For atmospheric methane a greenhouse gas, we got to recycle the
kitchen sink analogy yet again. But for CO2, it's more complicated.
If we're going to use the kitchen sink analogy to
understand, we have to do some construction work here.
This is what it would look like.
The atmosphere would be a sink.
And there'd be
a line coming into it as before.
But then there are those other sinks that are kind of connected to it
by these tubes that run from one to the next, sideways.
The fossil fuel, carbon is a source
to the atmosphere there. And it raises the water level
in this atmosphere sink way up. And the first thing that will happen is
that the water will run from this middle sink into those two other sinks.
I've drawn these other sinks as much bigger than the atmosphere sink.
Because that's the way it is in the real world.
What happens then is that the amount of carbon in the land reservoir, maybe
and in the ocean reservoir certainly, will rise as a result of this
fossil fuel carbon spreading out to those other reservoirs.
And then there is also the steady drip, drip of water coming into this
sink which would represent the volcanic degassing of the earth and this is the
consumption of CO2 back to the solid earth
by the weathering reaction, and so when this all of these
sinks have a water level which is raised up compared to what it
used to be, because we dumped a bunch of water into the center
sink, they will eventually all settle back to the original equilibrium water level.
In a very long time so there's different stages
to the response to a CO2 edition here.
First it spreads out and the whole
thing relaxes back to an equilibrium.
If we look at the airborne fraction of
a slug of carbon that's added to the atmosphere it starts out at at zero.
And then we'll call this the fossil fuel era when the CO2 is rising, and
at the peak, about half of the CO2
that had been emitted would be in the atmosphere.
That's where we are today actually.
If we were to stop emitting carbon dioxide to the atmosphere today, we would
be right here, with about half of the carbon we ever put, in the atmosphere.
Or if we stop in the year 2100, or whenever, it'll
be about 50%, given how quickly we're adding the CO2.
Then the recovery can commence. There will be a fairly fast
draw down of the atmosphere CO2 on a time scale of centuries
governed by the thousand year circulation time scale of the ocean.
And that will draw the CO2 down fairly fairly far.
It won't recover all the way to the
original atmospheric C02 concentration,
because this sink is bigger than that, but it's not infinitely bigger than that.
And so the atmosphere still gets some. The next stage is when calcium
carbonate dissolves to make carbonate ion,
which essentially replenishes the carbonate ion stock
in the ocean. This is a neutralization stage.
This calcium carbonate is an antacid, and
it's neutralizing the acid of the CO2, and
when it does that it replenishes the carbonate
ion, which as the reaction partner of CO2.
It's like anti-CO2.
when you replenish that carbonate ion, you allow the ocean to take up more, a little
bit more CO2.
Because you're restoring the chemical buffer of the ocean.
This process takes a few thousand years.
And it draws the CO2 down a little further, still not all the way to
the initial value, but down to 10% or so of the CO2.
And the last bit
is this very long recovery due to the CO2 weathering thermostat,
which because the fluxes are so low, 0.1 gigatons per year is the weathering rate
of igneous rocks, it will take a very long time for the ultimate recovery.
There's kind of a peak and a long tail to the CO2.
As opposed
to methane, which is just a simple single exponential decay,
this is mathematically, you would describe this as the sum
of several exponential terms, if that makes any sense to you.
If not, you can just look at the artwork.