Page 75 - Earth's Climate Past and Future

CHAPTER 3 • CO and Long-Term Climate 51
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even though the relatively warm temperatures in those typically cold, dry, and more sparsely vegetated, with
areas would otherwise favor it. Despite these com- more extensive snow and ice. An initial climate change
plications, temperature and precipitation generally act toward a colder, drier, less vegetated Earth should
together. A warmer Earth is likely to be a wetter Earth, reduce chemical weathering and slow the rate of
and both factors tend to act together to intensify chemi- removal of CO from the atmosphere. Slower CO
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cal weathering. removal should reduce the effect of the initial push
Vegetation also enhances chemical weathering. Plants toward climate cooling.
extract CO from the atmosphere through the process The action of these negative feedbacks does not
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of photosynthesis and deliver it to soils, where it com- mean that no climate change occurs at all. Any process
bines with groundwater to form carbonic acid. Although that initially acts to warm Earth succeeds in doing so,
H CO is a weak acid, it enhances the rate of chemical but by an amount smaller than would have been the
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breakdown of minerals. Scientists estimate that the pres- case without the negative feedback. Conversely, any
ence of vegetation on land can increase the rate of chemi- process that initially acts to cool Earth succeeds in
cal weathering by a factor of 2 to 10 over the rates typical doing so, but also to a reduced degree. The existence of
of land that lacks vegetation. a climate-dependent negative feedback due to chemical
Vegetation is closely linked to precipitation and weathering was proposed in 1981 by the geochemist
temperature (companion Web site, pp. 47–50). Dense
rain forests are found in regions with year-round rain-
fall, open forest or savannas in areas with a short dry
season, grasslands in places with a long dry season, and
deserts in areas with little or no rainfall. Each step in the Warmer
direction of greater rainfall is a step toward more vege- climate
tation and more total carbon biomass stored in vegeta- Initial Reduction of
tion and soils. change initial warming
In addition, the rate of production of carbon by Increased
photosynthesis across the planet is correlated with tem- temperature,
perature (Figure 3-6C). Cold, ice-covered regions pro- precipitation,
vegetation
duce little plant matter, and seasonally or permanently
frozen (but ice-free) polar regions produce only sparse Increased
CO removal
tundra vegetation. In comparison, production of carbon by weathering
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in warmer mid-latitude and tropical regions is much
greater. Increased
chemical
weathering
Is Chemical Weathering Earth’s
A
Thermostat?
Colder
Now we have in hand the components of a mechanism climate
that could act as Earth’s thermostat and moderate long-
term climate: the chemical weathering thermostat. Initial Reduction of
The global rate of chemical weathering is analogous to change initial cooling
a thermostat because it reacts to (depends on) the aver- Decreased
temperature,
age state of Earth’s climate and then alters that state by precipitation,
regulating the rate at which CO is removed from the
2 vegetation
atmosphere.
Consider what would happen if Earth’s climate Decreased
CO removal
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began to warm (Figure 3-7A). Any initial climate by weathering
change (for any reason) toward a warmer, moister, more
heavily vegetated greenhouse Earth should enhance Decreased
chemical
chemical weathering of silicate minerals, but the faster weathering
weathering in such a world should then speed up the
rate of removal of CO from the atmosphere. The B
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result should be a negative feedback that removes CO FIGURE 3-7 Negative feedback from chemical weathering
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and moderates the size of the imposed warming. Chemical weathering acts as a negative climate feedback by
The opposite sequence should happen if Earth’s cli- reducing the intensity of both (A) imposed climate warming
mate began to cool (Figure 3-7B). Icehouse climates are and (B) imposed climate cooling.