CO2 is the greenhouse
gas that we're most concerned about for climate change.
But we're going to talk about methane first because it's easier,
and makes it easier to understand CO2 which we'll do next.
Methane is the second most powerful anthropogenic green house gas.
It's not negligible anyway.
Methane is a transient gas.
It comes out of a gas burner and it can burn.
It's very high energy and it
burns in the atmosphere, too, naturally, just under the influence of sunlight.
The chemical reaction looks something like this.
The methane encounters this guy who is called a hydroxyl radical.
Remember in seawater we had all these things that had charges,
like the calcium that had two plus, and the carbonate ion that had two minus.
Those are ions.
Those can't really exist in gases.
Gases have these things which have no electrical charge, but they have extra
electrons that are hanging out there, and very reactive.
It's an unstable molecule,
and it's written as radical by putting this dot there.
The OH radical is formed from water losing
a hydrogen, and it really wants to get that hydrogen back.
If it encounters a methane, it'll steal one of methane's hydrogens,
leaving CH3, and the radical now on the carbon.
And now the OH has satisfied itself and become a water.
The average lifetime of methane in the atmosphere is ten years.
In the dark, like in the bubbles of ice cores,
you can measure the methane concentration from the atmosphere
from 100,000 years ago, and it won't have reacted at all.
The atmosphere is like a very slowly burning candle flame, which is
only kept going by the sunlight
that creates these radical compounds.
And actually a candle flame
works by the same radical chemistry as this.
There are different ways that the concentration
of methane in the atmosphere could be changed.
You could either change how quickly you're
releasing methane to the atmosphere, which we call the source of methane.
or you could change the amount of OH radical
that is around for the methane to react with.
If we try the first case, we start out where
the reaction of methane is balancing its production here initially.
But now suddenly, we add a whole bunch of
new methane to the atmosphere in an ongoing way,
increasing the rate of method emission.
At this point, you'd be putting more methane into the system
than you're taking out and so the methane concentration would rise.
And the rate
that methane will react will go up as you have more methane.
The reaction of methane
will grow in until it balances the source again.
And this takes about ten years that lifetime of methane in the atmosphere.
Alternatively if you did something that suddenly
depleted the the availability of this OH radical,
you could start out with an
equilibrium and then suddenly the reaction rate would drop,
and the concentration would rise until
the reaction rate again balanced the source.
This should be looking a little bit familiar to you because it's
basically our old friend the kitchen sink analogy
again, applied to a totally different thing.
Where the source of methane is the water coming out of the faucet, and
the rate at which the water goes down the drain depends on the water level.
The same way that the rate of methane reaction depends
on the amount of methane that's there. If we assume
that the amount of OH radical is staying the same, which it seems like it
pretty much is, we can draw a equilibrium
equation that says that the source should balance the reaction rate.
Or the reaction rate has to be balancing the
source, it's a slave to the source in equilibrium.
And that could be given by the concentration divided by this lifetime
of ten years.
The main way that humans are altering the methane concentration
of the atmosphere is by adding new sources of methane to the atmosphere.
There are natural sources that existed before us and exist still today.
And the main of these is swamps.
We talked before when we were talking about coal formation that when you
have water logged soil, it's hard for oxygen to get down in there.
And in that case, bacteria can produce methane from the organic carbon.
And so you hear methane called swamp gas for this reason.
If you step in a swamp, you'll see bubbles coming out, and then
maybe the bubbles can catch fire in the atmosphere and look like a UFO.
They blame UFOs on swamp gas, which is probably
kind of silly.
But swamps are a main natural source of methane to the atmosphere.
Termites and ruminant animals also produce methane in a natural way.
Humans have impacted this story by growing rice which we do in flooded fields.
It's not that the rice requires the flooding but it can
tolerate it, and it makes it much easier to keep the other
invasive plants out, if the field is flooded.
A flooded field tends to
make anaerobic conditions favorable to methane production,
so rice agriculture is a major source of methane to the atmosphere every year.
our domestic ruminant animals; cows also release methane.
They exhale it actually, it's not a, it's not a, ahhh...
The fossil fuel industry
is also responsible for some leakage of methane to the atmosphere.
Looking at the concentrations
of methane through time it is about twice as
high as the pre-anthroprogenic value which we can measure from bubbles in ice cores.
And it was rising through the decades until about 1990 when it seemed
to reach this plateau and stop changing.
Everything else about the human impact on the planet is growing.
And so its actually kind of a mystery why the methane concentration isn't going up.
There are multiple theories which is what always
happens when you don't really know for sure.
One possibility is that the earth has been drier
in the last few decades because of climate change,
whick cuts down the emission of methane from wetlands.
Another possibility is that the collapse of the eastern block
capped a lot of leaky methane to the atmosphere.
But it's not really very well known.
Just in the last few years, there has been an
uptick in the methane concentration, which is also not very well understood.
The future of atmospheric methane is
difficult to predict, given that we can't really tell you
in detail why it has done what it has done in the recent past.
However the the short lifetime of methane in the atmosphere means that if you want
methane to go up, you have to keep adding more and more and more, all the time.
And if you cut back on the emissions the concentration
will fall within a fairly short time frame.