Everyone knows carbon dioxide (CO2) traps heat and warms the Earth. But there’s a twist to the harmful gas that scientists have only now discovered. While warming the Earth, CO2 is simultaneously cooling the upper atmosphere at an alarming rate.
For decades, scientists knew this was happening but couldn’t fully explain why.
Now, a new study has finally laid out the mechanics behind the conflicting phenomena carried out by CO2.
“It explains a phenomenon that’s a fingerprint of climate change, has been known to occur for decades, and has not been understood,” said Robert Pincus, a research professor at Columbia’s Lamont-Doherty Earth Observatory and co-author of the study.
TWO ALTITUDES, TWO FUNCTIONS
Think of carbon dioxide as playing two very different roles depending on where it sits in the sky.
In the lower atmosphere, which is the air we breathe and fly through, CO2 acts like a blanket, trapping heat close to the Earth’s surface and driving global warming.
But roughly 11 to 50 kilometres above the ground, in a layer called the stratosphere, CO2 behaves more like a radiator. There, it absorbs heat rising from below and releases it out into space. Add more CO2, and that radiator becomes more efficient, pushing more heat out and making the stratosphere colder.
Researchers found that the stratosphere has already cooled by around 2°C since the mid-1980s, which is a rate estimated to be more than ten times what would have occurred without human-driven CO2 emissions.
“Here’s this process that we’ve known about for 50-plus years, and we had a pretty decent qualitative understanding of how it worked. However, we didn’t understand the details of what actually drove that process mechanistically,” said Sean Cohen, the study’s lead author and a postdoctoral researcher at Lamont-Doherty.
WAVELENGTH WHERE EVERYTHING CHANGES
The key to this breakthrough finding was identifying how CO2 interacts with different wavelengths of infrared light.
Not all wavelengths behave the same; some are far more efficient at driving stratospheric cooling than others.
The team found a particular “Goldilocks zone” of wavelengths that do most of the work, and as CO2 levels rise, that zone expands.
“It’s those changes in efficiency that are going to ultimately be what’s driving stratospheric cooling,” Cohen said.
The researchers have said that the findings could even help scientists understand the atmospheres of other planets, and dictate plans for future space missions.
But more details are needed to completely understand the phenomena and to extract possible applications.
“The existing theory was incredibly insightful, but at the moment we lack a quantitative theory for CO2-induced stratospheric cooling,” said Cohen.
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