Geoscience Reference
In-Depth Information
high for some reason, the weathering rate will increase, and CO
2
will
be more actively removed from the atmosphere. The increased removal
of CO
2
will in turn cause the CO
2
concentration in the atmosphere to
drop, reduce the greenhouse warming, and as a result, the temperature
will drop. Therefore, a balance point is reached between CO
2
concen-
tration, temperature, and the removal rate of CO
2
by weathering. Sup-
pose for some reason Earth becomes completely frozen. This may have
happened a few times during the course of Earth's history. If so, we
need not worry, at least when considering long geologic time scales.
Tectonic processes ensure that CO
2
will continuously be added to the
atmosphere. Without liquid water, there will be no CO
2
removal by
weathering, so the CO
2
concentration will build up until temperatures
rise to the point where ice melts, and weathering commences again.
During weathering, CO
2
is converted to a soluble ion known as bicar-
bonate (HCO
3
-
), which precipitates as minerals like calcite and dolo-
mite (think of clam shells and coral reefs) in the oceans. These minerals
are decomposed back to CO
2
during the subduction processes, thus
completing the cycle. To summarize then, Earth, through the cycling
of rocks (also known as the rock cycle), has an active control mechanism
for temperature, which is enabled by the churnings of the mantle and
the associated process of plate tectonics. Therefore, plate tectonics is also
critical in allowing Earth to enjoy a continuous record of water through
most of its long history.
This is a beautiful story, but is it true? I think that it must be, at least
in its broad detail. Some geological evidence, however, points to early-
Earth concentrations of atmospheric CO
2
that were too low to warm
an Earth illuminated by a less powerful Sun.
8
Jim Kasting has again
stepped into the discussion by suggesting that methane may have been,
harking back to Sagan and Mullen, a major greenhouse gas early in
Earth history. This would help explain the low CO
2
concentrations.
9
This may also be true, but the methane cycle does not obviously lend
itself to robust temperature control like CO
2
. Very recently, Minik Ros-
ing and colleagues (we meet Minik again in
chapter 7)
argued that
maybe we've been thinking about the problem incorrectly. They sug-
gest, in fact, that maybe the albedo of early Earth was much lower than
today,
10
so perhaps we didn't need as much greenhouse gas to warm the
planet. Jim Kasting isn't terribly happy with this idea, but lower con-
