It seems paradoxical: The greenhouse gas carbon dioxide heats up the earth – but makes its upper atmosphere colder. Physicists have now deciphered why this is the case and what mechanism lies behind this stratospheric cooling. Accordingly, increasing CO2 concentrations increase the wavelength range in which the CO2 molecules efficiently radiate warming infrared radiation into space – the stratosphere becomes colder. This hinders the release of heat into space and increases the greenhouse effect.
When the CO2 content of the atmosphere increases, this has two seemingly opposite consequences: the lower, weather-determining layer of the atmosphere, the troposphere, becomes warmer. The additional CO2 ensures that less long-wave infrared radiation is reflected back into space from the Earth’s surface, thus causing the greenhouse effect. In the stratosphere above, however, it is exactly the opposite: increasing CO2 levels make this layer of the atmosphere, which begins at an altitude of around 15 kilometers, colder.
The stratosphere paradox
“This cooling of the stratosphere is one of the most observable symptoms of rising CO2 levels and a fingerprint of global climate change,” explain Sean Cohen and his colleagues from the Lamont-Doherty Earth Observatory at Columbia University in New York. As early as the 1960s, Japanese climate researchers predicted this, at first glance, paradoxical reaction of the stratosphere to our greenhouse gas emissions.
Since then, satellite data and models have clearly confirmed this effect. Among other things, they show that the stratospheric CO concentrations and temperatures have a fixed relationship to one another: If the CO2 values of the stratosphere double, its upper edge cools by a further eight degrees. “This has significant consequences for the Earth’s energy balance,” say the researchers. The colder stratosphere hinders the Earth’s heat release into space, further fueling the greenhouse effect.
The spectral “Goldilocks” zone is crucial
The problem, however, is that previous theories can understand and describe this effect qualitatively. “But the mechanisms that determine the extent and vertical distribution of this stratospheric cooling were previously unclear,” write Cohen and his team. This made it difficult to model the effect of stratospheric CO2 quantitatively. The researchers have now managed to close this gap. To do this, they analyzed in more detail how the CO2 molecules interact with radiation under different pressure conditions and temperatures.
It turned out that the CO2 molecules in the stratosphere do not react the same way to all wavelengths of infrared radiation. Instead, there is a spectral zone in which they can radiate heat particularly efficiently into space. If the stratospheric CO2 increases, the radiation in this “Goldilocks” zone increases. This ensures particularly efficient cooling. “The vertical structure of stratospheric cooling is also primarily determined by how the Goldilocks zone changes with CO2 concentration,” explain Cohen and his team.
Important confirmation for climate research
The equations developed on the basis of these findings describe quantitatively and in detail for the first time why and how increasing CO2 emissions cool the earth’s stratosphere. “Our results suggest that stratospheric cooling is not caused by increased optical density of the greenhouse gas, but is instead due to the unique spectroscopic behavior of this gas,” the researchers write. This now makes it possible to determine even more precisely what effect this cooling in turn has on climate change.
But the new findings are important for another reason: “The work strengthens the foundations of climate science by showing that our understanding is not just empirical or numerical, but is based on basic physical principles,” explains geophysicist Nadir Jeevanjee from NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, who was not involved in the study. “If simple equations can reproduce the cooling patterns, this proves that basic physical principles are at work here – and not just peculiarities of complex climate models.”
Source: Sean Cohen et al. (Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York), Nature Geoscience, 2026; doi: 10.1038/s41561-026-01965-8