Climate change has a paradox hiding in plain sight. While temperatures rise at the surface and in the lower atmosphere, the upper atmosphere has been cooling dramatically.
Scientists have known about this for decades – it’s actually considered one of the clearest fingerprints of human-caused climate change – but the underlying physics has never been fully explained.
A new study conducted at Columbia University’s Lamont-Doherty Earth Observatory has shed more light on this phenomenon.
The research was led by Sean Cohen, a postdoctoral research scientist.
Robert Pincus, a research professor of ocean and climate physics, and Lorenzo Polvani, a geophysicist at Columbia Engineering, are co-authors of the study.
Two atmospheres, two stories
To understand the paradox, it helps to know that the atmosphere isn’t one uniform thing.
The atmosphere behaves very differently at different altitudes, and CO2 – the main driver of surface warming – plays opposite roles depending on where you are.
Down in the lower atmosphere, CO2 does what it’s famous for: it traps heat that would otherwise escape into space, warming the surface below.
But climb higher, into the stratosphere – the layer of atmosphere stretching from about 11 to 50 kilometers above the surface – and the dynamic flips entirely.
Up there, CO2 molecules act more like a radiator than a blanket. They absorb infrared energy coming up from below and emit some of it out into space.
Add more CO2 and the stratosphere radiates heat away more efficiently, cooling down.
Predictions of CO2-induced climate change
This effect was actually predicted back in the 1960s by climatologist Syukuro Manabe, whose models of CO2-induced climate change later earned him a Nobel Prize.
The stratosphere has since cooled by roughly 2 degrees Celsius since the mid-1980s – more than ten times what would have been expected without human-caused CO2 emissions.
“The existing theory was incredibly insightful, but at the moment we lack a quantitative theory for CO2-induced stratospheric cooling,” Cohen said.
Identifying the mechanism
The scientists worked out the details through a methodical, iterative process. They identified the key processes involved and assigned mathematical values to them.
The team compared their pen-and-paper models against comprehensive simulations and real-world data, adjusting the equations, and repeating the process.
What they found at the center of the process was the way CO2 interacts with infrared light (also known as longwave light).
Not all infrared wavelengths behave the same way as they pass through CO2 molecules. Some contribute to cooling far more efficiently than others.
Factors that were ruled out
The team identified a kind of Goldilocks zone of wavelengths that are particularly effective at driving stratospheric cooling. Crucially, as CO2 concentrations increase, that zone expands.
“It’s those changes in efficiency that are going to ultimately be what’s driving stratospheric cooling,” Cohen said.
The researchers also looked at ozone and water vapor, both of which are involved in similar processes. It turned out that compared to CO2, their influence on stratospheric cooling is relatively minor.
A twist that worsens surface warming
The equations the team developed fit neatly with several well-established observations.
Stratospheric cooling becomes more pronounced at higher altitudes. Each doubling of CO2 causes substantial cooling near the top of the stratosphere.
At the same time, a cooler stratosphere allows less infrared energy to escape into space overall.
CO2 makes the stratosphere better at radiating heat outward, which cools it. But because it becomes colder, it ends up radiating less total energy out to space than it otherwise would.
The net result is that more heat stays trapped in the Earth system overall, reinforcing the warming happening below.
CO2, in other words, is simultaneously cooling the stratosphere and making the surface warmer – and the two effects are connected.
“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,” Cohen said.
A new mechanistic understanding
The researchers are clear about what this study is and isn’t.
It’s not another piece of evidence for climate change – that case has long been settled.
What it offers instead is a clearer mechanistic understanding of a process that has been part of climate science for half a century without ever being fully explained.
“This is really telling us what is essential,” Pincus said.
Understanding which factors actually drive stratospheric cooling, and being able to express that mathematically, gives future researchers a more solid foundation to build on.
This foundation includes better models, more precise predictions, and a sharper picture of how the atmosphere actually works.
Applications beyond Earth’s climate
There’s also an unexpected reach beyond Earth. The same physics that governs CO2 behavior in our stratosphere applies, in principle, to the atmospheres of other planets.
A cleaner mathematical theory for stratospheric cooling could help scientists make sense of conditions on other worlds in the solar system and potentially on exoplanets orbiting other stars.
It’s a long way from a quirk in Earth’s temperature record to understanding alien atmospheres.
But that’s sometimes how basic science works. You set out to explain something that’s puzzled people for decades, and you end up with a tool that reaches further than you expected.
The study is published in the journal Nature Geoscience.
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