The water vapor feedback is a positive feedback that has an important impact
on Earth's climate with rising CO2 concentrations.
In fact, if it weren't for the water vapor feedback,
there really wouldn't be as much of a global warming problem at all.
The water vapor feedback roughly doubles the sensitivity
of the Earth to changes in greenhouse gas concentrations.
At the heart of it, there's actually a negative
feedback that controls the amount of water vapor in the atmosphere.
The state variable here is the water vapor.
And if the water vapor concentration is perturbed by
some external thing, like if you run a garden sprinkler,
it changes the rate of evaporation some place else, or else it just
rains out some place else because you put more water in the air.
The hydrological cycle leads to a negative feedback
that stabilizes the amount of water vapor in the air.
That's why running a garden sprinkler doesn't lead to global warming.
But the vapor leads to a positive feedback
operating on a state variable now of temperature.
The way this works is that if you
have some perturbation to the temperature, say by making the
sun brighter or adding more greenhouse gases, it changes
how much water vapor you want to hold in equilibrium.
It redefines 100% relative humidity essentially.
And so if you warm things up by some external means, that
leads to an increase in the amount of vapor pressure
in the air, and water vapor, as you know, is a greenhouse gas,
because it's got three atoms, you know it's a greenhouse gas, and it's asymmetrical.
Having more water vapor pressure means that there's more more greenhouse
gas forcing to warm the Earth even further.
The way this could go is you add some greenhouse gas
or you make the sun warmer, and then that makes the temperature go up by itself,
which causes the amount of water vapor in the air to go up,
because the warmer air can hold more water vapor,
moving in this direction on that pressure curve.
and then that's a greenhouse gas so that causes the temperature to go up even more.
It amplifies the temperature.
You may wonder: you have this loop of
cause and effect, you may wonder where does this end?
Well, the answer can be seen on a diagram such as this one which is a
phase diagram for water. It's got dimensions of temperature and
pressure of water vapor here, and then it has these different regions on the graph.
Up here where the temperature is fairly high and there's
a lot of water vapor, you can have liquid plus vapor.
If you have a lot of water vapor pressure,
but it's very cold, you're going to have vapor plus ice.
Or if the vapor pressure is very
low, then you may not have any other phase other than vapor, you'll
only have vapor in this region of the phase diagram.
We're going to do a thought experiment where we start out a planet
and we'll say, let's not put any water vapor in the atmosphere yet.
And we'll just have it be dry, and then we'll calculate what the temperature that
planet would be.
And so, for Earth that would lead us to a point right here.
If its got a liquid water, it's going to a evaporate,
liquid water wants to evaporate until it starts to be along this line here.
As the water vapor evaporates it causes the temperature
of the planet to rise because water vapor is a greenhouse gas, but ultimately this
trajectory hits the phase boundary to liquid and, at this point, it rains.
And so the runaway greenhouse, the water vapor feedback, is limited
on Earth by intersecting the phase boundary for a liquid water.
The hydrological cycle is stabilizing,
and it limits the the extent of the water vapor feedback.
This is Mars.
It starts out colder, and you add water vapor, and
it goes up and maybe it hits the phase diagram.
A phase boundary of ice plus vapor, so on Mars it would snow,
but again the water vapor feedback would be limited by this phase boundary.
Now Venus is a different story. Venus originally had water,
it doesn't anymore because Venus suffered a runaway greenhouse effect.
You start from a condition that's warmer and
you raise the water vapor concentration, and raise
the temperature, it can curve over and
miss the phase boundary for liquid, or solid entirely.
And this is called a runaway greenhouse effect.
All of the water on the planet would go into the atmosphere.
It would all boil.
And, this is a one way street.
If this ever happens on the planet it never can go back.
Because, if you get too much water vapor up in the upper
atmosphere where it's exposed to the ultra violet light from the sun,
this ultraviolet light can split the water molecule into Hydrogens and Oxygens.
And these Hydrogens are so small they
travel quickly with their allotted kinetic energy,
just put into that small mass they go very fast.
And so they can be lost to space.
A runaway greenhouse effect is the end of a biosphere for a planet.
Looking at a picture of Earth from
space is always astonishing to me because the atmosphere
is so thin and, and the ocean is
just right there spread out through the whole surface
of the planet, and what keeps this from happening on Earth is a cold trap.
The fact that it gets so much colder in the upper atmosphere,
the water vapor can't get through it,
it all rains or snows out. This is is very, very thin
layer of cold air that has protected and guarded all of Earth's water...
for these billions of years! It looks so fragile to me looking at a picture
of Earth from space.