Page 286 - Earth's Climate Past and Future

262 PART IV • Deglacial Climate Changes

Changes in mountain glaciers clouds alter local surface temperature in direct propor-
Advances Retreats Advances Retreats tion to how long they block the Sun.
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The glacial geologist Richard Alley has proposed
that millennial oscillations fall somewhere between the
extremes of cycles and red noise. His term for this
2000 behavior, stochastic resonance, is a composite of two
concepts that seem in conflict with each other. The
word stochastic means “random,” whereas resonance
implies cyclic behavior.
4000 In Alley’s view, resonance is evident in the fact that
oscillations at a period near 1500 years do appear now
Years ago 6000 and then in some records (Figure 14–13). At other times,
however, the climate system skips past (fails to register)

B 2500-year cycles ? individual oscillations at 1500 years because of interfer-
ence from the effects of random noise. When this hap-
pens, northern hemisphere climate drifts slowly toward
8000 colder conditions. Eventually, climate again responds
to one of the multiples of the 1500-year cycle with an
abrupt shift back to warmer climates. Because these
abrupt transitions can occur over a wide range of inter-
10,000 vals (after approximately 3000, 4500, 6000, 7500, or 9000
A 1500-year cycles?
FIGURE 14-11 Millennial-scale oscillations of mountain
glaciers Two attempts to synthesize advances and retreats Greenland
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of mountain glaciers over the last several millennia have 1470-year δ O ( )
produced different interpretations. (A) One hints that filter –42 –40 –38 –36
advances may have occurred at intervals of 1500 years,
but (B) the other indicates advances separated by about
2500 years. (A: Adapted from F. Rothlisberger, 10,000 Jahre
20,000
Gletschergeschichte der Erde [Frankfurt: Sauerländer, 1997].
B: Adapted from G. H. Denton and S. C. Porter,
“Neoglaciation,” Scientific American 222 [1970]: 100–10.)

actually ranges between about 1000 and 2000 years. 40,000
This signal is better described as “quasi-periodic” than
cyclic. In any case, as noted earlier, oscillations at or Years ago
near 1500 years are more often missing than present in
other records of the current interglaciation from the
high-latitude North Atlantic. 60,000
Alternatively, these oscillations may be red noise.
The word noise means that the fluctuations are random
and unpredictable, rather than cyclic and predictable.
The term red refers to a characteristic behavior in which
the longer-duration oscillations are larger in size than the
80,000
shorter-term oscillations. As an analogy, consider a late
spring afternoon with distinct clouds of many sizes
drifting across the sky. Small clouds that block the Sun
FIGURE 14-12 A millennial cycle in Greenland ice? Filtering
for a few minutes may cool temperatures at Earth’s of the full δ O record from Greenland GISP2 ice covering the
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surface in a barely noticeable way. Larger clouds that interval 85,000 to 10,000 years ago (right) reveals that a
block the sunlight for an hour or more cool the warm 1470-year cycle (left) is present at times but weak or absent
afternoon more obviously. A band of clouds that persists during others. (Adapted from M. Stuiver, T. F. Brazunias,
for most of the afternoon reduces the peak daily tem- P. M. Grootes, and G. A. Zielinski, “Is There Evidence for
perature even more. In this example of red noise, the Solar Forcing of Climate in the GISP2 Oxygen Isotope
passage of clouds in front of the Sun is random, and the Record?” Quaternary Research 48 [1997]: 259–66.)