Page 71 - Earth's Climate Past and Future

CHAPTER 3 • CO and Long-Term Climate 47
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tion, along with a slightly larger reservoir in soils, a The present rate of natural carbon input to the
much larger reservoir in the deep ocean, and an atmosphere from the rock reservoir is estimated at
immensely larger reservoir in rocks and sediments. approximately 0.15 gigatons of carbon per year (see
Carbon storage in these reservoirs is measured in Figure 3-3B). This value is uncertain by a factor of at
billions of tons (gigatons). least 2, because volcanic explosions are irregular in time
The rates of carbon exchange among these reser- and because the amount of CO released varies with
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voirs vary widely (Figure 3-3B). In general, an inverse each eruption. As we will see later, this natural rate of
relationship exists between the size of a reservoir and carbon input is roughly balanced by a similar rate of
the rate at which it exchanges carbon. The smaller natural removal. This balance between natural input
reservoirs (atmosphere, surface ocean, and vegetation) and removal rates helped to keep the size of the “nat-
all exchange carbon relatively quickly, while the huge ural” (preindustrial) atmospheric carbon reservoir at
rock reservoir gains and loses carbon much more ~600 gigatons.
slowly. As a result of the combined effects of reservoir But how likely is it that this balance could have
size and exchange rate, carbon can cycle through the persisted over immensely long intervals of geologic
smaller reservoirs at the surface within a few years but time? We can evaluate this question by a simple thought
moves much more slowly through the larger and deeper experiment. Using the reservoir concept introduced in
reservoirs. Chapter 2, we can calculate how long it would take for
Because all these reservoirs exchange carbon with the atmospheric CO level to fall to zero if all volcanic
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the atmosphere, each has the potential to alter atmos- release of carbon from Earth’s interior to the atmosphere
pheric CO concentrations and affect Earth’s climate. abruptly ceased but carbon continued to be removed
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The relative importance of each carbon reservoir in from the atmosphere at the same rate as before.
Earth’s climate history varies according to the time scale The answer, derived by dividing the preindustrial
under consideration. In this chapter, we are concerned atmospheric carbon reservoir of 600 gigatons by an
with very gradual climate changes over tens of millions annual rate of carbon removal of 0.15 gigaton, is 4000
of years. Over these very long (tectonic) time scales, the years. This number, although obviously well beyond the
effects of the slow carbon exchanges between the rocks length of a human lifetime, is remarkably brief in the
and the surface reservoirs produce large changes in the context of the several billion years of Earth’s existence.
amount of CO in the atmosphere. It tells us that changes in volcanic input persisting over
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that relatively “small” span of time could have a drastic
effect on the CO content of our atmosphere.
3-1 Volcanic Input of Carbon from Rocks to the In actuality, the atmosphere is not really this vulner-
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Atmosphere
able because rapid exchanges of carbon occur continu-
Carbon cycles constantly between Earth’s interior and ously between the atmosphere and several other carbon
its surface. It moves from the deep rock reservoir to the reservoirs. These rapid exchanges have the effect of
surface mainly as CO gas produced during volcanic slowing and reducing the impact of the loss of carbon
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eruptions and in the activity of hot springs (Figure 3-4). from Earth’s interior.

CO 2
CO 2

Volcano

Hot spring
FIGURE 3-4 Input of CO from
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volcanoes CO enters Earth’s
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atmosphere from deep in its interior
through release of gases in volcanoes
and at hot springs such as those
Melting found today at Yellowstone National
Park in Wyoming.