Page 76 - Earth's Climate Past and Future

52 PART II • Tectonic-Scale Climate Change

James Walker and his colleagues Paul Hays and James Slower rates of weathering would have left more CO
2
Kastings. in the atmosphere over much of Earth’s early history, per-
How do we apply this concept to the mystery of the haps 100 to 1000 times as much as today (Figure 3-8A).
faint young Sun paradox? Recall that Earth needed a The warmth produced by this high-CO atmosphere
2
global thermostat that made it warmer early in its his- could have countered most of the cooling caused by the
tory to counter the weakness of the early Sun, but that smaller amount of incoming solar radiation.
later throttled back on the warming as the strengthen- Then, as Earth began to receive more radiation
ing Sun provided greater heat. from the brightening Sun, its surface warmed and the
Earth’s environment very early in its history is rate of chemical weathering gradually increased. Faster
poorly known, but it is widely thought to have included chemical weathering began to draw more CO out of
2
active volcanism that caused large emissions of volatile the atmosphere, and the resulting drop in atmospheric
gases (including CO ) from its interior. Many scientists CO levels provided a cooling effect that counteracted
2 2
believe that Earth’s surface may even have been entirely the gradual increase in solar warming and kept Earth’s
molten for a few hundred million years after 4.55 Byr temperatures moderate (Figure 3-8B). The centerpiece
ago. In addition, ancient craters preserved on our moon of this explanation is that the slow warming of Earth by
and on other planets indicate that Earth was once under the strengthening Sun would have caused changes in
heavy bombardment by asteroids, meteors, and comets, weathering that moderated changes in climate.
and these collisions may have triggered greater volcan-
ism as well. Radioactive elements deep in Earth’s inte- IN SUMMARY, chemical weathering is an excellent
rior also released heat that could have increased the candidate for Earth’s thermostat.
amount of volcanism. Increased volcanic activity would
have delivered more CO to the atmosphere and helped If chemical weathering is Earth’s thermostat, we face
2
to make Earth hot. As noted earlier, however, it is very still another question: What happened to all that CO
2
unlikely that volcanism is the thermostat responsible that once resided in the atmosphere and kept Earth
for maintaining Earth’s moderate climate through all warm? The most likely answer is found by looking at
4.55 Byr of its existence. the size of the carbon reservoirs in Figure 3-3: the car-
Chemical weathering is a more promising explana- bon removed from today’s atmosphere by weathering is
tion. The weakness of the young Sun would have tended buried in ocean sediments that eventually turn into
to make the early Earth cooler than it is today, and the rocks. The same process would also have been at work
rate of CO removal from the atmosphere by weather- in the past, and over time it would have caused a slow
2
ing would have been slower because of the lower tem- but massive transfer of carbon from the atmosphere to
peratures. In addition, early continents are thought to rocks. If this interpretation is correct, most of Earth’s
have covered a smaller area than they do today. The early greenhouse atmosphere lies buried in its rocks
smaller area of the continents would also have favored instead of concentrated in the atmosphere, as on Venus.
slower CO removal from the atmosphere by weather- Some scientists have suggested that greater emissions
2
ing because less rock surface was available to weather. of methane (CH ) and ammonia (NH ) from Earth’s
4 3

Weaker solar radiation Stronger solar radiation

FIGURE 3-8 Earth’s thermostat
A plausible explanation of the faint
Stronger greenhouse Weaker greenhouse
young Sun paradox is that (A) the
weakness of the early Sun was
compensated for by a stronger CO
2
greenhouse effect in the
atmosphere. (B) Later, when the
Sun strengthened, increased
chemical weathering deposited the
excess atmospheric greenhouse
carbon in rocks, and the weakened
CO 2 in rocks greenhouse effect kept Earth’s
CO in atmosphere temperatures moderate. (Adapted
2 from W. Broecker and T.-H. Peng,
Greenhouse Puzzles [New York:
A Early Earth B Modern Earth Eldigio Press, 1993].)