This class describes the science of global warming and the forecast for humans’ impact on Earth’s climate. Intended for an audience without much scientific background but a healthy sense of curiosity, the class brings together insights and perspectives from physics, chemistry, biology, earth and atmospheric sciences, and even some economics—all based on a foundation of simple mathematics (algebra).
Clouds
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From the course by The University of Chicago
Global Warming I: The Science and Modeling of Climate Change
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From the lesson
Feedbacks
Feedbacks are loops of cause-and-effect that can either stabilize Earth's climate or amplify future climate changes. There is an exercise in Part II of this class where you solve for a planet's temperature by iteration, and in the process demonstrate a runaway ice albedo feedback that might have led to the Snowball Earth climate state 700 million years ago.
Meet the Instructors
David ArcherProfessor
Geophysical Sciences
[MUSIC]
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.
So 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 clouds affect the visible light is by a process known as scattering.
So we've talked before about light coming in and
being absorbed by, say, a droplet of water.
Which means tha the energy from the light has been translated into kinetic
energy of the molecules in the water, which is the temperature of the water.
So you absorb the light you warm up.
And then, the warmer droplet will shine energy out in the infrared.
So 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.
So the droplet of water can be a little bit plus one side,
and little bit minus on the other side.
And flipping back and forth in resonance with the light that's coming in.
And then, 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.
So 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're dark because they're absorbing the light, and
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 know instinctively, based on your experience,
that bright clouds don't rain as much, and this is why.
So, there are, basically, three different kinds of clouds.
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 ten 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.
So the distinctions between these clouds are that the cirrus clouds,
which are higher up, are in colder air.
So these cloud droplets are much colder than the ground.
Cirrus clouds are also optically thin.
You can, if all you have is 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, there 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.
I guess I didn't have anything behind there.
So, if we put these together, these two different settings,
we can deduce, or understand,
the impact that these different kinds of clouds have,
on the visible and infrared energy budgets.
So, for the different kinds of clouds, we have the infrared effect,
which is, basically, the greenhouse effect.
And thinking about these clouds,
the one's 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 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.
So a strong impact on the infrared,
is a strong warming effect on our own earth's climate.
Whereas, low clouds, because they're lower in the atmosphere, they're fairly warm.
They're not not that much colder than the ground.
So, if you have really bright infrared coming from the ground, and then,
you absorbed it and replace it with, almost as bright infrared,
from not that much higher from the ground,
you don't have a very strong effect on the infrared energy budget.
So 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 your 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, on the earth's temperature.
So, the different kinds of clouds, have different impacts 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 too 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.
So clouds can cut both ways.
On balance, it seems as clouds lead to an,
overall, cooling effect of 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 will 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 raindrops 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 principals.
Instead, they, what we call parameterize clouds.
So if the temperature is such and such, and you've got so much water vapor,
in those sorts of conditions, you tend to find this kinds of cloud or that kind of
cloud or no clouds, clouds with certain cloud sizes of droplets, that thing.
They can't base on the cloud properties on the first principles,
as they would like, they have to fake it.
And so, that's why modeling clouds is really the hardest part,
about the climate change forecast.
[MUSIC]
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