As the temperature of the Earth’s surface and lower atmosphere continues to rise, the opposite phenomenon is occurring in other parts of the Earth’s atmosphere. Far above the Earth’s surface, the upper atmosphere has cooled significantly over decades. Scientists have long known that this unusual contrast is one of the clearest signals of anthropogenic climate change, but the exact physics behind it remained unclear.
Now, researchers at Columbia University have announced that they have finally uncovered the mechanism responsible. Their new study explains how carbon dioxide (CO2) interacts with different wavelengths of light, warming the planet below while cooling the upper atmosphere.
“This describes a phenomenon that is a signature of climate change and has been known to occur for decades but was not understood,” said Robert Pincus, research professor of ocean and climate physics at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School, and co-author of the study published in 2006. natural earth science.
Why CO2 cools the stratosphere
Near the Earth’s surface, CO2 traps heat that would otherwise escape into space, contributing to global warming. However, the situation is very different above the atmosphere.
In the stratosphere, a layer of the atmosphere that extends approximately 7 to 50 miles above the Earth’s surface, CO2 acts like a cooling system. The molecules absorb infrared energy rising from below and release some of that energy into space. As CO2 levels increase in the atmosphere, the stratosphere becomes even more effective at releasing heat, reducing the temperature of the stratosphere.
Scientists first predicted this effect in the 1960s through a climate model developed by climatologist Shuo Manabe, who later won a Nobel Prize. Since the mid-1980s, the stratosphere has cooled by about 2 degrees Celsius. Researchers estimate that this cooling effect is more than 10 times greater than it would be without human CO2 emissions.
Although scientists understood the general idea behind stratospheric cooling, many of the detailed processes remained unresolved.
“While existing theory has been incredibly insightful, we currently lack a quantitative theory of stratospheric cooling due to CO2,” says Sean Cohen, a postdoctoral fellow at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School, and lead author of the study.
Infrared “Goldilocks Zone”
To solve this puzzle, Cohen collaborated with Pincus and Lorenzo Polvani, a geophysicist in Columbia Engineering’s Department of Applied Physics and Applied Mathematics. The research team built a mathematical model that identifies the key processes causing stratospheric cooling. They repeatedly compared their calculations to climate simulations and observational data, and spent months refining the equations until the model matched reality.
Their research pointed to an important factor in how CO2 molecules interact with infrared radiation, also known as longwave radiation.
Not all infrared wavelengths behave the same in the atmosphere. Researchers have found that some wavelengths are particularly effective at promoting cooling. They described this highly efficient range as the “Goldilocks Zone.” As CO2 concentration increases, this zone widens and the atmosphere becomes more efficient at cooling.
“It’s these changes in efficiency that ultimately drive cooling of the stratosphere,” Cohen said.
The researchers also investigated the effects of ozone and water vapor. Although both can influence heating and cooling processes in the atmosphere, their impact on stratospheric cooling was found to be relatively small compared to CO2.
How stratospheric cooling enhances subsurface warming
The team’s equations were successful in reproducing some of the known features of the atmosphere. They agreed with observations showing that cooling becomes stronger with increasing altitude, with the greatest cooling occurring near the top of the stratosphere. The calculations also confirmed that for every doubling of CO2, the stratosphere, the upper boundary of the stratosphere, cools by about 8 degrees Celsius.
The study also highlights important climate feedbacks. Increased CO2 helps make the stratospheric heat radiation more effective, but the resulting cooler temperatures ultimately mean the Earth system radiates less infrared energy throughout space. This strengthens the insulation near the earth’s surface and intensifies the warming of the lower atmosphere.
“This is this process that we’ve known for over 50 years, and we had a pretty qualitative understanding of how it worked, but we didn’t understand the details of what actually mechanically drives the process,” Cohen says.
Cohen and Pincus said the study was focused less on proving the existence of climate change and more on improving scientific understanding of how the atmosphere works.
“This really tells us what’s essential,” Pincus says.
This discovery may have applications beyond Earth. Researchers say the same principle could help scientists better understand the atmospheres of other planets and distant exoplanets.
“Maybe we can better understand what’s going on in the stratosphere of other planets in our solar system or exoplanets,” Cohen said.
