Page 138 - Earth's Climate Past and Future

114 PART II • Tectonic-Scale Climate Change

future, Earth’s climate won’t be getting there soon
Global
Uplift enough for it to cause you or me any concern. And in
cooling
Increased the meantime, other factors operating on shorter time
cooling scales will drive climate changes more relevant to
immediate human concerns, including the orbital-scale
Increased changes explored in Parts III and IV and the human-
ice induced changes examined in Part V.
Decreased
CO
2

Increased
rock fragmentation Key Terms
and weathering
permafrost (p. 99) ocean gateways (p. 103)
A Positive weathering feedback 18
δ O (p. 100) Monterey hypothesis
fractionation (p. 100) (p. 112, Box 6–2)

Meltback
Fragmented rock
Glacial position Review Questions
1. What kinds of changes in vegetation and ice show
that Earth has cooled in the last 50 Myr?
B Mountain glaciers
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2. What two things do changes in δ O values in the
last 50 Myr tell us about climate changes?
3. How well does the spreading rate (BLAG)
hypothesis explain the last 50 Myr of
Ice Meltback Glacial cooling?
position
4. How well does the uplift weathering
Fragmented rock hypothesis account for the last 50 Myr of
cooling?
C Continental ice sheets
5. Explain how chemical weathering could either
FIGURE 6-19 Positive feedback from ice? (A) Global
moderate or deepen long-term cooling.
cooling produces more ice on Earth, and the ice increases rock
fragmentation (B) in high mountain terrain and (C) near ice 6. The volume of water in the world ocean is
sheets. Chemical weathering of this fragmented debris may 48.5 times larger than the amount stored in the
cause further cooling by positive feedback. two largest ice sheets. The average δ O value of
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the ocean is near zero and the mean δ O value of
ice on Antarctica and Greenland is –50‰. Show a
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calculation indicating how much the mean δ O
value of ocean water would decrease if the two ice
sheets melted.
positive and negative. In any case, Earth’s long-term
“forecast” over tectonic time scales calls for colder tem- 7. Over the last 15 Myr, ice volume has increased,
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peratures and more ice, assuming that the current plate δ O values measured in deep-ocean foraminifera
c
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tectonic regime does not change. This cooling is hardly have increased, and long-term δ O values in
c
imminent. All tectonic-scale processes and feedbacks planktonic foraminifera from the tropical Pacific
operate at extremely slow rates, and the changes they Ocean have remained almost constant. What
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produce become evident only over millions of years. explains the difference between the two δ O c
Even though we seem to be headed toward a colder trends?