The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 277

18 O, and water containing 16 Oismorelikely to evap-
orate than is water containing 18 O. Because calcium
carbonate in shells contains oxygen from seawater and
because higher seawater temperatures correlate with
higher 18 O-to- 16 Oratios in seawater, higher 18 O-to- 16 O
ratios in shells correlate with higher water temperatures.
Because certain shelled organisms live within specified
temperature ranges, the distribution of shells in a verti-
cal core sample also gives insight into historical water
temperatures.

absolute quantities, partial pressures of CO 2 (g) 4.6 to
2.5 b.y.a. may have been 0.01 to 0.1 atm, or 30 to 300
times their current values (Garrels and Perry, 1974; Pol-
lack and Yung, 1980). The high partial pressures of
CO 2 (g) enhanced the greenhouse effect of the young
Earth.
Over time, chemical weathering in the newly formed
oceans converted CO 2 (g) to carbonate rocks. Bacteria
also consumed CO 2 (g) and fossilized when they died.
Carbonate rock formation may have been enhanced by
cyanobacteria and other autotrophic bacteria, which use
carbon dioxide as an energy source. Dead cyanobacteria
pile up to form laminated, bounded structures of trapped
carbonaceous material called stromatolites ,which are
usually found in shallow marine waters in warm regions,
and some are still being formed (Figure 12.16).
The Proterozoic eon (2.5-0.542 b.y.a.) was a period
during which the Earth was generally warm and ice free,
except for two periods of extended continental glacia-
tion. The first was the Huronian glaciation (2.4-2.1
b.y.a), due in part to the Great Oxygenation Event
(GOE) (Section 2.3.4), during which oxygen produced
by photosynthesizing bacteria increased slightly in the
air because rocks had become saturated with oxygen.
The increase in oxygen increased OH(g) and O( 1 D )(g),
which converted CH 4 (g), a strong greenhouse gas, to
CO 2 (g), a weaker one, cooling the climate. Simultane-
ously, CO 2 (g) decreased due to an increase in carbonate
rock formation. The drops in CH 4 (g) and CO 2 (g), cou-
pled with the low solar intensity at that time, may have
triggered the glaciation.
The second period of glaciation involved two sep-
arate ice ages that occurred during the Cryoge-
nian period (850-635 m.y.a.). These were the Stur-
tian glaciation (750-700 m.y.a.) and the Marinoan/
Varanger glaciation (660-635 m.y.a.). The two ice
ages were the most severe in the Earth's history, caus-
ing glaciers to extend on continents down to low lati-
tudes, as evidenced by rock analysis (Williams, 1975).
Some scientists speculate that the glaciations resulted
in a snowball Earth ,but there is no evidence of sea ice
appearing at low latitudes (Crowley and North, 1991),
indicating that the glaciations occurred over land only.
The glaciations may have been caused by a drop in
CO 2 (g) following a surge in weathering of exposed
volcanic rock due to the rifting and breakup of the bar-
ren and lifeless supercontinent, Rodinia , 750 m.y.a.
The remnants of Rodinia merged again 600 m.y.a. to
form the supercontinent Pannotia ,which itself split
into Gondwana (Africa, South America, Antarctica,
Australia), Laurentia (present-day North America),

12.3.2.3. From the Origin of the Earth
to 542 Million Years Ago
Figure 12.15 shows the Earth's geologic time scale.
Since the formation of the Earth 4.6 b.y.a., near-surface
air temperatures have gone through great swings.
Between 4.6 and 4.0 b.y.a., surface temperatures
escalated due to the conduction of energy from the
Earth's core to its surface, creating magma oceans (Sec-
tion 2.3.1) that resulted in air temperatures 300 ◦ Cto
400 ◦ Cgreater than those today (Crowley and North,
1991). During that time, energy released by meteorite
impacts also contributed to high surface temperatures.
Between 4.6 and 2.5 b.y.a. (the Hadean and Archean
eons ), the intensity of solar radiation incident upon
the Earth was about 20 to 30 percent lower than it
is today because the solar output of a young star is
low and gradually increases over time. Yet, even after
the magma oceans solidified to form the Earth's crust
3.8 to 4 b.y.a., air temperatures were still much higher
than today (58 ◦ Cvs.15 ◦ Ctoday). The low sun intensity
coupled with the high air temperatures following crustal
formation is referred to as the Faint Young Sun Para-
dox (Sagan and Mullen, 1972; Ulrich, 1975; Newman
and Rood, 1977; Gilliland, 1989; Crowley and North,
1991). The reason for the high temperatures may have
been an enhanced greenhouse effect due to both car-
bon dioxide and methane. During solidification of the
Earth's crust and mantle, outgassing, particularly of
water vapor, carbon dioxide, and methane, occurred.
Whereas much of the water vapor condensed to form
the oceans, most carbon dioxide and methane remained
in the air. The oceans had not formed sufficiently by 3.5
b.y.a. to remove carbon dioxide by chemical weather-
ing. Furthermore, anoxygenic photosynthesis (Section
2.3.3.4), which removes CO 2 (g) from the air and con-
verts it to cell material, did not develop until 3.5 b.y.a.
Figure 2.11 shows that the Earth's atmosphere prior
to the oxygen age (2.3 b.y.a.) may have contained 10
to 80 percent carbon dioxide by volume. In terms of