A key question was the origin of the logarithmic scale of the greenhouse effect: the 2 to 5 degree temperature increase that models predict will occur for each doubling of CO2One theory held that the scale was due to how quickly temperature dropped with altitude. But in 2022, a team of researchers used a simple model to show that the logarithmic scale was due to the shape of carbon dioxide’s absorption “spectrum” — that is, how its ability to absorb light varies with the wavelength of the light.
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 much worse) at absorbing light at those wavelengths. Absorption falls off on either side of the peak at just the right rate to give rise to the logarithmic scale.
“The shape of that spectrum is essential,” he said. David Rompsclimate physicist at the University of California, Berkeley, co-author of the 2022 paper. “If you flip it around, you don’t get the log scale.”
The shape of carbon’s spectrum is unusual — most gases absorb a much narrower range of wavelengths. “The question I had in the back of my mind was: Why is it shaped like this?” Romps said. “But I couldn’t pinpoint the reason.”
Consequential movements
Wordsworth and his co-authors Jacob Seeley and Keith Shine turned to quantum mechanics to find the answer.
Light is made up of packets of energy called photons. Molecules such as CO2 They can absorb them only when the packets have exactly the right amount of energy to bring the molecule into a different quantum mechanical state.
Carbon dioxide is usually found in its “ground state,” in which its three atoms form a line with the carbon atom at the center, equidistant from the others. The molecule also has “excited” states, in which its atoms wiggle or wobble.
A 15-micron photon of light contains exactly the right amount of energy for the carbon atom to spin around its center point in a kind of hula-hoop motion. Climate scientists have long blamed this hula-hoop state for the greenhouse effect, but, as Ångström anticipated, the effect requires an overly precise amount of energy, Wordsworth and his team found. The hula-hoop state cannot explain the relatively slow decline in the photon absorption rate beyond 15 microns, so it cannot explain climate change on its own.
The key, they discovered, is another kind of motion, in which the two oxygen atoms repeatedly move toward and away from the carbon center, as if stretching and compressing a spring connecting them. This motion requires too much energy to be induced by infrared photons from Earth alone.
But the authors found that the energy of the stretching motion is almost twice as high as that of the hula hoop motion, so both motion states are mixed. There are special combinations of both motions that require a little more or a little less than the exact energy of the hula hoop motion.
This unique phenomenon is called Fermi resonance after the famous physicist Enrico Fermi, who deduced it in a 1931 paper. But its connection to the Earth’s climate was only first established in A role Last year, Shine and his student did just that, and this spring’s paper is the first to lay it completely bare.
