Scientists have long known that something strange is happening in the Earth’s atmosphere. While greenhouse gases are warming the planet’s surface and lower atmosphere, the upper atmosphere is actually getting colder.
Now, researchers from Columbia University say they finally understand the physics behind this unusual effect.
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Their study, published in the journal Nature Geoscience, explains how carbon dioxide behaves differently depending on where in the atmosphere it is located, and how it interacts with infrared light.
Close to the Earth’s surface, carbon dioxide traps heat that would otherwise have escaped into space. This is the main reason why greenhouse gases contribute to global warming.
But higher up in the atmosphere – especially in the stratosphere – carbon dioxide behaves very differently.
The stratosphere extends from about 11 to 50 kilometers above the Earth. In this region, carbon dioxide molecules absorb infrared energy from below and then emit some of this energy into space. In practice, the gas behaves almost like a cooling radiator.
As more carbon dioxide builds up in the atmosphere, the stratosphere becomes even better at sending heat into space – causing it to cool down further.
An effect scientists have been predicting for decades
Scientists predicted this effect already decades ago. Climate models developed in the 1960s by Nobel Prize-winning climatologist Syukuro Manabe already suggested that increased carbon dioxide levels would warm the lower atmosphere while cooling the stratosphere.
Since the mid-1980s, the stratosphere has cooled by about 2 degrees Celsius. Scientists estimate that this cooling is more than ten times greater than what would have occurred naturally without man-made carbon dioxide emissions.
Although the researchers understood the basic idea, the detailed mechanism behind the cooling has been unclear – until now.
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The “Golden Hair Zone” that explains the mystery
The research team, led by Sean Cohen together with Robert Pincus and Lorenzo Polvani, developed new mathematical models to explain the process better.
They discovered that cooling depends largely on how carbon dioxide interacts with different wavelengths of infrared light.
Not all infrared radiation behaves the same. The team found that certain wavelengths fall into what they describe as a “Goldilocks zone” – where carbon dioxide is particularly efficient at sending heat into space.
As carbon dioxide levels rise, this highly efficient zone expands, increasing cooling in the stratosphere.
Less heat escapes – more heat is trapped downstairs
The researchers also investigated the roles of ozone and water vapour. Although these substances may contribute to cooling in the upper atmosphere, their effects were much smaller compared to carbon dioxide.
The study also helps to explain why the warming near the Earth’s surface is accelerating at the same time as the stratosphere is cooling.
As the stratosphere loses heat and gets colder, it emits less infrared energy overall. This means that less heat escapes completely from the Earth – which means that more heat remains trapped in the lower atmosphere.
In other words: Carbon dioxide cools the upper atmosphere while indirectly amplifying warming closer to the ground.
Why this is important
The researchers emphasize that their work is not important because it proves climate change once again – it is already well established. Instead, the study improves scientists’ understanding of exactly how the atmosphere responds to greenhouse gases.
They also believe that the findings could help scientists study the atmospheres of other planets — and even remove exoplanets outside our own solar system.
