Page 195 - Earth's Climate Past and Future

CHAPTER 9 • Insolation Control of Ice Sheets 171

BOX 9-2 LOOKING DEEPER INTO CLIMATE SCIENCE

CONTINUED

125,000-year reef 125,000-year reef
(+44 m)
(+256 m)
82,000-year reef
82,000-year reef
(+8 m) (+145 m)
Modern sea level

Barbados New Guinea

0
Modern reef at Modern reef at
modern sea level modern sea level
25,000

50,000

75,000 Calculated uplift correction Calculated uplift correction
Reef formed Reef formed
100,000 17 m below 17 m below
modern sea level Reef formed modern sea level
82,000 years ago at +6 m 82,000 years ago
125,000
Reef formed
at +6 m
0 10 20 30 40 0 50 100 150 200 250
Current reef elevation (m) Current reef elevation (m)

Reconstructing sea level from ancient reefs By subtracting the effects of slow tectonic uplift
of the islands of Barbados and New Guinea, scientists can reconstruct sea level at earlier times
when coral reefs formed.

highlights the mismatch between the rhythms present in tion even during many of the smaller insolation max-
18
the insolation signal and those found in the δ O (ice ima, ice sheets did not disappear as easily but persisted
volume) response (Figure 9–20 top, center). until a sufficiently strong maximum occurred to cause
Large Glaciation Phase (0.9 Myr ago to the complete melting. With a longer time span in which to
Present) As global cooling continued, the glaciation grow, these ice sheets became larger than the ones dur-
threshold line again shifted relative to the insolation ing the small glaciation phase.
curve. In this new regime, conditions favorable for ice This large glaciation phase corresponds in a general
18
accumulation rather than ablation prevailed most of the way with the interval of δ O changes after 0.9 Myr ago
time (Figure 9–19E). Through much of this interval, (see Figure 9–13). During the last 900,000 years, larger
the climate point remained over the land and allowed ice sheets have grown over longer intervals of time.
ice growth. It retreated to the Arctic Ocean only during Milankovitch would probably have been surprised by this
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unusually strong insolation maxima (Figure 9–19F). part of the δ O record. Although the continued presence
Because the land remained in a regime of ice accumula- of 23,000-year and 41,000-year glacial cycles during this