Physicists Pinpoint the Quantum Origin of the Greenhouse Effect
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Welcome to the Quantascience Podcast. Each episode we bring you stories about developments in science and mathematics. I'm Susan Vallett. Carbon dioxide's powerful heat-trapping effect has been traced to a quirk of its quantum structure. The finding may explain climate change better than any computer model. That's next.

It's season three of The Joy of Why, and I still have a lot of questions. Like, what is this thing we call time? Why does altruism exist? And where is Jana Levin? I'm here, astrophysicist and co-host, ready for anything. That's right. I'm bringing in the A-team. So brace yourselves. Get ready to learn. I'm Jana Levin. I'm Steve Strogatz. And this is... Quantum Magazine's podcast, The Joy of Why. New episodes drop every other Thursday.

In 1896, Swedish physicist Svante Arrønjös realized that carbon dioxide, or CO2, traps heat in Earth's atmosphere. The phenomenon is now called the greenhouse effect.

Since then, increasingly sophisticated modern climate models have verified Arianeos' central conclusion: that every time the CO2 concentration in the atmosphere doubles, Earth's temperature will rise between 2 and 5 degrees Celsius. Still, the physical reason why CO2 behaves this way has remained a mystery until recently.

First, in 2022, physicists settled a dispute over the origin of the logarithmic scaling of the greenhouse effect. That refers to the way Earth's temperature increases the same amount in response to any doubling of CO2, no matter the raw numbers.

Then, in spring of 2024, a team led by Robin Wordsworth of Harvard University figured out why the CO2 molecule is so good at trapping heat in the first place. The motivation for it was that

Although the physics of climate change is not in question, we all know that it's real. The models have shown how it works since the 1960s. And so the idea of doing an analytic derivation like this is not to replace models like that. It's instead to be able to explain to people, particularly people in other fields, exactly how everything holds together. And surprisingly, although all the pieces have been in place, I would say probably since at least the 1970s,

Nobody actually tried to do this before. Wordsworth and his fellow researchers identified a strange quirk of the molecule's quantum structure that explains why it's such a powerful greenhouse gas and why pumping more carbon into the sky drives climate change. The findings appeared in the Planetary Science Journal.

Raymond Pierre Humbert, an atmospheric physicist at the University of Oxford who wasn't involved in the study, calls it a really nice paper. He says it's a good answer to all those people who say that global warming is just something that comes out of computer models. To the contrary, global warming is tied to a numerical coincidence involving two different ways that CO2 can wiggle. Pierre Humbert says if it weren't for this accident, then a lot of things would be different.

How could Ari Unios understand the basics of the greenhouse effect before quantum mechanics was even discovered? It started with Joseph Fourier. He was a French mathematician and physicist who realized 200 years ago that Earth's atmosphere insulates the planet from the freezing cold of space. It's a discovery that launched the field of climate science.

Then, in 1856, an American, Eunice Foote, observed that carbon dioxide is particularly good at absorbing radiation. Next, Irish physicist John Tyndale measured the amount of infrared light that CO2 absorbs. This showed the effect which R. E. Eunice then quantified using basic knowledge about Earth.

Earth radiates heat in the form of infrared light. The gist of the greenhouse effect is that some of that light, instead of escaping straight to space, hits CO2 molecules in the atmosphere.

A molecule absorbs the light, then re-emits it. Then another does. Sometimes the light heads back down toward the surface. Sometimes it heads up to space, leaving the Earth one iota cooler, but only after traversing a jagged path to the cold upper reaches of the atmosphere.

Using a cruder version of the same mathematical approach climate scientists take today, Ariunos concluded that adding more CO2 would cause the planet's surface to get warmer. It's like adding insulation in your walls to keep your house warmer in the winter. Heat from your furnace enters at the same rate, but it escapes more slowly.

However, a few years later, Swedish physicist Knut Engström published a rebuttal. He argued that CO2 molecules only absorb a specific wavelength of infrared radiation, 15 microns. And there was already enough of the gas in the atmosphere to trap 100% of the 15 micron light Earth emits. So adding more CO2 would do nothing.

David Romps is a climate physicist at the University of California, Berkeley. There's a kernel of truth in what Engstrom said, in that where CO2 is strongly absorbing, it has no effect on the climate, to first order, as you increase the concentration of CO2. And that statement is true. What it means is that there are places where CO2, whether wavelengths or wavenumbers or frequencies,

at which CO2 is not currently strongly absorbing, but will become so as you crank up the concentration. And in fact, there are always wave numbers, wave lines, frequencies of CO2 waiting to become quote-unquote strongly absorbing.

as the concentration has increased. So what Angstrom missed was that CO2 can absorb wavelengths slightly shorter or longer than 15 microns, though less readily. This light gets captured fewer times along its trip to space. But that capture rate changes if the amount of carbon dioxide doubles. Now the light has twice the molecules to dodge before escaping, and it tends to get absorbed more times along the way.

It escapes from a higher, colder layer of the atmosphere, so the outflow of heat slows to a trickle. It's the heightened absorption of these near 15 micron wavelengths that's responsible for our changing climate. Despite the mistake, Engstrom's paper threw enough doubt on Arios' theory that discussion of climate change more or less exited the mainstream for half a century.

Even today, skeptics of the climate change consensus sometimes cite Angstrom's erroneous carbon saturation argument. In contrast to those early days, the modern era of climate science has moved forward largely by way of computational models that capture the many complex and chaotic facets of our messy, shifting atmosphere. For some, this makes the conclusions harder to understand.

Nader Jivangi is an atmospheric physicist at the National Oceanic and Atmospheric Administration, or NOAA. I've talked to a lot of skeptical physicists in my time, and including folks here at Princeton, and one of their objections is, you know, oh, you guys just run computer models, and then you take the answers from this black box calculation, and you don't understand it deeply, and, you know, I'm not sure why you trust it. And I feel like if we can reproduce to within 10-15%,

on a blackboard, the same numbers that we get from the computer models, that just demonstrates to the rest of the world, to the skeptical physicists and engineers, that we know our stuff and that the models are trustworthy. So I think that's the other reason why you want to have this kind of concentrated analytical understanding. It's a little unsatisfying not to be able to explain to someone on a chalkboard why we get the numbers we get.

And not being able to do that means we don't really understand which are the critical variables. Givenchy and others like him have set out to build a simpler understanding of the impact of CO2 concentration on the climate. A key question was the origin of the logarithmic scaling of the greenhouse effect, the 2 to 5 degree temperature rise that models predict will happen for every doubling of CO2.

One theory held that the scaling comes from how quickly the temperature drops with altitude. But in 2022, a team of researchers used a simple model to prove that the logarithmic scaling comes from the shape of carbon dioxide's absorption spectrum. That's how its ability to absorb light varies with the light's wavelength.

This goes back to those wavelengths that are slightly longer or shorter than 15 microns. A critical detail is that carbon dioxide is worse, but not too much worse, at absorbing light with those wavelengths. The absorption falls off on either side of the peak at just the right rate to give rise to the logarithmic scaling.

Climate physicist David Romps co-authored the 2022 paper. The shape of that spectrum is essential. If you change it, you don't get the logarithmic scaling. The carbon spectrum's shape is unusual. Most gases absorb a much narrower range of wavelengths. The question I had had in the back of my mind, and to ask people too, why does it have wavelengths?

this sheep. And I didn't know, I didn't get an answer to it. Wordsworth and his co-authors Jacob Seeley and Keith Schein turned to quantum mechanics to find the answer. Light is made of packets of energy called photons. Molecules like CO2 can absorb them only when the packets have exactly the right amount of energy to bump the molecule up to a different quantum mechanical state.

Carbon dioxide usually sits in its ground state, where its three atoms form a line with the carbon atom in the center, equidistant from the others. The molecule has excited states as well, in which its atoms undulate or swing about. A photon of 15 micron light contains the exact energy required to set the carbon atom swirling about the center point in a sort of hula hoop motion.

Climate scientists have long blamed this hula-hoop state for the greenhouse effect, but, as Engstrom anticipated, the effect requires too precise an amount of energy. That's what Wordsworth and his team found. The hula-hoop state can't explain the relatively slow decline in the absorption rate for photons further from 15 microns, so it can't explain climate change by itself.

The key turns out to be another type of motion, where the two oxygen atoms repeatedly bob toward and away from the carbon center, as if stretching and compressing a spring connecting them. This motion takes too much energy to be induced by Earth's infrared photons on their own. But the authors found that the energy of the stretching motion is so close to double that of the hula hoop motion that the two states of motion mix with one another.

Special combinations of the two motions exist, requiring slightly more or less than the exact energy of the hula hoop motion. This unique phenomenon is called Fermi resonance after the famous physicist Enrico Fermi, who derived it in a 1931 paper. But its connection to Earth's climate was only made for the first time in a paper in 2023 by Schein and his student.

and the paper last spring was the first to fully lay it bare. Here's Wordsworth. The moment when we wrote down the terms for this equation and saw that it all clicked together, it felt pretty incredible because it is a result that finally shows us directly how the quantum mechanics links to the bigger picture. And of course, it's not like

It changes predictions for climate change. It just allows us to explain things in a way that doesn't require some big computer model to be in the middle. And if you're fond of doing theory, as I am, it feels very satisfying to be able to do that. Wordsworth says in some ways, the calculation helps us understand climate change better than any computer model. It just seems a fundamentally important thing to be able to say in a field that we don't

can show from basic principles where everything comes from. Joanna Haig, an atmospheric physicist and emeritus professor at Imperial College London, agrees.

Haig says the paper adds rhetorical power to the case for climate change. The climate change deniers would say things like, oh, carbon dioxide, it's only 10,000th concentration in the atmosphere. How can it possibly do anything? And so you come up with this radiated spectra to show that it can. But here...

Tying that into the radiative forcing is now based on fundamental quantum mechanical concepts and established physics so that the deniers can't say CO2 can't cause global warming.

because it's very well established that it can, and by this relationship as well. In 2024, NOAA's Global Monitoring Laboratory reported that the concentration of CO2 in the atmosphere has risen from its pre-industrial level of 280 parts per million to a record high 419.3 parts per million as of 2023, triggering an estimated 1 degree Celsius of warming so far.

Arlene Santana helped with this episode. I'm Susan Vallett. For more on this story, read Joseph Howlett's full article, Physicists Pinpoint the Quantum Origin of the Greenhouse Effect, on our website, quantummagazine.org. Explore science mysteries in the quantum book, Alice and Bob Meet the Wall of Fire, published by the MIT Press. Available now at amazon.com, barnesandnoble.com, or your local bookstore.