- 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.