Page 77 - Earth's Climate Past and Future

CHAPTER 3 • CO and Long-Term Climate 53
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interior warmed the early Earth. In the modern atmos- Initial
phere, both of these gases tend to be broken down in the change Climate
atmosphere within a few years by chemical reactions, warming
which would seem to require large and continual addi- Increased
tions to the early atmosphere to maintain high levels. On warming
the other hand, the early atmosphere was devoid of oxy-
gen, which plays a primary role in removing methane and
ammonia today. These two gases could also have played a
role in warming the early atmosphere.
Increased
atmospheric
3-3 Greenhouse Role of Water Vapor water vapor
What about the role of water vapor in these longer-
term climate changes? Water vapor is by far the most Increased
important greenhouse gas in Earth’s atmosphere today. greenhouse
It accounts for most of the 33°C greenhouse effect that trapping
keeps our planet warm (companion Web site, pp. 2–3). of radiation
Why wouldn’t water vapor have played a major thermo-
stat role in Earth’s past? FIGURE 3-9 Water vapor feedback When climate warms,
The answer to this question is that water vapor and the atmosphere is able to hold more water vapor, which is the
CO work in fundamentally different ways in the cli- major greenhouse gas in the atmosphere. The increase in water
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mate system. Over tectonic time scales, CO acts as a vapor leads to further warming by means of positive feedback.
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negative feedback that suppresses the size of changes This feedback works in the same way (but opposite direction)
in climate. In contrast, the effect of water vapor is just during cooling.
the reverse: it acts as a positive feedback that amplifies
changes in climate.
The amount of water vapor in Earth’s atmosphere
varies over a wide range, from as little as 0.2% in very climate, yet we have also seen that the processes
cold dry air to more than 3% in humid tropical air. This involved are not strictly physical. Biological processes
natural link between air temperature and water vapor participate in the carbon cycle. For this reason, some
produces an important positive feedback in the climate scientists infer that life itself, rather than strictly physi-
system called water vapor feedback (Figure 3-9). cal-chemical factors, may be the thermostat that actu-
Assume that climate warms for some reason. Because ally regulates Earth’s climate. (In addition, a substantial
a warmer atmosphere can hold much more water vapor, amount of the carbon that moves through Earth’s reser-
the resulting increase in this greenhouse gas traps more voirs does so in organic form, as part of a separate and
heat. This enhanced greenhouse effect further warms smaller subcycle (Box 3-1).
Earth, amplifying the initial warming through a posi-
tive feedback loop. The same positive feedback process 3-4 Gaia Hypothesis
works in the opposite direction when climate cools: if an The biologists James Lovelock and Lynn Margulis pro-
initial cooling reduces the amount of water vapor held posed in the 1980s that life itself has been responsible
in the atmosphere, an additional cooling will result from for regulating Earth’s climate. They called their idea the
the reduced greenhouse effect. Because water vapor Gaia hypothesis, after the ancient Greek Earth god-
amplifies rather than moderates climatic changes, it can- dess. A crude analogy of how their hypothesis works
not have acted as Earth’s thermostat. The continuing is the way the fur on an animal fluffs out to create an
presence of running water shown by geologic deposits insulated layer and keep the creature warm when the
through Earth’s recorded history indicates that liquid weather turns cold. The animal in effect unconsciously
water has been available to feed water vapor to the regulates its own environment for its own good. The
atmosphere and provide positive feedback to tempera- Gaia hypothesis holds that life regulates climate on
ture changes.
Earth for its own good (Figure 3-10).
Supporters of this hypothesis cite features of the
Is Life the Ultimate Control on Earth’s chemical weathering thermostat that directly involve
Thermostat? the action of life-forms: (1) the fact that carbon is the
basis of the CO cycle; (2) the action of land plants in
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We have seen that chemical weathering provides a plau- photosynthesizing carbon dioxide and transferring it to
sible thermostat-like mechanism to moderate Earth’s the soil as part of the vegetation litter, thereby forming