Earth’s surface is warming, but far above the planet, temperatures in the upper atmosphere have been falling for years. Researchers now say they have identified the physical mechanism behind this long-observed phenomenon.
The study, published in Nature Geoscience by scientists from Columbia University, provides a more detailed explanation for a process climate researchers have tracked since the 20th century. The authors say the findings clarify one of the clearest atmospheric fingerprints associated with rising human-produced CO2 emissions.
The new research focused on how infrared radiation moves through the upper atmosphere and why some wavelengths are much more efficient at cooling than others. To reach their conclusions, the team repeatedly adjusted mathematical models and compared them with climate simulations and atmospheric observations until the results matched measured data.
CO2 Behaves Differently High Above Earth
The stratosphere stretches from roughly 11 to 50 kilometers above Earth’s surface. In that region, carbon dioxide absorbs infrared energy rising from lower layers of the atmosphere and then emits part of it back into space.
The study found that rising CO2 levels are making the stratosphere better at releasing heat, causing temperatures there to slowly drop over time. Researchers say the stratosphere has cooled by around 2°C since the mid-1980s.
The phenomenon itself was not unexpected. Scientists first predicted it in the 1960s through early climate models developed by climatologist Syukuro Manabe, whose later work earned a Nobel Prize. What remained unclear was the exact mechanism controlling the cooling rate.
“The existing theory was incredibly insightful, but at the moment we lack a quantitative theory for CO2-induced stratospheric cooling,” lead author Sean Cohen said through Columbia’s Lamont-Doherty Earth Observatory.
An Infrared “Goldilocks Zone” Has Been Identified
To better understand the process, the Columbia team analyzed how different infrared wavelengths behave in the atmosphere. They found that certain wavelengths are especially effective at driving cooling in the stratosphere.
The researchers described this range as a “Goldilocks zone,” where infrared radiation escapes into space with particularly high efficiency. As atmospheric CO2 levels continue to rise, that zone expands.
“It’s those changes in efficiency that are going to ultimately be what’s driving stratospheric cooling,” Cohen explained in comments accompanying the Nature Geoscience publication.
The researchers also examined ozone and water vapor, two gases that affect how heat moves through the atmosphere. But their analysis found that both play only a small role in stratospheric cooling compared with carbon dioxide.
The cooling also doesn’t happen evenly throughout the stratosphere. The models showed that temperatures drop much faster at higher altitudes, with the strongest cooling occurring near the stratopause, the upper boundary of the stratosphere.
A Key Climate Signal Is Becoming Clearer
The study also matched several atmospheric trends scientists have been seeing for years. One of the clearest findings was that every time atmospheric CO2 levels double, temperatures near the stratopause drop by about 8°C.
Researchers from the Columbia Climate School said the results also point to a feedback effect tied to Earth’s energy balance. As the stratosphere cools, the planet sends less infrared energy out into space, which means more heat staystrapped closer to Earth’s surface.
The team stressed that the study was not meant to prove global warming exists. Instead, the goal was to better understand how part of the atmosphere reacts as greenhouse gas levels keep rising.
They added that the work may also prove useful for scientists studying atmospheres beyond Earth, including those of planets elsewhere in the solar system and distant exoplanets.
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