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CHAPTER 11
Orbital-Scale
Interactions,
Feedbacks, and
Unsolved Problems
This chapter returns to the mystery of how ice sheets respond to changes in
orbital insolation. Over the last 2.75 Myr, the ice sheet response recorded in
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marine δ O signals contains the same periods as those in the orbital forcing,
but the dominant rhythms do not match. Summer insolation fluctuates mainly
at the 23,000-year cycle of orbital precession, with smaller changes at the
41,000-year tilt cycle. In contrast, between 2.75 and 0.9 Myr ago, the ice sheets
varied mainly at the 41,000-year period, and they have then oscillated at a
period centered on 100,000 years since 0.9 Myr ago. These disconnects
between the insolation forcing proposed by Milankovitch and the observed ice
sheet responses are a mystery that this chapter explores.
