Columbia Climate Scientists Solve a Long-Running Atmospheric Paradox: How Rising Carbon Dioxide Cools the Stratosphere by 2 Degrees Celsius While It Simultaneously Warms the Surface Below

Columbia University climate physicists have published what is being called the most complete explanation yet of one of atmospheric science's strangest signatures: the planet's stratosphere is cooling at roughly the same time its surface is warming, and the same gas drives both. The paper, 'Stratospheric cooling and amplification of radiative forcing with rising carbon dioxide,' appeared in Nature Geoscience on May 11, 2026, and was led by graduate researcher Sean Cohen with co-authors Robert Pincus and Lorenzo M. Polvani of the Lamont-Doherty Earth Observatory.

Measurements from satellite microwave instruments and radiosonde balloons have shown for decades that the stratosphere — the band of atmosphere between roughly 11 and 50 kilometers up that hosts the ozone layer — has been cooling steadily by about 0.3 degrees Celsius per decade, totaling around 2 degrees Celsius since the mid-1980s. The cooling is so distinctive that the U.S. National Academy of Sciences calls it a 'fingerprint' of human greenhouse-gas emissions, since natural variability cannot produce simultaneous lower-atmosphere warming and upper-atmosphere cooling. But the precise physics behind the divergence had never been pinned down quantitatively.

The Columbia team's answer comes down to how the carbon dioxide molecule interacts with infrared radiation at different altitudes. Near the surface, the air is thick and CO2 absorbs upwelling infrared from the warm ground; the energy is quickly redistributed through collisions, trapping heat and raising temperatures. Up in the thin stratosphere, those collisions are rare, so each CO2 molecule that absorbs infrared has time to re-emit it — and a substantial fraction of that re-emission travels straight out to space. The more CO2 you put up there, the more efficient the radiator becomes. 'The stratosphere is essentially turning into a more efficient heat-shedding layer,' Pincus said.

More striking, the Columbia paper argues that the very cooling of the stratosphere amplifies the warming felt at the surface — a positive-feedback wrinkle that earlier climate models had treated only loosely. A colder stratosphere emits less infrared back down toward the troposphere, which in turn leaves more energy trapped near the surface. The team's calculations, run on the high-resolution version of NCAR's Community Earth System Model, suggest the stratospheric-cooling feedback adds about 5% to the total surface warming response to a doubling of CO2. 'It's not the dominant feedback, but it's not zero,' Cohen said.

The result helps reconcile a small but persistent gap between observed stratospheric temperature trends and what global climate models predict, a discrepancy that has fueled scientific debate for nearly a decade. By more accurately handling the wavelength-by-wavelength radiative behavior of CO2 in thin air, the Columbia model brings simulated stratospheric cooling into agreement with the satellite microwave sounding record for the first time. The work also has implications for next-generation weather forecasting models, since stratospheric temperatures influence the polar vortex and, through it, mid-latitude winter cold outbreaks.

The stratospheric cooling fingerprint is also one of the few diagnostic tests that can distinguish anthropogenic warming from possible solar or natural causes — a cooling stratosphere is inconsistent with a warmer Sun, which would heat all layers of the atmosphere. 'This is part of why the scientific community has been so confident the warming is human-caused,' Polvani said. 'Now we have a much sharper quantitative picture of the underlying radiative physics.' The Lamont-Doherty team plans next to extend the analysis to the mesosphere, which is cooling even faster than the stratosphere and where ground-based lidar networks have detected temperature drops approaching 3 degrees Celsius since 1990.