Page 225 - Earth's Climate Past and Future

CHAPTER 11 • Orbital-Scale Interactions, Feedbacks, and Unsolved Problems 201

Colder Warmer the ice sheets had grown to a large size. With these con-
More ice Less ice straints, terminations would naturally tend to occur after
18
δ O (‰) either four or five 23,000-year insolation maximum, or
5 4 3 at intervals of ~92,000 or ~115,000 years.
0
In any case, the origin of the ~100,000-year oscilla-
tions remains an area of intensive research. The many
explanations that have been proposed fall into three
major groups.

11-5 Ice Interactions with Bedrock
1 0.9
Myr One suggestion is that Earth’s climate system has a
natural 100,000-year resonance, analogous to a bell that
rings with a characteristic period or frequency, called a
resonant response. In this view, any external force
that disturbs Earth’s climate (changes in tectonic con-
figurations, orbital insolation, or something else) will
produce a characteristic internal resonant response like
2
a ringing bell.
Myr ago of tectonic-scale changes that might create such a reso-
The problem with this explanation is that the kinds
nance have changed extremely slowly over the last few
million years (Chapter 4). Plates move at rates of cen-
2.75 timeters per year, which translate into a few tens of kilo-
Myr meters over a million years. Rates of net mountain
3
uplift and erosion are even slower. No one has yet

June 65°N
solar radiation
2
(cal/cm /day)
Minimum Maximum
800 900 1000 ice ice
4 0
100,000

18
18
FIGURE 11-14 Changes in δ O in the last 4.5 Myr A δ O
200,000
record from a sediment core in the eastern Pacific Ocean
18
shows a slow increase in δ O values over the last 4.5 Myr.
(Adapted from A. C. Mix et al., “Benthic Foraminifer Stable Years ago 300,000
Isotope Record from Site 849 [0–5Ma]: Local and Global Climate
Changes,” Ocean Drilling Program, Scientific Results 138 [1995]:
400,000
371–412.)
500,000
be produced at intervals of ~100,000 years from a highly
modulated 23,000-year cycle. 600,000
Although major deglaciations (terminations) occur at
an average spacing of ~100,000 years, none of the gaps
FIGURE 11-15 Strong summer insolation peaks pace rapid
between terminations is actually that long. The spacing deglaciations (Left) Strong summer insolation peaks at the
between the last five terminations was approximately precession cycle resulting from eccentricity modulation match
116,000, 117,000, 94,000, and 84,000 years. This some- (right) rapid deglacial terminations indicated by δ O
18
what irregular timing makes sense if the deglaciations (ice volume) signals. Adapted from W. S. Broecker,
were constrained to occur on or near one of the major “Terminations,” in Milankovitch and Climate, ed.
insolation maxima at the precession cycle and only after A. L. Berger et al. [Dordrecht: D. Reidel, 1984].)