Page 81 - Earth's Climate Past and Future

CHAPTER 3 • CO and Long-Term Climate 57
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capable of accelerating the weathering process appeared, this interval, evidence that ice sheets were present
accelerated the rates of weathering, and pulled CO out (Figure 3-12). These rocks contain ice-deposited mix-
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of the atmosphere to keep the climate system in approx- tures of coarse boulders and cobbles along with fine silts
imate balance. and clays (Chapter 2). Because these ancient deposits
are difficult to date and correlate accurately, scientists
IN SUMMARY, the Gaia hypothesis is fascinating and is have inferred that as few as two or as many as four
still being argued. Scientists generally agree about major glacial eras occurred during this long interval.
the “minimum” form of Gaia: the idea that living A critical question is whether these ice sheets existed
organisms have played a significant role in the near the poles or at lower latitudes. For at least one of
history of physical-chemical processes on Earth, the glacial intervals (but not some of the others), several
including chemical weathering. Still, the lines of evidence suggest that the glaciated continents
“maximum” claim embedded in the Gaia were in the tropics. This conclusion forms the basis of
hypothesis—that individual life-forms regulate their the novel idea that Earth was once nearly frozen—the
own evolution for the greater benefit of all life on snowball Earth hypothesis.
the planet—is not accepted by most scientists. One obvious cause contributing to a cooler Earth
Somewhere in between lies the answer to the role of was weaker solar heating from a Sun that was still 6%
biota in determining the presence of life on Earth. below its modern luminosity (see Figure 3-2). Accord-
ing to the thermostat concept, a cooler Earth would
have reduced the rate of chemical weathering and kept
Was There a “Thermostat Malfunction”? CO values higher and moderated global temperature.
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A Snowball Earth? In this case, however, climate models suggest that CO 2
concentrations would have had to have been lower than
Ice sheets occur today at high latitudes, yet they coexist today to permit ice sheets to exist in tropical latitudes.
with hot tropics where a strong overhead Sun heats the In this instance, the thermostat mechanism seems to
land and the tropical oceans. With the large pole-to- have malfunctioned, at least for a while.
equator gradient in temperature, polar ice sheets can The reason for the thermostat malfunction remains
easily coexist on a planet with tropical heat. unresolved. One explanation is that the continents were
For a continent-sized ice sheet to have existed near all clustered near the equator, where high temperatures,
the equator, temperatures in the normally hot tropics precipitation, and vegetation cover combined to drive
would have had to be near or below freezing through unusually strong chemical weathering. Paradoxically,
most of the year. Today’s frigid polar climates would this tropical clustering could have reduced CO con-
2
have had to have invaded the tropics to permit ice centrations and cooled the planet.
sheets to exist there. The debate about how cold this world was contin-
Some climate scientists have suggested that Earth ues. Some scientists feel that Earth was frozen “hard,”
came very close to freezing totally between approxi- with sea ice extending right to the equator. Model sim-
mately 750 and 550 Myr ago. Sedimentary deposits ulations generally point toward a “softer” freeze, with
from glaciers are found on several continents during sea ice reaching to middle latitudes but not into the

90°W 0° 90° 180°E

FIGURE 3-12 Snowball Earth?
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Evidence of several glaciations between
60° 800 and 550 Myr ago exists in rocks on
the continents of today’s Earth. If these
glaciated regions were located in the
30° tropics, Earth must have been much
colder than today. (Adapted from L.
0° A. Frakes, Climates Through Geologic Time
[Amsterdam: Elsevier, 1979] and from
30° J. G. Meert and R. van der Voo,
“Neoproterozoic (1000–540 Myr)
Glaciation 850–550 Myr ago Glacial Intervals: No More Snowball
60° Earth,” Earth and Planetary Science Letters
S 123 [1994]: 1–13.)