Transcript for: Understanding the Greenhouse Effect
We've all seen this a thousand times. - [Newscasters] The Earth is hurtling towards a climate danger. The atmosphere warming. Some of that energy will be absorbed. Trapped some of this energy. The greenhouse effect. Greenhouse gas emissions. The greenhouse effect. - But what does that even mean? Just saying, "because
the greenhouse effect," doesn't explain anything, really. The question you should be asking, the one that gets you to an actual answer, is what's so special about a
molecule of carbon dioxide? How does just 0.04% of this lead to this? (upbeat music) Got a longer shirt now. Hope you're happy, YouTube commenters. Fill up a glass box with regular air and shine sunlight on it and the air will heat up a little bit. Do the same thing with a glass box full of carbon dioxide and the
air will heat up a lot. This is the greenhouse
effect and it is old news. Eunice Foote first observed
it way back in 1856. Look, it says in carbonic acid gas, which is what they called it at the time. You can also do the opposite experiment. Fill up a glass box with just
oxygen and nitrogen molecules, no carbon dioxide, and the temperature
will not change at all. Somehow, the carbon
dioxide is able to absorb the sun's energy and heat up, while oxygen and nitrogen are not. Okay, but why? There are two ways that a
molecule can absorb energy. Here, I have a molecule of water
with some amount of energy, hence the motion. And the first way this
molecule can absorb energy is by getting smacked another molecule. - [Man] I get you on the face? - Yes.
(laughing) And the first way this
molecule can absorb energy is by getting smacked by another molecule. This molecule transfers some
of its energy to this one, which makes this one move around faster. These kinds of models are great for showing molecular
collisions, but I can't show you the second way a molecule
can absorb energy with a model like this
because it would just- (model clanking) What we really need is a
different kind of model. So I'm gonna make one. (upbeat music) That should do it. And this is the same
molecule as before, water. This ball right here is oxygen. And these two balls right
here are the hydrogens. The springs represent the bonds between the oxygen and the hydrogens. And what this model demonstrates
is a fundamental truth of chemistry and physics, which is that all molecules
vibrate all the time. A molecule cannot be
perfectly still because that would violate a key tenant
of quantum mechanics called the Heisenberg
Uncertainty Principle. And if you wanna deeply
understand that sentence, and I mean deeply, I highly recommend the Stanford Encyclopedia Philosophy's 14,000 word article about this, which I have linked in the description. Anyway, in this ball bearing
and spring model of water, the energy that drives
these vibrations is me. I'm stretching or compressing bonds. In an actual water molecule, these vibrations are
driven by the molecule's kinetic energy. Kinetic just means movement. So kinetic energy is the energy of motion. The more kinetic energy a
group of molecules have, the higher their temperature. Now all molecules have some base amount of kinetic energy and they
show that energy by vibrating or rotating or moving through
space or all of the above. But we are just gonna focus
on the vibrations here. All molecules can also
absorb additional energy on top of what they started with. We already talked about
the first way they do this, which is by getting smacked. And now let's talk about the second way, which is they can absorb
a photon of light. There are entire subfields
of chemistry and physics devoted to how molecules
interact with light. Because what happens to
the molecule depends a lot on how much energy the light has. If a low energy photon hits a molecule, it might just make the
molecule spin a little faster. If a super high energy
photon hits a molecule, it might knock an electron clean off. If a medium energy photon, also known as an infrared
photon, hits a molecule, it can make the molecule vibrate faster, depending on the molecule. For example, let's look at the
most abundant greenhouse gas in the atmosphere, carbon di- Water, yeah, water. Now, a water molecule
can vibrate in three ways and I don't have three hands, so I can't show you the ways myself, but someone who can is
friend of the channel, Linus Pauling. (bell dinging) - [Pauling] Since the water
molecule contains three atoms, it has three normal modes of vibration. An asymmetric stretching motion, another motion which
is symmetric stretching and partly bending of the molecular bond. And a third motion,
which is mostly bending. - But wait a second, surely water can vibrate in
lots of other ways, right? For example, couldn't this
one hydrogen just move back and forth on its own like this? Amazingly, the answer is no, it can't. Again, because of quantum mechanics. Now I know this is deeply unsatisfying and I promise I am only
stonewalling you because if I were to explain it from
scratch, we'd have to start with the Schrodinger equation
for a harmonic oscillator and work our way up to
the Hermite polynomials. And this not being a math channel, we're just gonna stick
with because I said so. I hate because I said so. Anyway, a water molecule vibrating in any of these three ways, or any
combination of these three ways, can absorb a photon of infrared
light, absorbing its energy and causing it to vibrate faster. Simple, right? Okay, so now we're gonna
move on to carbon di- Oxygen, oxygen. We're not at carbon dioxide yet. We need to talk about oxygen
and also nitrogen, first. It turns out that oxygen can't absorb infrared photons and neither can nitrogen, which is weird, because both of these
molecules have two atoms and a molecule with two
atoms can definitely vibrate. So what is it that's different about this vibration from this one? Remember I talked about a
photon of light colliding with a water molecule? That is the language of particles. But electrons and light
can also behave as fields, which you can think of as waves. So let's look at the photon, the molecule, and the collision using the language of waves instead. First, instead of a photon, let's consider this little bit
of light to be a short pulse of oscillating electric
and magnetic waves. And instead of little balls of negative charge zooming
around the nucleus, let's think of electrons as being a
cloud of negative charge. Now water's oxygen is
more electro-negative than the hydrogens. So it pulls electron
density towards itself. And that means that this molecule has what's
called a dipole moment, which is a fancy way of saying the molecule has an electric field caused by the separation of two opposite charges. And also remember that a water molecule, like every other molecule
in the entire universe, is always vibrating, which means that its dipole
moment is constantly changing. Okay, now remember that a photon
of light in this wave model is partially an
oscillating electric field. So when the photons constantly
changing electric field and the molecules constantly
changing electric field occupy the same space, they can couple. And the energy from the
photon is transferred to the water molecule and
the photon ceases to exist. This is the fundamental principle
a molecule needs to obey if it's gonna absorb a photon of light and use that energy to vibrate faster. It needs to have a dipole moment that changes during the vibration. Okay, now let's go back to oxygen. There are just two atoms here,
and they're the same atoms. So each one pulls exactly the same amount of electron density towards itself, which means there's no
separation of charge, which means there's no dipole moment. And when oxygen vibrates, it can only do it in one way, like this. Throughout this entire vibration, the molecule stays perfectly symmetrical. Neither side is more positive
or more negative, which means that oxygen doesn't have a
dipole moment that changes during its vibration. Because it doesn't have
a changing dipole moment, there's no changing electric
field for the photon's oscillating electric field to couple to. So the light just passes right through. Same deal for nitrogen. It's kind of like how this
magnet produces a magnetic field so it can interact with
ferromagnetic objects but not with non-ferromagnetic ones. And now, finally, we can
talk about carbon dioxide. Now, carbon dioxide has
three atoms, like water, but those three atoms
are arranged in a line, like oxygen or nitrogen. Oxygen is more
electronegative than carbon. So just like in water, the oxygens pull electron
density towards themselves. So you might think, okay, carbon dioxide has a dipole moment, but the two oxygens are
exactly opposite each other and each one's pull effectively
cancels the others out. So if molecules of carbon
dioxide were perfectly still like this, they would
not have a dipole moment. But remember, all molecules
are vibrating all the time. And rather than try and show this to you, let's just cut back to Linus here. - [Pauling] The vibrations of
carbon dioxide can be resolved into four normal modes. - In this vibration, the
molecule stays symmetrical throughout the entire vibration. So there's no changing dipole moment for light to interact with. But look at this vibration. Okay, pause here. See how the carbon is closer to this oxygen than to this one? And this means that the molecule
is no longer symmetrical, and thus has a dipole moment. Here's another example. Okay, pause here. The molecule is bent to the
point where it almost looks like a water molecule, which
remember, has a dipole moment. So when carbon dioxide vibrates in a way that changes its dipole moment, it can absorb infrared light. And when it absorbs infrared
light, it vibrates faster. Now that it's converted
the energy from a photon into extra movement, there are
two ways the CO2 can return to its normal resting state. It can release a new
infrared photon shortly after absorbing the old one, or it can bump into, say,
an oxygen or a nitrogen molecule, causing that
molecule to vibrate faster. Once the CO2 has
transferred its extra energy to another molecule or
released a photon, it's back to its resting state and ready
to absorb another photon. Now, the important thing to realize here is that this is a cycle. The CO2 is not getting
used up by this process. Quite the opposite. It can do this bazillions
of times per second. So it's actually wrong to think of CO2 as a molecule that absorbs infrared light as if it were a sponge absorbing water. It behaves more like a conduit. It takes infrared light
that would otherwise be lost to space, and it uses that energy to make nearby molecules move faster. And remember, faster motion
means higher temperature. So essentially, CO2 behaves
as a heating element, catching infrared photons
and transferring that energy off to oxygen and nitrogen, which can't catch infrared
energy themselves. If you do some highly simplified math, and we're talking math the
IPCC could do in second grade, you'll see that the carbon
dioxide that we've added to just a U.S. sized chunk
of the atmosphere since 1750 can serve as a conduit for roughly 22 trillion Joules of energy per second. Per second. That'd be like detonating,
28,500 of the nuclear bombs dropped on Hiroshima every single day. So when you see a graph of
CO2 concentration over time, 420 parts per million today might not seem like all that much, but
compared to an atmosphere with only 280 parts per million,
that's a staggering amount of light energy being absorbed
and converted to heat. Imagine a Hiroshima-size
nuclear bomb going off somewhere over the
U.S. every three seconds. That is a lot of energy. Can we turn up rainfall
like we do a thermostat? To find out, check out Nova's latest video about cloud seating and how
it could help with drought. Head over to the link in the description and tell them Reactions sent you. Also, don't forget to check out the rest of the awesome content in
PBS's Earth Month lineup.