Earth’s Upper Atmosphere Is Cooling Fast and Scientists Finally Know Why

Earth’s surface and lower atmosphere are getting hotter, but far above the planet, another dramatic change has been unfolding in the opposite direction. The upper atmosphere has been steadily cooling for decades, creating one of the most recognizable signs of human-driven climate change. Scientists have understood that this was happening, but the detailed physics behind it remained unclear.

Now, researchers at Columbia University say they have identified the mechanism responsible. Their new study shows that the cooling is closely tied to how carbon dioxide (CO₂) interacts with different wavelengths of light in the upper atmosphere.

“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.

Why CO₂ Heats Earth but Cools the Stratosphere

Near Earth’s surface, CO₂ traps heat that would otherwise escape into space, helping drive global warming. But conditions change dramatically higher in the atmosphere.

In the stratosphere, which stretches from roughly 11km to 50 km above Earth’s surface, CO₂ behaves less like a blanket and more like a cooling system. The molecules absorb infrared energy rising from below and then emit part of that energy back into space. As CO₂ concentrations increase, the stratosphere becomes even better at releasing heat, causing temperatures there to fall.

Scientists first predicted this effect in the 1960s through pioneering climate models developed by climatologist Syukuro Manabe, whose work later earned a Nobel Prize. Since the mid-1980s, the stratosphere has cooled by about 2 degrees Celsius. Researchers estimate this cooling is more than 10 times greater than what would have occurred without human-produced CO₂ emissions.

Even so, many details of the process remained unresolved.

“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.

The Infrared “Goldilocks Zone”

To better understand the phenomenon, Cohen worked alongside Pincus and Lorenzo Polvani, a geophysicist in Columbia Engineering’s Department of Applied Physics and Applied Mathematics. The team repeatedly refined mathematical models that simulated the processes involved in stratospheric cooling. They compared their calculations with advanced climate simulations and real-world observations, adjusting the equations over several months until the results aligned.

The researchers found that one factor stood out above all others: how CO₂ interacts with infrared light, also known as longwave radiation.

Different infrared wavelengths behave differently in the atmosphere. Some are far more effective at driving cooling than others. The team identified a particularly efficient range of wavelengths they described as a “Goldilocks zone.” As atmospheric CO₂ levels continue rising, this zone expands.

“It’s those changes in efficiency that are going to ultimately be what’s driving stratospheric cooling,” says Cohen.

The study also measured the influence of ozone and water vapor. Both gases play similar roles by trapping heat lower in the atmosphere while contributing to cooling higher up through heat radiation. However, the researchers found their overall impact on stratospheric cooling is small compared with CO₂.

How Upper Atmospheric Cooling Strengthens Warming Below

The team’s equations successfully reproduced several long-observed atmospheric patterns. The models showed that cooling becomes stronger with altitude, with the weakest cooling lower in the stratosphere and the strongest near its upper boundary. They also confirmed that every doubling of CO₂ produces about 8 degrees Celsius of cooling at the stratopause, the upper edge of the stratosphere.

The findings also reveal a climate feedback effect. Increasing CO₂ helps the stratosphere radiate heat more efficiently, which cools that region. But as the stratosphere cools, the Earth system actually releases less infrared energy into space overall, allowing more heat to remain trapped closer to the surface.

“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.

According to Cohen and Pincus, the study is not about proving global warming exists. Instead, it provides a clearer understanding of one of climate change’s most important atmospheric processes.

“This is really telling us what is essential,” says Pincus.

The research could also help scientists studying atmospheres beyond Earth, including those of planets elsewhere in the solar system and distant exoplanets.

“Maybe we can better understand what’s going on in the stratospheres of other planets in our solar system or exoplanets,” says Cohen.

Reference: “Stratospheric cooling and amplification of radiative forcing with rising carbon dioxide” by Sean Cohen, Robert Pincus and Lorenzo M. Polvani, 11 May 2026, Nature Geoscience.
DOI: 10.1038/s41561-026-01965-8

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