Page 91 - Earth's Climate Past and Future

CHAPTER 4 • Plate Tectonics and Long-Term Climate 67

BOX 4-1 LOOKING DEEPER INTO CLIMATE SCIENCE
Brief Glaciation 440 Myr Ago

n ice sheet comparable in size to that on the conti- enough to explain it. Volcanoes and chemical weathering
Anent of Antarctica today covered the North African rates can gradually drive CO levels low enough to pro-
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part of the Gondwana continent near 440 Myr ago. Until duce glaciation, but this episode appears to require a
recently this glaciation was thought to have lasted at least mechanism capable of dropping and then raising CO val-
2
10 Myr, and it was attributed to a combination of factors: ues within 1 Myr.
the general cooling effect from a Sun that was 4% weaker One mechanism under consideration is an abrupt
than today, the positioning of the North African part of increase in the rate of burial of organic carbon. The organic
Gondwana directly over the South Pole, and a reduction carbon subcycle (see Box 3–1) meets several requirements
of atmospheric CO values caused by some combination for explaining a large but rapid climate cooling. Because it
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of slower CO input by volcanoes and faster chemical carries one-fifth of the total flow of carbon through the
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weathering. Faster weathering may have been caused by upper parts of Earth, this subcycle has the potential to alter
small continental collisions prior to the ones that later the global carbon balance and atmospheric CO levels. Also
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formed the supercontinent Pangaea, perhaps aided by the favoring this explanation is the fact that large amounts of
first appearance of vegetation on land and its effect in organic carbon can be quickly buried in the sedimentary
enhancing weathering (Chapter 3). record, causing a rapid reduction of CO levels.
2
More recent dating of the geologic record suggests Several kinds of changes can cause rapid burial of organic
that this glaciation may have lasted much less than 10 Myr, carbon: changes in wind direction that cause increased
possibly 1 Myr or less—a very brief episode in comparison upwelling along coastal margins; an increase in the amount
with the 35 Myr of the present glacial era and the glacia- of organic carbon and nutrients delivered to the ocean; a
tion that lasted from 325 to 240 Myr ago. If this glaciation change toward wetter climates on continental margins,
was indeed only a million years long, neither seafloor where low relief naturally favors formation of vegetation-
spreading nor chemical weathering seems likely to have rich swamps; or the isolation of small ocean basins in regions
changed the CO concentration in the atmosphere fast of high rainfall that generates carbon-rich river runoff.
2

Pangaea provides climate scientists with a very different
IN SUMMARY, the polar position hypothesis may be and yet real Earth for testing the performance of cli-
part of the story, but some other factor must also be mate models.
at work, a factor that controls climate in such a way
as to allow ice sheets to form over polar continents 4–4 Input to the Model Simulation of Climate on
during some intervals and prohibit them from doing
so during others. One likely cause is changes in Pangaea
concentrations of greenhouse gases. Recall from Chapter 2 that GCM runs require the
major physical aspects of a past world to be specified
Modeling Climate on the Supercontinent in advance as boundary condition input in order to run
Pangaea simulations of past climates. The most basic physical
constraint is the distribution of land and sea. Pangaea
One fortunate aspect of studying the history of Earth’s remained intact from the time it formed (250 Myr ago)
climate on tectonic time scales is the number of natural until it broke up after 180 Myr ago. The focus here is
climate experiments Earth has run by greatly altering its on this long interval of relatively stable land-sea geome-
geography. Because the locations of continents are try. The only tectonic change of significance during this
accurately known for the past 300 Myr, climate scien- time was a very slow northward movement of Pangaea.
tists can use general circulation models (GCMs) to eval- At 200 Myr ago, Pangaea stretched from high
uate the impact of geographic factors on climate. Here northern to high southern latitudes and was almost
we examine a time near 200 Myr ago when collisions of symmetrical around the equator (Figure 4–10A). The
continents had formed the giant supercontinent Pan- landmasses of Gondwana (Antarctica, Australia, Africa,
gaea. Because this configuration differs considerably Arabia, South America, and India) formed its southern
from the more dispersed locations of continents today, part. Northern Pangaea consisted of the remaining