The human impact on the earth's carbon cycle is to dig up
some of the carbon in the solid earth reservoir and put it in the atmosphere,
a rate of about 8 gigatons per year. And also, we are
cutting down trees on land, which results in a flux
of CO2 to the atmosphere of about another 2 gigatons a year
The amount of CO2 in the atmosphere is going up.
But it's actually not going up as quickly as it
should be, given how much carbon we're putting into it.
It's only going up at about four or five gigatons per year.
There's a whole bunch of carbon that we're
putting in the atmosphere, which is not showing up.
It's going away into some combination of the land and the ocean.
The time scale for solid earth
to take up carbon is very long.
That's not really helping us that much, presumably.
It's hard to measure how much carbon
is going in to the land, because the land is so heterogeneous.
One tract of forest is very different from another.
And to know exactly how much carbon is being
absorbed, is a very difficult measurement to make.
Actually, interestingly they can measure the carbon uptake
in the ocean much more easily, much more precisely.
In the early days of thinking about this, back in the
1950s and before, people basically figured that the ocean would control the
atmospheric CO2 concentration and that we wouldn't really be able to change
atmospheric CO2, because the ocean, after all, has 70% of the earth's surface.
And it's also got a lot more carbon than the atmosphere does.
About 40 times more, and so it's a really heavy hitter in the carbon cycle.
The reason why the ocean doesn't just immediately mop up all of our extra CO2
has to do mostly with the way that the ocean water circulates,
the way it moves from the surface where it can see the atmosphere
down into the deep which is where most of the ocean water is.
The way the ocean circulates is
that in the high latitudes, in Antarctica and in the
North Atlantic, where it gets very cold, the water at
the surface gets cold enough that it can sink
by the same convection process we were talking about before,
but now upside down because we're cooling it from above.
And so you have this very focused downward flow in very specific locations.
And this is balanced by vague and diffuse
upward velocities every place else, or other parts of the ocean.
The overall time scale for the circulation of the ocean is about a 1000 years.
And that actually limits how quickly the CO2
can equilibrate between the atmosphere and the ocean.