How can such a small amount of carbon dioxide in the atmosphere—only around 420 parts per million—cause so much warming?

LHF: Hello, and welcome back to Today I Learned: Climate. I’m Laur Hesse Fisher of the MIT Environmental Solutions Initiative, and today we are kicking off our fifth season with a very important episode. If you want to get smarter on how the greenhouse effect actually works, today’s show is for you.

You probably know that today’s climate change is caused by certain gases—what scientists call greenhouse gases—that human activity has been adding to our atmosphere. The prime example is carbon dioxide, CO2, and we’ve sometimes referred to CO2 and other greenhouse gases as acting like a “blanket” over the Earth, keeping our planet warm. But—how do these gases actually keep heat from escaping into space? And why these gases in particular?
To help us answer these questions, we invited an environmental chemist to the show.

DP: My name is Desiree Plata and I'm an associate professor in the Department of Civil and Environmental Engineering at MIT and the director of the MIT Methane Network.

LHF: Methane is also a greenhouse gas, and you’ll learn some cool things about methane later in this episode.

A little wonkiness warning for this episode: we’re going to talk some physics and some chemistry. But I promise—pinkie swear—we’re going to make it easy to understand, and, in fact, fascinating.

Ready? Let’s get started.

Imagine you’re standing outside on a clear day, looking up toward the sun. It’s blindingly bright, and it feels warm on your face. What you’re experiencing is actually a few different kinds of radiation coming from the sun: visible light, which is what we see; and infrared, which we feel as heat. There’s also a dash of ultraviolet light thrown in, which is what causes sunburn.

Now imagine the sun has gone down. You can’t feel that nice warmth on your face anymore, but you’re not in the frozen void of space either. You are, in fact, still feeling that nice, warm infrared radiation. How?

DP: So if we think about solar energy coming to the earth, it comes to the Earth mostly in the ultraviolet and the visible range, loses a little bit of energy and is reradiated to outer space as infrared.

LHF: Essentially, the land, oceans, trees… everything on Earth absorbs visible and UV light from the sun and converts it into warm infrared radiation. You know night-vision goggles? They work because they show that infrared radiation that we can’t see with our eyes—infrared that’s radiating from the Earth, not just from the sun.

So all that infrared that’s radiating off of the Earth – where does it all go?

DP: That infrared outgoing solar energy is in a sweet spot for absorption by methane and CO2 and other greenhouse gases in our atmosphere. So what happens is that infrared energy gets absorbed by the molecule. And that molecule enters an excited state. As it's relaxing back down, it releases some of that energy as heat, and that heat can go to outer space or can come back to planet Earth.

LHF: These greenhouse gases in the atmosphere only grab infrared particles for an instant before they let them go. But the direction in which they release those particles is random; some go out into space, and some of them end up coming back down to us, here on Earth.

This effect is so powerful that we get almost twice as much heat from our own atmosphere as we get directly from the sun.

And this is the greenhouse effect: this infrared radiation bouncing around between the Earth’s surface and the atmosphere, 24 hours a day. It’s why our planet is, on average, almost 60° F instead of in the freeze of space. It’s critical; it’s why our planet is able to support life. And scientists have actually known all this for almost 200 years.
What’s new is that human activity is adding more greenhouse gases to the atmosphere, which keeps more infrared heat energy bouncing around us.

But if you’re like me, you want to know: why? Why are these greenhouse gas molecules so good at grabbing infrared?

Prof. Plata told us that it actually comes down to the atoms in the molecules having some freedom to move around.

DP: CO2 is a central carbon atom surrounded by two oxygen atoms. And if you think about these geometries, it influences kind of what I like to call, um, the “dance moves” that the molecule has access to. Now scientists call those dance moves, vibrational and rotational modes, and the energy transitions associated with those modes overlap with the energy that can be absorbed in the infrared.

LHF: If you imagine a bunch of infrared particles leaping across the dance floor, CO2 has the right moves to reach out and catch their hands as they’re passing by. CO2 isn’t all that complex as molecules go—just three atoms—but it’s more complex than nitrogen or oxygen, which only have two atoms each and together make up more than 90% of our atmosphere. Those gases don’t have many dance moves, and they leave infrared alone. CO2, with a few more moves, catches its little dance partner and twirls it around—giving off some heat in the process.

And of course there are far  more complex molecules than CO2.

DP: Methane is a central carbon atom surrounded by four hydrogen atoms. If you wanna imagine looking at it from the outside, it would make, um, a pyramid that has a triangular base. So methane, because it's got a different geometry and more things attached to it, it's got a lot more dance moves than carbon dioxide.

It makes it a much more potent greenhouse gas. Instantaneously, methane is about 120 times more warming than CO2, pound for pound.

LHF: And if you look at some of the other greenhouse gases out there—especially human-made ones like hydrofluorocarbons (also called HFCs) or sulfur hexafluoride, things we mostly use in appliances and industry—these are the quadruple-jointed tango champions of the atmosphere, catching infrared at many hundreds or thousands of times the rate of CO2.

So wait, why do we talk so much about CO2 and so little about these other, much more powerful gases? Well, one reason is that there’s so much more CO2. There’s about 200 times more CO2 in the atmosphere than the next most common greenhouse gas, methane. After that we have nitrous oxide, which is about 5 times rarer still, and then a whole bunch of human-made stuff that’s much rarer than that.

There is, as an aside, a greenhouse gas even more common than CO2. Do you know what it is? Water vapor. Yeah, clouds. All those H2O molecules up there do have some dance moves, and they help the Earth keep in heat. But if you put more water in the atmosphere, it comes right back down as rain. That’s why you don’t have to worry about warming the planet when you boil a pot of water—our water cycle is so fast that the steam you create won’t stick around to contribute to climate change.

So CO2 is the most common greenhouse gas that’s contributing to climate change. But there’s a second reason we pay so much attention to CO2, and that’s to do with the way it behaves over time.

DP: CO2 is the low and slow, steady warmer. It stays in the atmosphere for many thousands of years. So CO2 is kind of like the thin blanket. You might accumulate lots of thin blankets over time, and that would impact the total amount of warming that you're experiencing. Methane is a really thick blanket that's going to warm you really quickly. So methane is what's called a short-lived climate pollutant. And what this means is that its atmospheric lifetime isn't very long. It lives in the atmosphere for about 12 years after the point that it's emitted. The fate of methane in the atmosphere is actually to turn into CO2, which is one of the reasons that it becomes less and less of a climate forcer over time.

LHF: Imagine these gases are little quilters, sewing blankets to add to the big pile that warms the Earth. For every watt of heat one of these gases re-emits to Earth, it adds a square to the blanket. If you release an equal amount of methane and CO2 into the atmosphere, a minute later, the methane will have 120 squares for every 1 square the CO2 has.

But over time, the methane decays into CO2, and it slows down. Meanwhile, CO2 just methodically sews its squares at the same rate year after year. After a couple of decades, it’s starting to catch up.

So, how do we compare the climate change impact of methane versus CO2? That depends on time: how far out you want to look. And there’s actually a semi-official way to resolve this. The United Nations convenes a body of thousands of scientists that analyze the latest science on climate change, called the Intergovernmental Panel on Climate Change, or IPCC. That group usually looks a full one hundred years out.

DP: And this isn't totally arbitrary, you know, some definitions of sustainability are actually drawn around good stewardship of the environment for say, seven generations is a common one that people hear. So the Intergovernmental Panel on Climate Change made this decision to look on a hundred year timeframe.

LHF: On a 100-year timeline, methane is about 28 times as powerful as CO2. Which is a lot, but remember, there’s 200 times more CO2 in the atmosphere than methane, so long-lasting CO2 more than makes up that gap in volume. If we look at all the warming that we’re going to experience over the next hundred years, 75% of it will be caused by the CO2 that human activity has already added to the atmosphere. So yeah, it does make sense to give most of our attention to CO2.

That is, assuming you agree that 100 years is the right time frame to look at.

DP: If you look on just a 10 year time horizon, methane and CO2 actually warm the atmosphere just as much. They're contributing equally to the climate forcing. And this is a really underappreciated fact.

LHF: The world was a pretty different place when that 100-year standard was created. The IPCC published its first report comparing different greenhouse gases in 1990: when climate change was well known, but its effects weren’t so visible and it seemed possible to stop it before it profoundly affected our lives.

Now we’re dealing with the effects of climate change – more intense wildfires, droughts, hurricanes… and there’s good reason to be very concerned about the warming that will happen in just the next 10 or 20 years.

DP: People have known for a long time that methane is an important climate forcer, and I think there's been this perception that it'll just take care of itself. You know, because it's short-lived in the atmosphere, it will go away. But we're realizing that we're a little late to the game for putting the brakes on our greenhouse gas emissions. And one of the unique ways that we could start to change the rate of global warming in our lifetime is actually to target short-lived climate pollutants.

If you think about strategies to kind of cool yourself in the near term, you would want to start pulling off those really thick blankets quickly.

The reality is that we have to think of ourselves as in a couple of races here. We're in a sprint to abate methane and we're in a marathon to try and reduce CO2 emissions and reduce atmospheric CO2 levels.

LHF: And that’s where we’ll pick up next week: with the sprint to deal with methane. And the week after that, we’ll stick with this theme and introduce another class of greenhouse gases, refrigerants. But for now, I hope you feel smarter about greenhouse gases, what they’re doing in the atmosphere, and why it’s so important to stop adding more of them.

If you want to bring this lesson into the classroom, our Educator Guide for this episode will include hands-on activities as well as a deeper dive for high school students. We also have some links in our show notes to help you understand the greenhouse effect and compare these different gases to each other. You can find all that at tilclimate.org.

TILclimate is produced by the MIT Environmental Solutions Initiative at the Massachusetts Institute of Technology. David Lishansky is our Editor and Producer. Aaron Krol is our Scriptwriter and Associate Producer — and did our artwork. Michelle Harris is our fact-checker. Sylvia Scharf is our Climate Education Specialist. The music is by Blue Dot Sessions. And I’m your Host and Producer, Laur Hesse Fisher.

Thank you to Prof. Desiree Plata for joining us, and thank you for listening.