The oceans are another major player in the earth's carbon cycle,
and it also exchanges carbon with the atmosphere, but on a very
different rhythm, a very different time scale than that of the solid earth.
Basically there's a large amount of
carbon dissolved in the ocean, much more than
in the atmosphere, and some parts of the ocean are releasing CO2 to the atmosphere.
In some places CO2 is degassing or evaporating from the ocean.
In other places the ocean is taking up carbon dioxide.
Say, where the water is colder so the CO2 dissolves
more readily into the water than in warm water, for example.
The back and forth fluxes of carbon
between the atmosphere and the ocean are huge.
They are something like 1,000
times higher than the back and forth fluxes with
the solid Earth. The unit that we use to talk
about carbon, or one set of units that we use, is gigatons.
Giga means billion, and ton in this case means metric ton which is 1,000 kilograms.
A gigaton of carbon is also about
ten to the 15th grams (1E15), if you find that helpful.
Or another way to think of it is all of
the people on Earth probably weigh less than a gigaton.
There's much more carbon going back and forth between the atmosphere
and the ocean every year, by a factor of 100 or more,
than the total mass of all the people on earth.
And this is much bigger than the flux to the solid earth.
The oceans are thought to be responsible for changes in atmospheric
CO2 through the glacial inter-glacial cycles, which
are documented from records preserved in ice cores in Antarctica and Greenland.
From little bubbles of air trapped in
those ice cores we know that atmospheric CO2 concentration
is generally about 280 parts per million
during warm times like today, during interglacial periods.
And then during cold times like the last glacial maximum 20,000 years ago,
the CO2 concentration was lower, about 200 parts per million.
And there's this astonishingly good correlation between the
CO2 from the ice cores and temperature estimates
from those same ice cores.
It makes it very clear that CO2 plays a very strong role in Earth's climate.
And the the timing of the ice ages is extremely
well-correlated with fluctuations in Earth's orbit around the sun.
And in particular what seems to matter is how bright the sunlight intensity
is during the Northern Hemisphere's summer.
The Northern Hemisphere is what's important, because that's
where the ice sheets have room to form.
There's more land in the Northern Hemisphere
than in the Southern Hemisphere.
And the summer is important, because that's when you decide whether to melt
last winter's snow or keep it around and start to accumulate an ice sheet.
There's a very good correlation between the orbital fluctuations and
the amount of ice on earth.
Twenty thirty years ago, that seemed like the whole story, but then
they discovered that the CO2 concentration is also going up and down as the ice sheets
are going up and down, so this seems like it must be a
a positive feedback, that the orbit of the earth says "let there be ice sheets".
And then the carbon cycle says "Okay",
"if you're going to have ice sheets, I'm going to"
"pull the CO2 down and amplify that cooling from the ice sheets."
And so that's what makes the ice ages global instead
of just from one hemisphere or the other, is the CO2 effect.
On time scales of thousands of years and longer the ocean seemed to act like
a positive feedback to the climate system.
However, today we are releasing CO2 to the atmosphere.
And so the oceans are taking up some of that CO2,
so, on the timescale of decades today, the oceans are acting as a negative feedback.
They're helping clean up,
they're dampening, they're stabilizing Earth's climate.
And so it's kind of an open question whether on
time scales of 1,000 years or longer the oceans might
revert to their positive feedback role, and amplify our climate excursion.