Page 292 - Earth's Climate Past and Future

268 PART IV • Deglacial Climate Changes

a range of indicators suggests that millennial oscilla- Key Terms
tions in the northern hemisphere were much larger in
winter than in summer. This pattern is consistent with millennial oscillations red noise (p. 262)
propagation of the millennial oscillation during the (p. 251) stochastic resonance
strong atmospheric flow of winter. Dansgaard-Oeschger (p. 262)
oscillations (p. 253) bedrock pinning points
IN SUMMARY, the origin of millennial climatic sediment drifts (p. 253) (p. 265)
oscillations remains unknown, but considerable Heinrich events (p. 253) bipolar seesaw (p. 267)
progress has been made in defining their time-and-
space “footprint” in the climate system. The
oscillations appear to be largely random rather than
cyclic. Their amplitude is largest when ice sheets are Review Questions
present on North America and Eurasia, and the
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largest oscillations are centered near Greenland and 1. How do the processes that control δ O changes
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the North Atlantic Ocean. Similar (but smaller) in ice sheets differ from those that control δ O
changes with nearly opposite timing occur over fluctuations in ocean cores?
Antarctica and in other regions in the southern
hemisphere, including the southeastern Pacific 2. Why is it difficult to correlate millennial climatic
Ocean off the coast of Chile. The bipolar seesaw oscillations in records from different regions?
is a promising candidate for an internal oscillation 3. What other regions show millennial oscillations
of natural origin. like those in the North Atlantic and Greenland?
4. Are millennial oscillations true cycles?
14-10 Implications for Future Climate 5. How strong is the evidence that solar changes
drive millennial oscillations?
Because millennial oscillations occur much faster than
orbital-scale changes, they have the potential to have a 6. What is the evidence for and against internal ice
more immediate impact on our climatic future. Scientists sheet processes causing millennial oscillations?
and policy planners would like to know whether natural 7. How could ocean flow cause opposite millennial
oscillations could cause climate to warm or to cool in oscillations north and south of the equator?
future decades. Some scientists have speculated that a
natural millennial-scale warming could be underway
now. If this view is correct, it means that the observed
warming of the last century or so could in part be the
result of natural processes rather than human activities. Additional Resources
This claim is unjustifiable for several reasons.
Because millennial oscillations are either completely Basic Reading
random (red noise) or at best quasi-periodic, their pre- Alley, R. B. 2000. The Two-Mile Time Machine.
sent and future course cannot be predicted with any Princeton, NJ: Princeton University Press.
confidence. More critically, the largest oscillations
occurred only during glacial climates, whereas the Advanced Reading
changes during the past 8000 years of warm interglacial Beer, J., et al. 1988. “Information on Past Solar
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climate have been small and local in scale. This observa- Activity and Geomagnetism from Be in the
tion argues against natural oscillations playing a major Camp Century Ice Core.” Nature 331: 675–79.
role in present and future climate change. Bond, G., W. S. Broecker, S. J. Johnsen, J. McManus,
Of course, the Greenland and Antarctic ice sheets L. D. Labeyrie, J. Jouzel, and G. Bonani. 1993.
are still in place, and they remain susceptible to some “Correlations Between Climatic Records from
degree of melting in the warmer climate of the future. North Atlantic Sediments and Greenland Ice.”
Because ice sheets appear to have played a role in at Nature 365:143–47.
least some of the glacial-age millennial oscillations, par- Boyle, E. A. 2000. “Is Ocean Thermohaline
tial melting of today’s ice sheets because of human activ- Circulation Linked to Abrupt Stadial-Interstadial
ities could conceivably trigger changes in the climate Transitions?” Quaternary Science Reviews 19:
system in the future, even if at a smaller scale. In this 255–72.
case, however, the cause of these changes will be human Brook, E. J., et al. 2005. “Timing of Millennial-Scale
activities, not natural variations in the climate system. Climate Change at Siple Dome, West Antarctica,