View of Earth taken during International Space Station Expedition 66
Even as temperatures rise on Earth’s surface and in the lower atmosphere, the planet’s upper atmosphere has cooled dramatically. This paradoxical pattern is a well-known sign of humanity’s climate impacts—but until now, the underlying physics has remained a mystery.
In a new study, researchers from Columbia University describe the phenomenon’s mechanics, illuminating how it is largely determined by the way carbon dioxide (CO2) interacts with different wavelengths of light.
“It explains a phenomenon that’s a fingerprint of climate change, has been known to occur for decades, and has not been understood,” says Robert Pincus, a research professor of ocean and climate physics at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and co-author of the study published in Nature Geoscience.
In the lower atmosphere, CO2 molecules trap heat that would otherwise escape into space. Higher in the atmosphere, though, the dynamics change. In the stratosphere—the atmospheric layer that extends from about 11km to 50 km above Earth’s surface—CO2 molecules function almost like a radiator, absorbing infrared energy from below and emitting some of that energy into space. When more CO2 is added, the stratosphere radiates heat away more efficiently and it cools.
This was predicted in the 1960s by climatologist Syukuro Manabe’s Nobel Prize-winning models of Earth’s climate and CO2-induced global warming. The stratosphere has cooled by roughly 2 degrees Celsius since the mid-1980s. That’s estimated to be more than 10 times the amount of cooling that would have occurred in the absence of human-caused CO2 emissions.
However, though the basic principles of stratospheric cooling are understood, the specifics have remained cloudy. “The existing theory was incredibly insightful, but at the moment we lack a quantitative theory for CO2-induced stratospheric cooling,” says Sean Cohen, a postdoctoral research scientist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and the study’s lead author.
Cohen, Pincus, and Lorenzo Polvani, a geophysicist in Columbia Engineering’s Department of Applied Physics and Applied Mathematics, developed their theory through an iterative method of identifying the key processes involved in stratospheric cooling, assigning mathematical values to them, comparing the results of their pen-and-paper models to comprehensive simulations and real-world data, tweaking their equations and repeating. Over several months they deduced the equations that best fit.
The researchers arrived at a central factor: how CO2 molecules interact with light, and in particular infrared—also known as longwave—light. Not every infrared wavelength passes through them in the same way. Some wavelengths contribute to cooling more than others, and the team determined that wavelengths in a certain “Goldilocks zone” are especially efficient. As CO2 accumulates in the atmosphere, that zone expands.
“It’s those changes in efficiency that are going to ultimately be what’s driving stratospheric cooling,” says Cohen.
The researchers also quantified the roles played by ozone and water vapor. These are implicated in similar processes as CO2—they too can trap heat in the lower atmosphere but contribute to cooling in the stratosphere by radiating heat—but turn out to have little influence compared with CO2.
The researchers’ equations fit with three well-described phenomena: How stratospheric cooling varies by altitude, with the least cooling occurring at its lowest level and the most at its highest level; how each doubling of CO2 translates to a cooling of 8 degrees Celsius at the stratopause, or the stratosphere’s upper reaches; and how a cooler stratosphere lets less infrared energy escape to space, increasing CO2’s heat-trapping effect. In other words: CO2 makes the stratosphere better at radiating, which cools it—but because it becomes colder, the Earth system ends up losing less heat to space overall, strengthening warming below.
“Here’s this process that we’ve known about for 50-plus years, and we had a pretty decent qualitative understanding of how it worked. However, we didn’t understand the details of what actually drove that process mechanistically,” says Cohen.
Cohen and Pincus say the implications of the work are less about adding one more piece of evidence to support global warming—that reality is already clear—than developing a better understanding of the mechanisms involved in stratospheric cooling. “This is really telling us what is essential,” says Pincus, and it can inform future research on the process. The findings may also help scientists studying conditions outside of Earth.
“Maybe we can better understand what’s going on in the stratospheres of other planets in our solar system or exoplanets,” says Cohen.
