Summary:
Carbon dioxide is widely known for warming Earth’s surface and lower atmosphere, but in the upper atmosphere it produces the opposite effect. A study published in Nature Geoscience has now explained in detail why rising CO₂ levels cool the stratosphere, helping resolve a long-standing question in climate science.
Researchers from Columbia University found that the effect is linked to how carbon dioxide interacts with infrared radiation at different wavelengths. In the stratosphere, which extends from roughly 11 to 50 km above Earth’s surface, CO₂ absorbs infrared energy and releases part of it into space. As atmospheric CO₂ concentrations rise, this process becomes more efficient, causing the stratosphere to cool even as warming increases closer to the surface.
The team developed a mathematical framework showing that certain infrared wavelengths play a particularly important role in the cooling process. Their results also explain why cooling intensifies at higher altitudes in the stratosphere and why each doubling of CO₂ can produce up to 8 °C of cooling near the stratopause.
The researchers say the findings improve understanding of a key atmospheric signal linked to human-driven climate change. The work may also help scientists study the atmospheres of other planets and exoplanets.
— Press Release —
A new study explains how carbon dioxide cools the upper atmosphere and warms earth below
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.
Journal Reference:
Cohen, S., Pincus, R. & Polvani, L.M., ‘Stratospheric cooling and amplification of radiative forcing with rising carbon dioxide’, Nature Geoscience 19, 507–512 (2026). DOI: 10.1038/s41561-026-01965-8
Article Source:
Press Release/Material by Columbia Climate School | Columbia University
Featured image credit: NASA
