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CHAPTER 11 • Orbital-Scale Interactions, Feedbacks, and Unsolved Problems 205

Several factors contribute to the observed pac- 4. How could the northern hemisphere ice sheets
ing of deglaciations at an average period of 100,000 affect climate in the southern hemisphere?
years. One contribution noted earlier is the timing of
clusters of high-insolation maxima at ~100,000 years see 5. What evidence suggests that orbital-scale changes
(Figure 11–15). These peaks are large mainly because of in northern hemisphere ice volume drive changes
eccentricity modulation of the 23,000-year precession in atmospheric CO rather than the opposite?
2
cycle and they tend to position major deglaciations at 6. How could 100,000-year cycles in the size of
intervals of either ~92,000 years or ~115,000 years. northern hemisphere ice sheets be paced by
Some insolation peaks are particularly large because summer insolation changes that occur only at
they are closely aligned with high insolation caused by cycles of 41,000 and 23,000 years?
maxima at the tilt cycle.
Another factor that affects the timing of deglacia- 7. If a sine wave CO signal with a period of
2
tions is the net growth of ice sheets at 41,000-year 100,000 years forces an ice sheet response at the
intervals. A relatively large volume of ice obviously has same period and if the ice has a time constant of
to accumulate to create a major deglaciation. This 10,000 years, what should be the size of the ice
constraint tends to position terminations at intervals of sheet lag behind the forcing?
either 82,000 or 123,000 years, after either two or three
intervals of ice growth. Together, these two constraints Additional Resources
combine to position terminations within intervals of
either 82,000–92,000 years or 115,000–123,000 years. Basic Reading
The separations between the last five terminations all Imbrie, J., and K. P. Imbrie. 1979. Ice Ages: Solving the
fall within one of these two time clusters. As a result, ice Mystery. Short Hills, NJ: Enslow.
sheets that grew during 41,000-year episodes melted at Ruddiman, W. F. 2005. Plows, Plagues and Petroleum,
intervals near 100,000 years. Chapters 3–5. Princeton, NJ: Princeton University
Press.
IN SUMMARY, several possible explanations for the
~100,000-year glacial world are being explored by Advanced Reading
climate scientists. In all the proposed explanations, Broecker, W. S. 1984. “Terminations.” In A. L. Berger
the large ice sheets produce internal responses et al., eds., Milankovitch and Climate, pp. 687–98.
(either of bedrock or in the climate system) that Dordrecht: Reidel.
hasten their own destruction during intervals whose Broccoli, A. J., and Manabe, S. 1987. “The Influence of
timing is paced by changes in summer insolation. Continental Ice, Atmospheric CO , and Land
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The internal processes that destroy the ice sheets act Albedo on the Climate of the Last Glacial
as positive feedbacks that accelerate ice melting Maximum.” Climate Dynamics 1: 87–100.
initiated by rising insolation. Huybers. P. 2006. “Early Pleistocene Glacial Cycles
and the Integrated Summer Insolation Forcing.”
Science 313: 508–11.
Key Terms Manabe, S., and A. J. Broccoli. 1985. “The Influence of
Continental Ice Sheets on the Climate of an Ice
ice-driven responses resonant response Age.” Journal of Geophysical Research 90: 2167–90.
(p. 192) (p. 201) Raymo, M. E., Lisiecki, L. E., and Nisancogliu, K. H.
2006. “Plio-Pleistocene Ice Volume, Antarctic
Climate, and the Global δ O Signal.” Science 313:
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Review Questions 492–95.
Rind, D., D. Peteet, W. S. Broecker, A. McIntyre,
1. In what sense are ice sheets both a climatic and W. F. Ruddiman. 1986. “The Impact of
response and a source of climatic forcing? Cold North Atlantic Sea-Surface Temperatures
on Climate: Implications for the Younger
2. Name an ice-driven response and explain its origin.
Dryas Cooling (11–10K).” Climate Dynamics 1:
3. Summarize three possible explanations for the 3–33.
unexpected strength of the 41,000-year response Ruddiman, W. F., 2006. “Ice-Driven CO Feedback
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of ice sheets between 2.75 and 0.9 Myr ago. on Ice Volume.” Climate of the Past 2: 43–78.