You've heard about the carbon cycle as a series of reservoirs.
All of which inhale and exhale carbon dioxide to the atmosphere, which
is the center of the carbon cycle in some ways.
But there's another trick that the carbon cycle
has done on Earth, which involves this chemical reaction.
CO2 plus water combined with energy can be put together,
into a molecule of the family of CH2O.
Which is a very, very simple
representation of what chemists call organic carbon.
Organic carbon is what plants and people
and things like that are made out of.
And originally, organic carbon could only be made
by life, which is why it's called "organic" carbon.
Nowadays,
if you go to the grocery store and you buy some organic fruit,
what it means is that
they didn't use any industrial chemicals to produce it.
But actually a lot of those industrial
chemicals, like pesticides, can actually be made out
of carbon in this configuration
that a chemist would actually call organic carbon.
There's different uses for the word organic, that actually
can be in conflict with each other, in different situations.
It takes energy to do this chemical reaction,
to make organic carbon out of CO2.
And where the energy comes from is light,
to power photosynthesis, so it takes plants to do this chemical reaction.
The other chemical on this side of the equation is oxygen.
This is O2, just like the oxygen in our atmosphere.
It turns out that when organic carbon is
buried in the earth, it leaves behind the oxygen it produced.
And this is where the oxygen in the atmosphere comes from.
Actually, most of the organic carbon that's produced
by photosynthesis is respired back to CO2 because somebody eats it.
If we eat our lunch we are doing respiration, or if bacteria degrade a
leaf in the soil it sends it back this way.
But, in very special conditions this organic carbon can be protected from
being eaten by being buried in the mud in the bottom of the ocean, or in a swamp.
In which case, it leaves behind oxygen in the atmosphere.
What's happened is that the biosphere, the life on earth,
has charged up the surface of the earth like a giant battery.
And we are creatures of the battery, we eat organic carbon and we breath oxygen in
order to have everything we need on this side of the reaction, to do respiration
which is how we get energy back.
There are really two reasons why why life does photosynthesis.
One is to store energy in this form of organic carbon.
And another is to put carbon in a
chemical state where it has lots and lots of
versatility to do all the different things that
need to be done in the bio-molecular machinery.
The amazing thing about this battery of
the biosphere is that it's so remarkably stable.
There's no simple chemical reason why we have
20% O2 in the atmosphere instead of 10 times as much, or ten times less.
It's kind of an accident of how much of this organic carbon
has gotten buried in the mud in the Earth.
It'd be very easy for this system, it seems like conceptually,
to have built up to 10 times more oxygen in the atmosphere.
In which case a spark of lightning could start a continent wide forest fire.
No wet wood would burn. Nothing could put it out.
Or having 10 times less oxygen in the atmosphere.
in which case, all of us multi-cellular types would would choke and die.
And we know that for the last more than half a
billion years there has been mutilcellular life on earth.
The oxygen concentration in the atmosphere is
somehow regulated in a very miraculous way.
It has to be something like, if oxygen in the atmosphere gets too low, then you get
anoxic conditions in the ocean, which
make it easier to bury organic carbon, which then
acts as an extra source of oxygen to the atmosphere.
But scientists don't understand this very well, this
this uncanny, this eerie stability of earth's biosphere.