Page 70 - Earth's Climate Past and Future

46 PART II • Tectonic-Scale Climate Change

The solution to the faint young Sun paradox appears quently decreased in abundance, that would have pro-
to require a process that works the same way a thermo- vided a thermostat-like control.
stat works in a house. When outside temperatures fall in Our earlier comparison of Earth and Venus lends
winter, the thermostat detects the cooling and turns on credibility to this explanation. Earth’s carbon is mainly
a heat source that keeps the house warm. When temper- stored in its rocks, while carbon on Venus is mostly in
atures become too hot outside in summer, the thermo- its atmosphere. If carbon can reside in different reser-
stat activates a cooling source that keeps the house cool. voirs on different planets, why couldn’t it move among
The thermostat moderates extreme swings in tempera- reservoirs during the history of a single planet? More
ture. Such a thermostat must have been at work through specifically, could the early Earth have held more car-
Earth’s history, warming its climate very early on when it bon in its atmosphere (like Venus) and then transferred
would otherwise have frozen under a weak Sun and later it to its rocks later in its history?
on cutting back on the heat provided from the strength-
ening Sun. Carbon Exchanges between Rocks and
One possibility is that greenhouse gases have been
part of the mechanism that acts as Earth’s thermostat. the Atmosphere
Our modern concentrations of greenhouse gases do not To understand how carbon may have shifted among
provide enough warming to have counteracted the Earth’s reservoirs, we need to examine the present car-
effects of a weak early Sun, but if these gases had been bon cycle (Figure 3-3A). Small amounts of carbon exist
more abundant earlier in Earth’s history and subse- in the atmosphere, in the surface ocean, and in vegeta-

Vegetation: 610
Atmosphere: 600
(preindustrial)

Ocean mixed layer: 1000
Soils: 1560

Deep ocean: 38,000
Sediments and rocks:
66,000,000

A Major carbon reservoirs (gigatons)
FIGURE 3-3 Carbon exchanges
100 Atmosphere with Earth’s rocks (A) The largest
50
50 reservoir of carbon on Earth lies in its
Vegetation
rocks, not in its atmosphere,
50 74.6 vegetation, or ocean. (B) All of
0.6 74 Earth’s reservoirs exchange carbon.
0.8
Over intervals of millions of years,
Soil Ocean mixed layer
slow exchanges among the rock and
surface reservoirs can cause large
37
changes in atmospheric CO levels.
2
0.2 (Adapted from J. Horel and J. Geisler,
37 Global Environmental Change [New York:
Wiley, 1997] and from National
Deep ocean Research Council Board on
Sediments and rocks Atmospheric Sciences and Climate,
0.2
Changing Climate, Report of the
Carbon Dioxide Assessment
Committee [Washington, D.C.:
B Carbon exchange rates (gigatons/year) National Academy Press, 1993].)