Environmental Engineering Reference
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300
1
S
/
S
0
Freezing point of water
275
.9
T
S
250
.8
T
e
.7
225
4
3
2
1
0
Billion of years before present
Figure 3.4.2
Solar luminosity
The solar luminosity normalized by the current luminosity as a function of time. The
lower blue curve is the earth's effective radiating temperature,
T
e
. The upper blue curve
represents the calculated mean global surface temperature,
T
s
. Both curves have been
calculated using a model in which the atmosphere has the current (300 ppm) CO
2
con-
centration.
Figure based on Kasting and Catling
[3.10].
temperature of the earth. In these calculations, it is assumed that the earth
has its current atmosphere. This graph predicts (erroneously) that liquid
water could only be present on the earth's surface 1.5 Ga ago. The incon-
sistency of this prediction with experimental evidence of the existence of
liquid water 4.4 Ga ago is known as the faint young sun paradox [3.11].
The hypothesis is that the greenhouse effect must have been much
larger in early earth history. In fact, the amount of CO
2
must have been
orders of magnitude larger at those times in order for the greenhouse effect
to keep the earth's surface warm enough for liquid water. An important
question is how the earth has been able to regulate CO
2
on such a massive
scale. Not surprisingly, this regulation needs to involve very large amounts of
carbon. The largest carbon pool is comprised of the carbonate minerals, and
their central role in the carbon cycle is called the “inorganic carbon cycle.”
The inorganic carbon cycle
The
inorganic carbon cycle
is illustrated in
Figure 3.4.3.
CO
2
and water
form slightly acidic rain that slowly weathers rocks. The weathering reac-
tions can be summarized as:
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