The carbon cycle on Earth is amazing.
You never would have believed it if you hadn't seen it for yourself.
There's this uncanny stability.
The climate of the Earth is very stable through geologic
time, even though the sun has been getting warmer through time.
and there are other aspects too, of the
carbon cycle, like the oxygen concentration
of the atmosphere, which has this
incredible stability. We're going to think of the carbon
cycle as a series of these three reservoirs
of carbon, each of which exchange CO2 with the atmosphere.
The atmosphere is kind of like the Grand Central Station of the carbon cycle.
It's the smallest of these reservoirs, but it's the
way that carbon goes from one reservoir to another.
In this lecture we're going to focus on
the solid Earth as it affects the atmosphere, because this
sets the stage for why the three terrestrial
planets Earth, Venus, and Mars are the way they are.
Exchange of CO2, of carbon between the solid
earth and the atmosphere is governed by a chemical
reaction called the Urey reaction.
On this side, we have a very simple chemical formula for an igneous rock.
This is a kind of a rock that froze from lava, or
from magma, which is what they call lava if it's underground, crystalline rock.
Calcium Silica o3. And here's carbon dioxide.
And on the other side of the reaction, we have calcium
carbonate, which is limestone, and SIO2.
These are the two main components of sedimentary rocks.
Like all chemical reactions, there is an
equilibrium, a state of lowest energy. And this particular reaction,
because one of the chemicals is a gas
it has a very strong temperature dependence.
If you take a bunch of these elements and put them together at
very high temperature it favors this side of the chemical reaction.
It wants the carbon to be free as a gas if it's hot.
The same way that water vapor there's more water
vapor if if it's warm. Because the free form wants
to find itself in the higher temperatures.
whereas at colder temperatures this side of the chemical reaction is favored.
This sets up a kind of a dynamic duality between
the deep Earth where it's very high temperature which
basically wants all of the carbon to be in the atmosphere, like Venus.
All of Venus' carbon is in the atmosphere, there's 70 atmospheres of CO2 on Venus.
It's almost more like an ocean than an atmosphere, Venus.
The low temperatures at the Earth's surface would really
want carbon to be in the form of calcium carbonate.
The equilibrium according to the low temperature
at the surface, would only have about ten
parts per million of CO2 in the atmosphere.
Whereas the atmosphere in the natural world before the
fossil fuel era was about 280 parts per million.
And today it's just crossed the boundary of
400 parts per million because of human activity.
The real CO2 in the atmosphere is
in between what the deep earth wants, and what the shallow Earth wants.
This is a diagram that shows the basic
playing field for how the deep Earth and the shallow,
surface Earth try to relate to each other in
deciding what the CO2 concentration of the atmosphere should be.
You have a cycle of carbon, that goes
out from the solid Earth, and then back to the solid Earth.
Coming out of the solid Earth, you have CO2 degassing from volcanoes.
And also the hot springs
of the bottom of the ocean have a lot of dissolved carbon in them.
And then the CO2 reacts with igneous rocks.
Here, I've written the Urey Reaction in a more simple way, I've
got ridden of the SIO two on both sides, because that doesn't really.
I mean it makes CaO look more like a real rock, but it doesn't
really affect the carbon cycle other than that, so just stripping it down to
its essentials.
Well, we have igneous rock here, and the CO2, making the calcium carbonate.
Those 2 reacting together to
make carbonate, that's a process called weathering.
And then the calcium carbonate is deposited on the bottom of the ocean.
And eventually, as the ocean crust is subducted down
into the Earth, the calcium carbonate gets carried
down into the hot interior of the Earth.
Where the high temperature end says, I don't want
that carbon to be as calcium carbonate anymore, I
want to cook that stuff off and remake some igneous
rock, and release the CO2 back to the atmosphere.
This sets up a whole cycle of carbon coming
out of the earth and then going back into the earth.
The rate of weathering depends on rainfall, runoff.
Because in order to dissolve a rock, you have to have water to dissolve it in.
To a first approximation every gallon of water that runs down off and
to the ocean is carrying about as much dissolved rock as it can.
And so the way to make this chemical reaction happen
more quickly is to have more gallons of water washing over
the land.
And the rate of run off from the
land is a very strong function of the temperature.
The biggest river in the world is the Amazon, it's in Brazil in the tropics.
much more, many more inches of runoff per
year in the tropics than in the high latitudes.
And the Earth's temperature ultimately depends on carbon dioxide.
The more carbon dioxide you have the warmer it
is, the faster you can do this weathering reaction.
This ends up being another beautiful
negative feedback system, just like our kitchen sink.
We can recycle this analogy just again
and again for all kinds of different things.
In this particular application of the analogy, the
water coming out of the faucet into the sink is
the volcanic degassing rate.
That's what's driving the system.
And then the rate of weathering depends on
the water level in the sink.
The rate of water going down the
drain depends on the water level in the sink.
The rate of weathering depends on the amount of CO2 in the atmosphere.
What happens then
is that if you have some
volcanic degassing rate, some certain number of molecules
of CO2 every year coming out of the earth,
and if the rate of weathering is higher than that, that means you're
taking carbon dioxide out of the
atmosphere faster than you're putting it in.
Just like if it was going down the drain faster than coming out of the faucet.
And so the CO2 would tend to drop, until it approached and
ultimately equaled in an average way
the rate of degassing from volcanoes.
Or, if you started with no CO2, you'd have more coming
out of the Earth than is going back into the Earth.
The CO2 would build up until
it approached this equilibrium from the other side.
This sounds really cool.
It sounds as though the this thermostat mechanism,
this negative feedback, will clean up
our global warming mess by absorbing our CO2.
The trouble with that idea is this time scale.
How long it takes to happen is, on the order of a million years.
It's not something that we can wait around for.