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The carbon cycle on Earth is amazing.

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You never would have believed it if you
hadn't seen it for yourself.

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There's this uncanny stability.

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The climate of the Earth is very stable
through geologic

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time, even though the sun has been getting
warmer through time.

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and there are other aspects too, 
of the

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carbon cycle, like the oxygen
concentration

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of the atmosphere, which has this

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incredible stability.
We're going to think of the carbon

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cycle as a series of these three
reservoirs

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of carbon, each of which exchange CO2 with
the atmosphere.

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The atmosphere is kind of like the
Grand Central Station of the carbon cycle.

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It's the smallest of these reservoirs, but
it's the

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way that carbon goes from one reservoir to
another.

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In this lecture we're going to focus
on

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the solid Earth as it affects the
atmosphere, because this

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sets the stage for why the three
terrestrial

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planets Earth, Venus, and Mars are the way
they are.

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Exchange of CO2, of carbon between the
solid

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earth and the atmosphere is governed by a
chemical

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reaction called the Urey reaction.

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On this side, we have a very simple
chemical formula for an igneous rock.

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This is a kind of a rock that froze
from lava, or

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from magma, which is what they call lava
if it's underground, crystalline rock.

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Calcium Silica o3.
And here's carbon dioxide.

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And on the other side of the reaction, we
have calcium

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carbonate, which is limestone, and SIO2.

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These are the two main components of
sedimentary rocks.

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Like all chemical reactions, there is
an

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equilibrium, a state of lowest energy.
And this particular reaction,

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because one of the chemicals is a gas

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it has a very strong
temperature dependence.

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If you take a bunch of these
elements and put them together at

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very high temperature it favors this side
of the chemical reaction.

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It wants the carbon to be free as a
gas if it's hot.

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The same way that water vapor there's more
water

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vapor if if it's warm.
Because the free form wants

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to find itself in the
higher temperatures.

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whereas at colder temperatures this side
of the chemical reaction is favored.

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This sets up a kind of a dynamic
duality between

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the deep Earth where it's very high
temperature which

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basically wants all of the carbon to be in
the atmosphere, like Venus.

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All of Venus' carbon is in the atmosphere,
there's 70 atmospheres of CO2 on Venus.

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It's almost more like an ocean than an
atmosphere, Venus.

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The low temperatures at the Earth's
surface would really

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want carbon to be in the form of calcium
carbonate.

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The equilibrium according to the low
temperature

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at the surface, would only have about ten

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parts per million of CO2 in the
atmosphere.

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Whereas the atmosphere in the natural
world before the

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fossil fuel era was about 280 parts
per million.

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And today it's just crossed the boundary
of

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400 parts per million because of human
activity.

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The real CO2 in the atmosphere is

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in between what the deep earth wants, and
what the shallow Earth wants.

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This is a diagram that shows the basic

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playing field for how the deep Earth and
the shallow,

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surface Earth try to relate to each
other in

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deciding what the CO2 concentration of the
atmosphere should be.

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You have a cycle of carbon,
that goes

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out from the solid Earth, and then back to
the solid Earth.

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Coming out of the solid Earth, you have
CO2 degassing from volcanoes.

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And also the hot springs

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of the bottom of the ocean have a lot of
dissolved carbon in them.

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And then the CO2 reacts with igneous
rocks.

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Here, I've written the Urey Reaction in a
more simple way, I've

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got ridden of the SIO two on both sides,
because that doesn't really.

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I mean it makes CaO look more like a real
rock, but it doesn't

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really affect the carbon cycle other than
that, so just stripping it down to

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its essentials.

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Well, we have igneous rock here, and the
CO2, making the calcium carbonate.

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Those 2 reacting together to

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make carbonate, that's a process called
weathering.

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And then the calcium carbonate is
deposited on the bottom of the ocean.

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And eventually, as the ocean crust is
subducted down

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into the Earth, the calcium carbonate gets
carried

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down into the hot interior of the Earth.

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Where the high temperature end says, I
don't want

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that carbon to be as calcium carbonate
anymore, I

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want to cook that stuff off and remake
some igneous

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rock, and release the CO2 back to the
atmosphere.

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This sets up a whole cycle of carbon
coming

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out of the earth and then going back into
the earth.

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The rate of weathering depends on
rainfall, runoff.

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Because in order to dissolve a rock, you
have to have water to dissolve it in.

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To a first approximation every gallon of
water that runs down off and

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to the ocean is carrying about as much
dissolved rock as it can.

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And so the way to make this chemical
reaction happen

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more quickly is to have more gallons of
water washing over

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the land.

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And the rate of run off from the

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land is a very strong function of the
temperature.

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The biggest river in the world is the
Amazon, it's in Brazil in the tropics.

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much more, many more inches of runoff
per

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year in the tropics than in the high
latitudes.

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And the Earth's temperature ultimately
depends on carbon dioxide.

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The more carbon dioxide you have the
warmer it

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is, the faster you can do this weathering
reaction.

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This ends up being another beautiful

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negative feedback system, just like our
kitchen sink.

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We can recycle this analogy just again

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and again for all kinds of different
things.

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In this particular application of the
analogy, the

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water coming out of the faucet into the
sink is

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the volcanic degassing
rate.

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That's  what's driving the
system.

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And then the rate of weathering depends on

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the water level in the
sink.

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The rate of water going down the

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drain depends on the water level in the
sink.

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The rate of weathering depends
on the amount of CO2 in the atmosphere.

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What happens then

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is that if you have some

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volcanic degassing rate, some certain
number of molecules

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of CO2 every year coming out of the earth,

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and if the rate of weathering is higher
than that, that means you're

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taking carbon dioxide out of the

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atmosphere faster than you're putting it
in.

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Just like if it was going down the drain
faster than coming out of the faucet.

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And so the CO2 would tend to drop, until
it approached and

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ultimately equaled in an average way

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the rate of degassing from
volcanoes.

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Or, if you started with no CO2, you'd have
more coming

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out of the Earth than is going back into
the Earth.

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The CO2 would build up until

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it approached this equilibrium from the
other side.

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This sounds really cool.

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It sounds as though the this
thermostat mechanism,

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this negative feedback, will
clean up

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our global warming mess by absorbing our
CO2.

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The trouble with that idea is this time
scale.

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How long it takes to happen is, on the
order of a million years.

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It's not something that we can wait
around for.