Earth’s surface and lower atmosphere are getting hotter, but far above the planet, another dramatic change has been unfolding in the opposite direction. The upper atmosphere has been steadily cooling for decades, creating one of the most recognizable signs of human-driven climate change. Scientists have understood that this was happening, but the detailed physics behind it remained unclear.
Now, researchers at Columbia University say they have identified the mechanism responsible. Their new study shows that the cooling is closely tied to how carbon dioxide (CO2) interacts with different wavelengths of light in the upper atmosphere.
“It explains a phenomenon that’s a fingerprint of climate change, has been known to occur for decades, and has not been understood,” says Robert Pincus, a research professor of ocean and climate physics at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and co-author of the study published in Nature Geoscience.
Why CO2 Heats Earth but Cools the Stratosphere
Near Earth’s surface, CO2 traps heat that would otherwise escape into space, helping drive global warming. But conditions change dramatically higher in the atmosphere.
In the stratosphere, which stretches from roughly 11km to 50 km above Earth’s surface, CO2 behaves less like a blanket and more like a cooling system. The molecules absorb infrared energy rising from below and then emit part of that energy back into space. As CO2 concentrations increase, the stratosphere becomes even better at releasing heat, causing temperatures there to fall.
Scientists first predicted this effect in the 1960s through pioneering climate models developed by climatologist Syukuro Manabe, whose work later earned a Nobel Prize. Since the mid-1980s, the stratosphere has cooled by about 2 degrees Celsius. Researchers estimate this cooling is more than 10 times greater than what would have occurred without human-produced CO2 emissions.
Even so, many details of the process remained unresolved.
“The existing theory was incredibly insightful, but at the moment we lack a quantitative theory for CO2-induced stratospheric cooling,” says Sean Cohen, a postdoctoral research scientist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and the study’s lead author.
