THE FITNESS OF THE
ATMOSPHERE
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 As we have seen, Earth's
physical features-–mass, structure, temperature
and so on–are "just right for life". Such features
alone are not enough to allow life to exist on Earth
however. Another vital factor is the composition
of the atmosphere. We
noted above how science-fiction movies sometimes
mislead people. One example of how they do this
is how easily space travelers and explorers come
across planets with breathable atmospheres: they
seem to be lying all over the place. If we could
explore the real universe, we'd discover that
this isn't true at all: the possibility of another
planet's having an atmosphere that we could breathe
is most unlikely. That's because the atmosphere
of Earth is specially designed to support life
in a number of crucial ways.
The atmosphere
of Earth is composed of 77% nitrogen, 21% oxygen,
and 1% carbon dioxide. Let's start with the most
important gas: oxygen. Oxygen is vitally important
to life because it enters into most of the chemical
reactions that release the energy that all complex
life-forms require.
Carbon compounds
react with oxygen. As a result of these reactions,
water, carbon dioxide, and energy are produced.
Small "bundles" of energy that are called ATP
(adenosine triphosphate) and are used in living
cells are generated by these reactions. This is
why we constantly need oxygen to live and why
we breathe to satisfy that need.
The interesting
aspect of this business is that the percentage
of oxygen in the air we breathe is very precisely
determined. Michael Denton writes on this point:
Could your atmosphere
contain more oxygen and still support life?
No! Oxygen is a very reactive element. Even
the current percentage of oxygen in the atmosphere,
21 percent, is close to the upper limit of safety
for life at ambient temperatures. The probability
of a forest fire being ignited increases by
as much as 70 percent for every 1 percent increase
in the percentage of oxygen in the atmosphere.1
Even a 5% increase in the amount of oxygen
in our planet's atmosphere would result
in fires that would destroy much of its
forests. |
According to the British
biochemist James Lovelock:
Above 25% very little of our present land
vegetation could survive the raging conflagrations
which would destroy tropical rain forests
and arctic tundra alike... The present oxygen
level is at a point where risk and benefit
nicely balance.2
That the proportion of oxygen
in the atmosphere remains at this precise value
is the result of a marvelous "recycling" system:
Animals constantly consume oxygen and produce
carbon dioxide, which, for them, is not breathable.
Plants do just the opposite: they take in carbon
dioxide, which they need to live, and release
oxygen instead. Thanks to this system, life
goes on. Plants release millions of tons of
oxygen into the atmosphere every day.
Without the cooperation and
balance of these two different groups of living
things, our planet would be unlivable. For example,
if living things only took in carbon dioxide
and released oxygen, the earth's atmosphere
would support combustion much more easily than
it does and even a tiny spark could set off
enormous fires. Similarly, if both took in oxygen
and released carbon dioxide, life would eventually
die out when all the oxygen had been used up.
In fact, the
atmosphere is in a state of equilibrium in which,
as Lovelock says, risk and benefit are nicely
balanced.
Another finely-tuned
aspect of our atmosphere is its density, which
is ideally suited for us to breathe. |
