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While our emissions are trapping heat near the surface of the Earth, they are having the opposite effect in the upper atmosphere. For decades, the stratosphere has been cooling. A new study helps explain why.
Down below, carbon dioxide absorbs infrared light and radiates heat, which drives warming. Higher up, carbon dioxide does the same, but because the air is thinner at altitude, much of the radiated heat escapes into space, with a cooling effect.
Researchers have long been aware of this phenomenon, but they have been puzzled by the extent of cooling. Since the 1980s, temperatures in the stratosphere have dropped by roughly 2 degrees C, far more than they have risen down below.
The new modeling study sheds light on this mystery. It finds that as carbon dioxide accumulates, infrared light conveys heat more efficiently. “It’s those changes in efficiency that are going to ultimately be what’s driving stratospheric cooling,” said lead author Sean Cohen, a postdoctoral researcher at Columbia University.
The study, published in Nature Geoscience, “explains a phenomenon that’s a fingerprint of climate change, has been known to occur for decades, and has not been understood,” said coauthor Robert Pincus, also of Columbia.
Scientists say the subject is deserving of further study. Experts warn that the cooling of the stratosphere could slow the healing of the ozone layer, impact orbiting satellites, and fuel more chaotic weather down below.
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