The Quantum Mechanics of the Greenhouse Effect

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 reemits 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, Arrhenius 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.

A few years later, however, the Swedish physicist Knut Ångström 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 percent of the 15-micron light Earth emits, so adding more CO₂ would do nothing.

What Ångström 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, Ångström’s paper threw enough doubt on Arrhenius’s theory among his contemporaries 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 Ångström’s erroneous carbon “saturation” argument.

Back to Basics

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.

“I’ve talked to a lot of skeptical physicists, and one of their objections is ‘You guys just run computer models, and then you take the answers from this black-box calculation, and you don’t understand it deeply,’” said Nadir Jeevanjee, an atmospheric physicist at the National Oceanic and Atmospheric Administration (NOAA). “It’s a little unsatisfying not to be able to explain to someone on a chalkboard why we get the numbers we get.”

Jeevanjee and others like him have set out to build a simpler understanding of the impact of CO₂ concentration on the climate.