Researchers note stratospheric cooling from CO2

Sean Cohen, Robert Pincus and Lorenzo M. Polvani published a study in *Nature Geoscience* on May 11 that tackles a question climate scientists have pointed to for decades: why rising carbon dioxide cools the stratosphere even as it warms the surface and lower atmosphere. The paper uses idealized spectroscopy and radiative-transfer models to explain the vertical pattern of that cooling and how it feeds back into carbon dioxide’s warming effect. A Columbia University release on May 12 said the work addressed a long-standing atmospheric puzzle and tied the cooling to how CO2 interacts with infrared wavelengths. ### If carbon dioxide traps heat, why does the stratosphere cool? The stratosphere sits above the troposphere, where most weather occurs, and its energy balance works differently from the air near Earth’s surface. The new paper says the sensitivity of stratospheric temperature to CO2 is driven mainly by mass absorption coefficients in the primary CO2 infrared band, with water vapor and ozone emissions in other parts of the spectrum modifying the response. (nature.com) Nature Geoscience said on May 12 that higher carbon dioxide levels widen the part of the heat spectrum that can emit energy to space. That process cools the stratosphere while strengthening the warming effect of CO2 below, according to the journal’s research summary. ### What did the researchers actually calculate? The paper says each doubling of atmospheric CO2 produces roughly zero to eight kelvin of cooling across the depth of the stratosphere. (researchsquare.com) The authors wrote that the cooling is stronger higher up, a vertical structure they said follows from the spectral physics of CO2 and the modulating role of water vapor and ozone. (nature.com) The same analysis says stratospheric cooling increases the radiative forcing of CO2 by about 75%. In the paper’s framing, that means the cooling is not only a response to rising carbon dioxide but also part of the mechanism that increases the gas’s overall heat-trapping effect. ### Is this a new observation or a new explanation? Models have long predicted, and satellites have long observed, stratospheric cooling from rising anthropogenic carbon dioxide, according to a related *Nature Geoscience* research highlight published this week. (researchsquare.com) That highlight said the magnitude and vertical structure of the cooling had lacked a robust theoretical explanation until now. A 2023 PNAS study led by Benjamin Santer had already compared models and satellite-era observations and found tropospheric warming alongside stratospheric cooling over 1986 to 2022. That earlier work described the cooling pattern as a fingerprint of human influence and reported that the signal intensified with altitude in the stratosphere. ### What evidence underpins the broader claim? (nature.com) The new May 2026 paper is primarily a theory and radiative-transfer study rather than a fresh satellite data release. The authors explicitly say observations and models have already shown that increases in atmospheric carbon dioxide cause globally averaged stratospheric temperatures to decrease, and they set out to explain the mechanisms controlling the magnitude and vertical structure of that response. (pnas.org) Columbia University’s May 12 release said the researchers developed the theory by iterating between simplified calculations and more complete radiative-transfer models. The release said the findings help explain how stratospheric cooling strengthens CO2’s heat-trapping effect. (researchsquare.com) ### Where did this work first appear? Research Square posted a preprint of the study on September 25, 2025 under the title “Why increases in CO2 cool the stratosphere and how this amplifies radiative forcing.” The preprint page says the version of record was later published in *Nature Geoscience* on May 11, 2026 under the title “Stratospheric cooling and amplification of radiative forcing with rising carbon dioxide.” (news.climate.columbia.edu) The next citable version is the journal article published May 11, 2026, and the Columbia release dated May 12 identifies Cohen, Pincus and Polvani as the named researchers behind the work. (nature.com) (researchsquare.com)