Geoscience Reference
In-Depth Information
centrations of atmospheric CO
2
can satisfy both the geological evidence
for ancient CO
2
levels and produce enough of a greenhouse effect to
warm the planet in the presence of a faint young Sun. Therefore, the
CO
2
control mechanism as originally described by Walker and Kasting
can still work to regulate Earth's temperature through time, even if an-
cient CO
2
levels were lower than we once thought.
Now back to the original question. It's one thing to have water, but
it's another thing to support an abundant biosphere. As mentioned in
the beginning of this chapter, life is nearly everywhere on Earth's sur-
face. But how does our planet support it? Let's try some calculations.
Photosynthetic life on Earth, working at present rates of photosynthe-
sis, would deplete all of the CO
2
in the atmosphere in nine years.
11
Likewise, photosynthetic life in the oceans would deplete all of the
available phosphorus, a key nutrient in making aquatic plants and
algae, in just 86 years.
12
If this is true, how can we support so much life
over long time scales? Part of the answer is that most of the CO
2
and
nutrients tied up in plants and algae are liberated back to the environ-
ment as these organisms die and are consumed and decomposed by all
manner of creatures from giant pandas to bacteria. Okay, but still some
plant material and phosphorus aren't liberated back to the environ-
ment, and instead these things get buried in sediments and formed into
rock. If we redo our calculations based on these rates of loss, we find that
CO
2
would be depleted in 13,000 years,
13
and phosphorus in 29,000
years. These are still pretty short time scales compared to the billions of
years that life has existed on the planet and the hundreds of millions
of years that plants and animals have populated the land surface. How
do we explain this?
The answer is actually quite simple. We appeal to the same tectonic
processes we used to explain the role of CO
2
in solving the faint young
Sun paradox. Luckily, when materials are sequestered into marine sedi-
ments on Earth, they are not permanently trapped there. The tectonic
movements of the planet ensure that they are not. Through the pro-
cesses of subduction, mountain building, and sea-level change (sea level
is influenced by both tectonics and climate) most of these materials will
be exposed again to the weathering environment. During weathering,
organic matter is turned back to CO
2
, phosphorus is liberated again to
solution, and a whole host of other ingredients for life become available
