The land surface is the third component,
the third reservoir, of Earth's carbon cycle.
The difference between the land's surface and the solid earth
is the land surface is just the carbon that's exposed
at the surface which can respond quickly, as opposed to
solid earth, I mean stuff that's buried for millions of years.
The land carbon cycle is governed by plants mostly.
Plants grow in the spring and summer and they
take up CO2, and then in the fall and in
the winter, they drop their leaves, which decompose, or
turn back into CO2, and are released back to the atmosphere.
There's a very strong seasonal cycle in the CO2
concentration of the atmosphere which is driven by the land surface.
It goes up in the winter
and down in the summer. You can see that in
the Southern hemisphere the seasonal cycle is much less intense
in CO2 because there's less land in the southern hemisphere.
You have much wider swings
of atmospheric CO2 concentration in the Northern hemisphere.
In the Southern hemisphere they're out of phase with the north because
the seasons are out of phase. And then on top of the seasonal cycles
from those two regions you can also see a
secular trend. The rise in the CO2 concentration with
time which is due to fossil fuels.
There are a couple of ways that the land biosphere can affect the CO2 concentration
with the fossil fuel CO2 and all. One is deforestation.
If you go from a forested landscape and then
you cut down the trees and make an agricultural landscape
you have less carbon per square meter of area in
this land after it's been cut than you had before.
That actually provides a net source of carbon to the atmosphere.
It's not as much carbon as our fossil fuel carbon
but it's a significant amount of carbon released to the atmosphere.
Something else that's going on that's sort of more mysterious is that in places where
there's not active deforestation going on there appears to be a
natural uptake of carbon by the terrestrial biosphere, they call greening.
And it's not clear what's driving this.
It could be that with the climate change we've experienced already,
a longer growing season is allowing more plants to grow.
Or maybe it's allowing area
that use to be low-carbon tundra to convert into higher-carbon forests.
It could be actually CO2 fertilization effect.
Plants actually grow better if there's more CO2.
That's one of the purposes of a
greenhouse, is to have higher CO2 concentration.
It makes the plants grow better. It could be fire suppression.
Actually, it's not even known. Even though there's a
fairly large amount of carbon going into the natural
landscape someplace, we can't really find it for sure.
Don't know where it is.
The landscape is serving as a negative
feedback today, because it's taking up carbon that we
are releasing to the atmosphere, so it's stabilizing the
climate and acting as a negative feedback.
However, if you look at this map
of the distribution of soil carbon on earth, what you see is that there's much
more soil carbon in the high latitudes then there is in the low latitudes.
Rain forests look like very carbon-rich, lush places, and there
is a lot of carbon in rain forests but not in the soils.
The soils in rain forests are bleached out.
There's no carbon really there. It's because a leaf
that falls to the ground in the tropics decomposes
so quickly it doesn't accumulate in the soils.
Whereas in cooler climates you have build-up of more carbon in the soils.
And the end member case is in the very high latitudes in the permafrost
regions, the soil is actually frozen, and
so the carbon is very well preserved there.
You can find intact mammoths
from thousands of years ago that are still frozen and even
approaching edible after all those years, because it's been preserved by freezing.
If on a timescale of centuries we melt that
permafrost and create a more tropical
climate in the higher latitudes that could result
in the net release of a considerable amount of carbon.
Again, the land biosphere, like the ocean, has the
potential to switch from being a negative feedback that's stabilizing
and saving us from ourselves today, to being an amplifying
positive feedback in the fullness of time in the future.