Clouds represent the largest uncertainty in the climate forecast
and they're really the hardest part of the whole thing.
The biggest difference between different climate models,
and how much they forecast a change in climate,
is pretty much due to different ways
in which different climate models treat clouds.
Clouds have two possible ways of impacting the energy budget of the Earth.
One is by visible light and the other is by infrared light.
The way the clouds affect visible light is by a process known as scattering.
We've talked before about light coming in
and being absorbed by a droplet of water.
Which means that the energy from the light has been
translated into kinetic energy of the molecules in the water.
Which is the temperature
of the water. You absorb the light, you warm up.
And then the warmer droplet will shine energy out in the infrared.
We've seen this before, but another thing
that can happen is that the visible light comes
in and it's got this electrical dipole,
and it can actually induce an electrical dipole in the droplet.
The droplet of water can be a little bit plus on one
side and a little bit minus on the other side,
and flipping back and forth in resonance with the light that's coming in.
This oscillating dipole, which is chained to the frequency
of the light that comes in, can create light that then shines out.
And, it's because the frequency is the same as
the light that comes in, what leaves is visible light as well,
so this is called scattering.
And scattering works best if the size of the cloud droplet is comparable
to the size of the wavelength, lambda, of the light.
And it turns out that most cloud droplets are
actually much bigger than the wavelength of visible light.
The smaller the cloud droplet is, the better it can scatter.
If cloud droplets are really big, that's when you see dark clouds.
They are dark because they are absorbing the light,
if you see dark clouds, you've got to take your umbrella, because it might rain.
Whereas if the clouds are really bright, that means
that the cloud droplets are small and that, you just
know instinctively, based on your experience, that
bright clouds don't rain as much, and this is why.
There are basically three different kinds of
clouds, I mean, there's about a bazillion kinds
of clouds if you draw the
distinctions fine enough, but basically there are three kinds.
There are these really high altitude, very thin, wispy clouds called cirrus clouds.
They tend to be found at about an altitude of maybe 10 kilometers or so.
And then the other two main kinds are cumulus and stratus clouds,
which are found somewhere between about zero and four kilometers altitude.
And these are much thicker, much denser clouds.
with lots more water in them, they are all formed, basically,
when the air is rising, so cumulus clouds are formed by a focused upward
motion of air, whereas stratus clouds, which are extremely
wide and broad over large areas, are formed by broader scale upward motions.
The distinctions between these clouds are that,
the cirrus clouds, which are higher up, are in colder air.
These cloud droplets are much colder than the ground.
Cirrus clouds are also optically thin.
cirrus clouds and maybe a few airplane contrails, you don't even
really think of it as a cloudy day, because usually the sun
can come right through cirrus clouds, they just don't absorb that much light.
They's just not that much water up there.
Whereas, these other clouds are lower altitude, where the air is warmer,
and they also tend to be much
more optically thick.
Lucky I didn't have anything behind there.
If we put these together these two different settings,
We can deduce, or understand, the the impact that these
different kinds of clouds have on the visible and infrared energy budgets.
For the different kinds of clouds, we have
the infrared effect, which is basically the greenhouse effect,
and thinking about these clouds, the ones that are going to have the strongest
infrared effect, are the ones that are
high up in the atmosphere because they're colder.
And so, they're absorbing the warm intense infrared light
from the warm ground and replacing it with cold infrared
light from up where they are, and so they
have a very strong effect on the infrared energy balance.
And any effect you have on the infrared, as it's affecting
the light leaving the planet, tends to make the planet warmer.
A strong impact on the the infrared is a strong warming
effect on on earth's climate. Whereas low clouds,
because they're lower in the atmosphere, they're fairly warm.
They're not that much colder than the ground.
If you have really bright infrared coming from the ground
and then you absorb it and replace it with almost as bright
infrared from not that high, much higher than the ground, you
don't have a very strong effect on the infrared energy budget.
This is a much weaker
effect from the low clouds In the visible,
the high clouds tend to be optically very thin.
You can see through them and so they don't really scatter that much light.
And so they have a weak effect on the visible energy budget.
Now if you mess with the visible energy, what you're doing
is you're reflecting incoming sunlight back out to space, you're increasing
the albedo.
That tends to always make the planet cooler.
Lower clouds, because they are optically thick, have a stronger cooling
effect on the Earth's planet, so, on the Earth's temperature.
The different kinds of clouds have different impacts
on on the Earth. The high clouds usually
are warming because the infrared effect is strong, and the visible effect is weak.
The infrared effect is to warming, and so high clouds tend to warm the planet.
Low clouds, on the other hand, have a
weak infrared effect, a weak warming effect, and they
have a strong effect on the visible, because
they're so optically thick, they scatter more light, and
so they tend to cool the planet. Clouds can cut both ways.
On balance, clouds
lead to an overall cooling affect on Earth's climate
The reason why it's so hard to simulate
clouds in climate models, and why different climate
models get different answers for, what they think
the clouds would do in a changing climate,
Is because in a grided climate model you don't really
have enough grid points to simulate all of the processes that happen inside a cloud.
Little gusts of wind bring the rain drops together.
They coagulate, they fall out, they evaporate,
all kinds of small-scale processes happen that determine how clouds work.
And you just can't put all of that stuff into a climate
model the way you would like to do it, from first principles.
Instead, they, what we call, parameterize clouds.
You know, if the temperature is such and such
and you've got so much water vapor in those
sorts of conditions you tend to find these kinds
of clouds, or that kind of cloud, or no clouds.
Clouds with certain cloud sizes of droplets.
They can't base the cloud properties
on first principles as they would like.
They have to kind of fake it.
And so, that's why modeling clouds is really
the hardest part about the climate change forecast.