Page 72 - Earth's Climate Past and Future

48 PART II • Tectonic-Scale Climate Change

In our hypothetical example of a sudden cessation of function as the thermostat? The answer is no. The basic
volcanic CO input to the atmosphere, the actual sce- operating principle of a thermostat is that it first reacts
2
nario might develop more like this: As CO levels in the to external changes and then acts to moderate their
2
atmosphere begin to fall, the other surface reservoirs effects: a thermostat detects the chill of a cold night and
(vegetation, surface ocean, soils) would begin to surren- sends a signal that turns on the heat.
der some of their carbon to the atmosphere, slowing its Volcanic processes are not thought to operate in this
rate of loss. For the fast-reacting reservoirs, changes in way. The volcanic activity that has occurred on Earth
one are felt by the others almost immediately because of throughout its history has been driven mainly by heat
the rapid exchange rates. sources located deep in its interior and generally far
The combined size of all the near-surface reser- removed from contact with the climate system. Climat-
voirs (atmosphere, vegetation, soil, and surface ocean) ically driven changes in temperature penetrate only
is 3700 gigatons, more than six times larger than the the outermost few meters or tens of meters of the land
atmospheric reservoir alone. It would take roughly (or seafloor). As a result, climate changes confined to
24,700 years after volcanism ceased for these reservoirs Earth’s surface have no physical way to alter deep-
to lose all their carbon (3700 gigatons divided by seated processes in Earth’s interior. Without such a link,
0.15 gigaton/yr). no thermostat-like changes in volcanic activity and CO
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In addition, over time spans of centuries, the large delivery to the surface can occur.
deep-ocean carbon reservoir would begin to play a Earth’s thermostat lies elsewhere. It must be found
role. If the surface reservoirs were all losing significant in a process that responds directly to the climate condi-
amounts of carbon, the deep ocean would feed some of tions at Earth’s surface.
its carbon to the surface ocean, from which it would be
redistributed to the atmosphere and the vegetation. If 3-2 Removal of CO from the Atmosphere by
2
we take the large deep-ocean reservoir into account, the Chemical Weathering
total size of these reservoirs amounts to 41,700 gigatons.
It would take 278,000 years for a total shutdown of vol- To avoid a long-term buildup of CO levels, CO input
2 2
canic carbon input to deplete these combined reservoirs to the atmosphere by volcanoes must be countered by
completely (41,700 gigatons divided by 0.15 gigaton/yr). CO removal. The major long-term process of CO
2 2
At this point it might seem that we have shown that removal is tied to chemical weathering of continental
Earth’s surface reservoirs, including the atmosphere, are rocks (Chapter 2). Two major types of chemical weath-
actually not particularly vulnerable to changes in the ering occur on continents: hydrolysis and dissolution.
amount of carbon coming out of (or going into) its Hydrolysis Hydrolysis is the main mechanism for
rocks, but this conclusion would be incorrect. Even a removing CO from the atmosphere. The three key
2
time span as long as 278,000 years represents less than ingredients in the process of hydrolysis are the minerals
one–ten thousandth of Earth’s 4.55-Byr age. Because that make up typical continental rocks, water derived
Earth is so old, plenty of time is still available for the from rain, and CO derived from the atmosphere
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slow carbon exchanges with Earth’s rock reservoirs to (Figure 3-5).
alter the amount of carbon in the surface reservoirs by Most of the continental crust consists of rocks, such
large amounts. as granite, made of silicate minerals like quartz and
With Earth’s great antiquity taken into account, it feldspar. Silicate minerals typically are made up of posi-
+1
+2
+2
+3
is still amazing that over this immense span of time tively charged cations (Na , K , Fe , Mg , Al , and
+1
Earth’s volcanoes have somehow managed to keep deliv- Ca ) that are chemically bonded to negatively charged
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ering just enough carbon from Earth’s interior to keep SiO (silicate) structures. These silicate minerals are
4
the atmosphere from running out of CO but not so slowly attacked by groundwater containing carbonic
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much as to overheat the planet. This achievement acid (H CO ) formed by combining atmospheric CO
2 3 2
requires a very delicate balance. Even more amazing is with rainwater.
the fact that this balancing act had to be maintained as Part of the weathered rock is chemically converted
the faint young Sun was slowly increasing in strength. A to clay minerals (compounds of Si, Al, O, and H) and
simple analogy for this long-term balancing act is a left as soils. Chemical weathering also produces several
tightrope walker who has to stay balanced on a narrow types of dissolved ions and ion complexes, including
+1
wire that slopes uphill over a very long distance. HCO , CO , H SiO , and H . These ions are car-
–1
–2
3 3 2 4
We noted earlier that this balancing act requires ried by rivers to the ocean, and some are incorporated
some kind of natural thermostat to moderate Earth’s in the shells of planktic organisms (see Figure 3-5).
temperature. Could the rate of volcanic input of CO Dozens of chemical equations describe the process
2
from Earth’s interior have varied in such a way as to of chemical weathering—in fact, there is one equation