is another case in which the lesson learned from Earth carries over to other planets,
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